ANSCO COMPANY BINGHAMTQN.NEWYOR- 
 
 Please mention Practical Photography when writing Advertisers < 
 
HOW TO CHOOSE AND 
 USE A LENS 
 
 Practical Photography, No. 3 
 
 EDITED BY 
 
 FRANK R. FRAPRIE, S. M., F. R. P. S. 
 
 Editor of AMERICAN PHOTOGRAPHY and POPULAR PHOTOGRAPHY 
 
 American Photographic Publishing Co, 
 
 Boston, Mass. 
 
 1915 
 
Copyright, 1915, 
 
 by 
 AMERICAN PHOTOGRAPHIC PUBLISHING Co. 
 
 Stanbope jprcss 
 
 H.GILSON COMPANY 
 BOSTON, U.S.A. 
 
HOW TO CHOOSE AND 
 USE A LENS 
 
 Introductory. The purchase of a camera neces- 
 sarily involves the choice of a lens, and while few 
 beginners in photography, whether they select their 
 cameras for themselves or acquire them by gift, 
 make any very logical decision as to the capabilities 
 of the outfit they desire, the time is sure to come to 
 every amateur when intelligent knowledge of what 
 a lens will do is necessary. We will assume that the 
 reader is about to buy a camera, and that he knows 
 nothing about photography. We will endeavor to 
 show him what a lens is, and what each type now 
 obtainable will do. With this knowledge he can 
 study the catalogues of lens and camera manu- 
 facturers with more confidence, and decide for him- 
 self whether the expenditure of five dollars or fifty 
 dollars for a camera and lens making a given size of 
 picture is best for his necessities. We may say, 
 however, that practically every step in increased 
 price of either lens or camera is fully justified by 
 advance in careful workmanship, increased facility 
 of operation, or more versatile performance, and that 
 the higher-priced outfits in case of forced sale usually 
 bring a greater percentage of original cost. He who 
 begins with an outfit of limited capacity and con- 
 
 3 
 
 328323 
 
- TQ CHOOSE 
 
 tinues in photography invariably desires a more 
 flexible equipment. We therefore unqualifiedly ad- 
 vise the original purchase of the best outfit which 
 the buyer's means will command, as the most satis- 
 factory and economical course in the end. 
 
 In the majority of cases, the selection of one's 
 first lens means the purchase of a camera complete 
 and ready for use, but this scarcely limits the scope 
 for choice, for every grade of lens may be had today 
 on the moderate priced cameras of all makers, and 
 only in the cheapest styles is there no choice of lens 
 equipment. We will begin with a consideration of 
 the simplest forms of lenses, but we must first con- 
 sider the elementary principles of optics. 
 
 Light and Its Transmission. To the physicist, 
 light presents itself as vibrations of the luminiferous 
 ether, a perfectly elastic medium which is supposed 
 to fill space, and the principles of optics are deduced 
 from a mathematical consideration of the changes of 
 direction and velocity of these waves or vibrations 
 when passing from one medium to another. A much 
 simpler conception is to assume that every luminous 
 point emits rays of light which travel in absolutely 
 straight lines in every possible direction as long as 
 they remain in the same medium. In passing from 
 a rarer medium, such as air, to a denser one, such as 
 glass, the speed of the rays is lessened, and from 
 this fact follows the possibility of a lens, as will 
 appear later. 
 
 That light travels in a straight line may be easily 
 observed. Place a dark screen through which a pin- 
 hole has been pierced between the eye and a distant 
 arc light. But one position of the screen can be 
 
AND USE A LENS. 5 
 
 found through which the light will be visible, and 
 this is in a straight line between the eye and the 
 light. This can be mathematically proved, as can 
 all the facts about light and lenses which will follow, 
 but the reader who wants such proof must seek it in 
 more extensive treatises than this. 
 
 Pinhole Images. If we make a pinhole in the 
 light-tight shutter of a room from which all daylight 
 is excluded, and place behind this a white card, we 
 will see upon the card an image in natural colors of 
 outside objects. Observing this, we will see that the 
 image is inverted, the sky being at the bottom, and 
 objects at the right outdoors appearing on the left 
 side. This is the natural and only possible conse- 
 quence of the fact that light travels in straight lines. 
 Rays from the top and bottom of any object con- 
 verge at the pinhole, cross there, and reach the card 
 to form the inverted image. Lenses also cause the 
 rays to cross, and so the image on the ground glass 
 of the camera is inverted. 
 
 The Pinhole Camera. The beauty of such an 
 image produced in a dark room inspires the thought 
 that it might be recorded by photography, and in 
 fact the simplest apparatus for making photographs 
 is a light-tight box which contains a minute opening 
 in the centre of one end. At the opposite end a 
 sensitive plate is placed to receive the inverted image 
 of external objects produced by this minute opening. 
 The nearer the plate is to the pinhole, the smaller 
 the image of any given external object, but the 
 larger the area of the view outside which is recorded 
 on the plate. 
 
 The sharpness of the image produced in a pinhole 
 
6 HOW TO CHOOSE 
 
 camera depends partly on the fineness of the aper- 
 ture, and partly on the distance of the plate from it, 
 but mainly on the first. If the aperture could be 
 made so small that only a single ray without thick- 
 ness could pass through it, the image would be ab- 
 solutely sharp. A decrease of the area of the aper- 
 ture means a corresponding loss in brightness of the 
 image, and if it is made too small there is also a lack 
 of sharpness from the phenomenon known as dif- 
 fraction, which is a bending from their course of 
 part of the rays of light which strike the edge of 
 the opening. This, though small in amount with a 
 large opening, deflects an appreciable proportion of 
 the light passed by a very small opening. In practice, 
 therefore, the pinhole must have a fairly large di- 
 ameter to produce an image bright enough to impress a 
 plate in a reasonable time, and the practical problem 
 is to find the mean between lack of sharpness from 
 diffraction and that caused by too large an aperture, 
 which allows a large number of rays from any given 
 point to pass through, spreading out as they go and 
 causing unsharp contours. At best the pictures made 
 with a pinhole camera are far from sharp, and the 
 method is curious rather than useful, though it has 
 been much practiced in recent years, and several 
 books have been written on the subject. There is 
 considerable skill involved in making a successful 
 pinhole, and those interested in the subject would 
 find the purchase of the proper apparatus nearly as 
 cheap as its manufacture. 
 
 Pinhole Attachments. There are on the market 
 several styles of pinhole attachments which may be 
 substituted for the usual lens on any camera from 
 
AND USE A LENS. 7 
 
 which the objective is detachable. One style, selling 
 at sixty cents, has an opening listed as No. 6 on the 
 scale given below. A more expensive form, listing 
 at $2, has four pinholes and an aperture for the 
 purpose of focusing. The holes are numbered ac- 
 cording to the system devised by Alfred Watkins on 
 D'Arcy Power's suggestion, and called by Watkins 
 " Watkins-Power " numbers. The complete list is 
 as follows: 
 
 W.-P. No. 
 
 Diameter in inches 
 
 Nearest Needle No. 
 
 Best Distance for Plate 
 
 3 
 
 0-053 
 
 No. i 
 
 40 inches 
 
 4 
 
 0.040 
 
 No. 4 
 
 20 inches 
 
 5 
 
 0.032 
 
 No. 5 
 
 15 inches 
 
 6 
 
 0.027 
 
 No. 7 
 
 10 inches 
 
 7 
 
 0.023 
 
 No. 8 
 
 8 inches 
 
 8 
 
 0.020 
 
 No. 10 
 
 5 inches 
 
 10 
 
 0.016 
 
 No. 12 
 
 32 inches 
 
 12 
 
 0.013 
 
 No. 13 
 
 2$ inches 
 
 Using the W.-P. Number. Multiply the W.-P. 
 number by the distance from pinhole to plate and 
 use the result as the / value of the pinhole. Then 
 whatever the exposure would be for that / number 
 with a lens in seconds or fractions thereof is the ex- 
 posure for the pinhole in minutes or fractions thereof. 
 For example, W.-P. hole No. 6 used at a distance 
 of 10 inches from the plate is regarded as /:6o. If 
 the exposure, as determined by The American Photog- 
 raphy Exposure Tables, or by an exposure meter, 
 for a lens working at /:6o under the given conditions 
 is 2 seconds, then 2 minutes should be given for the 
 pinhole exposure. This system of calculating pinhole 
 exposures is the most convenient yet devised. 
 
8 HOW TO CHOOSE 
 
 Utility of the Pinhole. As indicated in the table 
 given above, there is a most favorable distance for 
 the use of any given pinhole, but it is not absolutely 
 necessary to have more than one, and a No. 6 pinhole 
 may be used on an ordinary camera at any distance 
 within the capacity of the bellows, with results vary- 
 ing very little. The pinhole camera finds a certain 
 application in architectural photography, especially 
 in cramped quarters, for it may be used to cover an 
 extremely wide angle if the plate is very near the 
 opening. The image is perfectly rectilinear. Owing 
 to the slight diffusion and consequent obliteration of 
 the finer detail inseparable from pinhole exposures, 
 pictures thus made are much in vogue among workers 
 of artistic temperament. 
 
 Refraction of Light. The reason that we cannot 
 get a perfectly sharp image with a pinhole is that, as 
 it is larger than a mathematical point, the rays from 
 any point in the object form a cone in passing through 
 it, and continue to diverge indefinitely. We there- 
 fore need some means of once more bringing these 
 rays back to a point, and this we do by means of a 
 lens, the utility of which is due to the phenomenon 
 known as refraction. As we have previously stated, 
 when rays of light pass from a rarer medium to a 
 denser, their speed is retarded; on the other hand, 
 the velocity is increased in passing from a denser 
 medium to a rarer one. When the passage from one 
 medium to another takes place in a direction per- 
 pendicular to the surface dividing the two media, 
 there is no change in the direction of the rays, though 
 their velocity is changed. When the direction of the 
 rays is oblique to this surface, the course they follow 
 
AND USE A LENS. 9 
 
 is bent or refracted. We will not go into any detailed 
 explanation of the laws of refraction; it will be 
 sufficient for us to know at present that when a ray 
 of light passes from air through a piece of glass the 
 sides of which are parallel, the direction of the ray 
 after emerging is parallel to its direction when it 
 entered, while if the sides of the glass are not parallel, 
 so that it forms a wedge or prism, the direction of 
 the ray is changed, and its course is bent or refracted 
 away from the angle of the prism and toward its 
 base. If we take two prisms and set them base to 
 base, it is evident that two parallel rays, one of 
 which passes through each prism and is duly re- 
 fracted toward its base, must eventually cross, and 
 these two prisms are the germ of the lens. Parallel 
 rays passing through one of these two prisms would 
 emerge with their direction changed, but still parallel, 
 and would cross the corresponding rays bent by the 
 other prism at variable distances from the prisms. 
 The amount of the bending depends on the angle in- 
 cluded by the prism, increasing as this does, and so 
 if we have a number of very thin slices of prisms of 
 continually increasing angles piled on each other, we 
 could make successive parallel rays bend more and 
 more toward the base, so that they would all cross 
 corresponding rays from the similar prisms on the 
 other side at a single point. Now, an ordinary lens 
 is just such a collection of portions of prisms, for all 
 curved surfaces made by mechanical means are com- 
 posed of small surfaces joining each other at large 
 angles, instead of being truly continuous curves. 
 Thus we have found that by placing a lens of glass of 
 the proper form in the place of our pinhole, we can 
 
io HOW TO CHOOSE 
 
 cause the rays to come together instead of separat- 
 ing, and it is a matter of common experience that 
 thereby a sharp image can be formed, instead of the 
 diffused one produced by the pinhole. 
 
 Simple Lenses. From the two prisms placed 
 base to base may be derived three forms of lenses, all 
 of which are thicker at the centre than at the edges. 
 They are called the double convex, with two convex 
 surfaces, the plano-convex, with one convex surface 
 and one flat surface, and the convex meniscus, with 
 one convex and one concave surface, but thicker at 
 the middle than at the edges. All of these forms of 
 lenses cause the light rays to converge until they 
 meet upon the axis of the lens, an imaginary line 
 drawn perpendicularly through the centre of the lens 
 surfaces, and they are therefore called converging or 
 positive lenses. If we assume that the two prisms 
 are placed together point to point instead of base to 
 base, we get the foundation for three more kinds of 
 lenses, the double concave, the plano-concave, and 
 the concave meniscus. All these lenses are thicker at 
 the edges than in the middle, cause rays of light to 
 diverge from the axis, and are called diverging or 
 negative lenses. These six forms of lenses are all 
 used in photography, and from them in combination 
 all types of photographic lenses are constructed. 
 The curved surfaces which bound them are in practice 
 always spherical, for in the process of grinding it is 
 mechanically easy to produce nearly perfect spherical 
 surfaces, whereas the construction of a lens whose 
 surfaces should be sections of a paraboloid, hyper- 
 boloid, ellipsoid, or other geometrical solid more com- 
 plex than the sphere, while not impossible, would be 
 
AND USE A LENS. n 
 
 a very expensive and laborious task. It is neverthe- 
 less probable that lenses with surfaces of this more 
 complicated description will be produced in the 
 future, for it seems impossible at the present time to 
 introduce much further refinement into present day 
 lenses that are made with spherical surfaces alone. 
 
 Dispersion of Light. We have now seen that it 
 is possible to make rays of light cross each other on 
 the axis of a lens by the use of a double convex or 
 other converging lens. It is, however, no more pos- 
 sible to make a perfect and satisfactory photograph 
 with a single lens of this character than we have 
 seen it to be with a pinhole. In order to understand 
 the most elementary reason therefor, we must revert 
 to our illustration of the prism. In considering this, 
 we saw that when a ray of light falls upon a prism 
 the ray is bent out of its course. A further observation 
 can be made of the effect of a prism upon light. If a 
 beam of sunlight falls upon the prism, and is bent 
 from its course, upon leaving the prism it no longer 
 appears as a beam of white light, but if we place 
 beyond the prism a sheet of white paper, we will find 
 upon it instead of white light, a band of light of 
 different colors. In other words, the prism has split 
 up or dispersed the beam of white light into a band of 
 colors called the spectrum, in which the light passes 
 by gradual change from one color to another, in the 
 following order: violet, blue, green, yellow, orange, 
 red. The order of these colors is always the same, 
 and the violet ray is refracted the most while the red 
 ray is refracted the least. 
 
 Chromatic Aberration. A lens of course acts in 
 the same way as the prism and separates the white 
 
12 HOW TO CHOOSE 
 
 rays of light which pass through it into their colored 
 components. The violet rays therefore cross on the 
 axis of the lens at a point nearer the lens than do the 
 red rays. The point at which any given rays come 
 together on the axis of the lens is called the focus of 
 these rays, and thus in the use of a simple lens we 
 find that the focus of the violet rays is nearer the 
 lens than the focus of the red rays. Photographically, 
 this is unfortunate, for the image which we see with 
 the eye on the ground glass of the camera is composed 
 mostly of yellow rays, which have the greatest visual 
 luminosity, that is, affect the eye the most. On the 
 other hand, the photographic plate is most affected 
 by the violet rays, and when we have focused on the 
 image formed by the yellow rays, and put a plate in 
 place of the ground glass, the image produced by the 
 violet rays is brought to a focus nearer the camera 
 than the plate, and the image which we photograph 
 is out of focus, diffused or blurred. It is possible by 
 means of a simple calculation to find out how much it 
 is necessary to move the plate after focusing, approxi- 
 mately ^V of the focal length of the lens, in order to 
 place it at the focus of the violet or actinic rays, as 
 they are called, instead of at the focus of the yellow 
 rays which the eye easily perceives, but this is a 
 clumsy manipulation, and a simpler method has been 
 found to obviate this difficulty. Opticians have dis- 
 covered that glasses of different chemical constitution 
 have different dispersions, that is, split up white 
 light to different extents, and that by the combination 
 of a positive or collecting lens, of a kind of glass 
 known as crown, with a negative or dispersing lens of 
 another kind of glass known as flint, of a different 
 
 
AND USE A LENS. 13 
 
 dispersive power, both the chemical and the visual 
 rays can be brought together at a single point. When 
 this has been done, the lens is said to be achromatic, 
 and practically all modern lenses are achromatized 
 for two points in the spectrum, one in the yellow and 
 one in the violet. For certain purposes it is neces- 
 sary to produce lenses of finer color correction than 
 this, and lenses for three-color work, for instance, are 
 corrected so that there is a common focus for three 
 different points in the solar spectrum, corresponding 
 to the three colors chosen to produce the image. 
 Such lenses are known as apochromats. 
 
 Spherical Aberration. If we were to separate 
 from the solar spectrum light of a single color, and 
 allow this to fall upon the whole surface of a simple 
 collecting lens, we would find that the lens would 
 be unable to bring all the rays of light of a single 
 color to a single focus. The rays passing through the 
 edge of the lens would be bent a little more sharply 
 and would come to a crossing on the axis somewhat 
 sooner than those passing through the lens nearer the 
 centre. This property is inherent in all lenses having 
 spherical surfaces, and is hence called spherical aber- 
 ration. It may be partly corrected by using a 
 diaphragm which will cut off the rays of light passing 
 through the outside of the lens and allow only those 
 striking the central portion to be used, but it is 
 more completely corrected in practice by combining 
 a positive and a negative lens, of opposing spherical 
 aberrations, in such a way that one counteracts the 
 other. Modern glasses are of such diverse proper- 
 ties that it is possible to correct a lens simultane- 
 ously for chromatic and spherical aberration. When 
 
I 4 HOW TO CHOOSE 
 
 a single combination has been so corrected that it will 
 produce sharp focus, with a comparatively large work- 
 ing aperture, it is said to be aplanatic or an aplanat. 
 
 The Achromatic Meniscus Lens. The simplest 
 type of lens used in photography of any importance 
 today is that corrected for chromatic aberration, and 
 known as the achromatic meniscus. This is the kind 
 of lens which is used in most of the cheap so-called 
 fixed-focus cameras, which sell from $i up. Certain 
 very cheap cameras may be equipped with a simple 
 spectacle lens, but they are of so little photographic 
 importance that this type of lens does not need to 
 be considered at length. The achromatic meniscus 
 works well in bright summer sunlight, with a snap- 
 shot shutter working at about ^ of a second and 
 gives a reasonably well-defined image for small direct 
 prints. This type of lens, however, though partly 
 corrected, still has a number of defects, which make 
 it impossible to obtain minutely sharp definition ex- 
 cept with a very small stop, and even if such stops 
 are provided on the camera, they can rarely be used 
 except in the very brightest light. 
 
 Usefulness of the Meniscus Lens. The user of a 
 cheap type of camera almost invariably desires to get 
 a large section of the view before him on his film or 
 print, and for this reason achromatic meniscus lenses 
 are usually of very short focus, thereby taking in a 
 large angle of view. As a result of this, and the ad- 
 ditional fact that the lens is stopped down so as to 
 utilize only the centre and best working part of the 
 lens, a camera of this type will give a fairly sharp 
 image of objects from about eight feet from the 
 camera to the limit of visibility, without any pro- 
 
AND USE A LENS. 15 
 
 vision for focusing. Consequently such cameras are 
 usually called " fixed-focus " cameras, a term evi- 
 dently designed by some advertising man to make 
 capital from the fact that it is not necessary to focus 
 such cameras for ordinary landscape pictures. Prob- 
 ably the term " average-focus " would be a better 
 one for a camera of this type, but whether the name 
 be proper or not, such instruments always give fairly 
 sharp images of objects at a reasonable distance from 
 the camera, and answer admirably for beginners and 
 those who do not care to give much thought to their 
 photography. They are excellent for vacation pic- 
 tures, and for those who desire to make mementoes 
 from time to time without becoming familiar with the 
 difficult manipulation of a better lens, and as a 
 matter of fact, the quality of achromatic meniscus 
 lenses sold in the United States is so high that many 
 pictures made with such lenses compare favorably 
 with those made with much more expensive lenses. 
 It is an ordinary experience for an amateur who has 
 been taking pictures with such a camera to turn out 
 much less successful work when he begins to use a 
 focusing camera with a better lens. This, however, is 
 merely due to his inexperience with the more compli- 
 cated apparatus, and as soon as he has learned to 
 manipulate it properly, he is sure to get better results 
 from the higher-grade outfit. 
 
 Defects of Meniscus Lenses. The meniscus lens 
 usually has only two aberrations even partly corrected. 
 There are three other important aberrations of lenses, 
 and these are not corrected in the meniscus lens. 
 The meniscus cannot render straight lines as such 
 when they are near the margin of the picture, and is 
 
16 HOW TO CHOOSE 
 
 consequently not suitable for taking pictures of build- 
 ings, though in landscape work this fault is unim- 
 portant. This defect is called curvilinear distortion. 
 The meniscus lens also suffers seriously from curva- 
 ture of field and astigmatism, two defects which will 
 be more fully explained further on, and the cheaper 
 grades will have some spherical aberration not cor- 
 rected. All of these shortcomings render it impossible 
 for such a lens to render the finest detail absolutely 
 sharp even in the centre of the field, and this is even 
 more the case at the corners. Finally, in order to 
 make it work at all, it has to be stopped down to such 
 a small aperture that the lens will not allow short 
 exposures to be made, and consequently is useless 
 under many circumstances. 
 
 Meniscus or Rapid Rectilinear? Ordinarily the 
 choice of a first lens for the beginner consists of de- 
 ciding between two cameras of the same model, one 
 fitted with a single meniscus achromatic and the 
 other with a rapid rectilinear lens, a type which we 
 will fully describe a little further on. The difference 
 in price is usually from two to six dollars. There are 
 a few cameras, the smaller and cheaper sorts more 
 particularly, which are offered with meniscus lenses 
 only. Such instruments are supplied with time and 
 instantaneous shutters and are of the fixed-focus type. 
 Naturally as the price is lower and the user does not 
 need to bother with focusing, these simple cameras 
 have been sold by thousands where the more elaborate 
 ones have been disposed of in hundreds. It is evident 
 that they must give good results, or they would not be 
 so popular. Let us, then, run over the advantages of 
 the meniscus achromatic lens and see what its field is. 
 
AND USE A LENS. 17 
 
 The meniscus lens is seldom used for picture sizes 
 larger than 4X5 or 3i X 5l (postcard) and the 
 focal length or distance from the film to the lens is 
 rarely more than 6j inches. The stop or opening 
 through which the light passes is seldom any larger 
 than /:i6, which, for a 6j-inch lens, is 6| 4- 16, or 
 i% inch. Such a stop is only of medium size, 
 compared to the focus of the lens, and it has the ad- 
 vantage that it allows a good average focus to be 
 obtained. The maker carefully adjusts the camera 
 until it makes a reasonably sharp image of all objects 
 from 8 feet to the far distance, and then arranges the 
 instrument so that when it is ready for use the lens 
 will always be at the right position to make this 
 average focus sure. A few postcard-size cameras 
 with meniscus lenses are, however, provided with an 
 adjustable front with pointer and focusing scale, so 
 that the operator can focus. Distances in feet are 
 marked on the scale, for example, 50, 15, and 6. It 
 is not necessary to have a focusing scale for the smaller 
 picture sizes, such as 2j X 2\ or 2\ X 3i, because the 
 lenses of shorter focal length used for them have 
 more depth of field and can, in accordance with 
 optical laws, focus at once objects near and far. 
 
 Finding the Hyperfocal Distance. The formula 
 by which the depth of sharp field can be found is: 
 Multiply the square of the focal length in inches by 
 100 and divide by the / number of the stop. For 
 a 4-inch lens working at /:8 this would be 
 
 4 X 4 X 100 
 
 = 200 inches = 16 feet 8 inches. 
 
 8 
 
 The distance found by this formula is called the 
 hyperfocal distance for that particular stop. If the 
 
1 8 HOW TO CHOOSE 
 
 lens is focused (by trial) on an object at this distance, 
 it will be found that the depth of sharp field is from 
 half the hyperfocal distance to the limit of vision. 
 Thus, in the example given, the 4-inch lens will re- 
 produce all objects more than 8 feet 4 inches from the 
 camera with reasonable distinctness. If the smaller 
 stop of /:i6, which is the largest usually supplied with 
 meniscus lenses, is used, the hyperfocal distance will 
 become 8 feet 2 inches, and the nearest object in 
 focus will be 4 feet I inch away. Camera makers, 
 however, usually set the lens so that it will not focus 
 objects closer than 6 feet, in order to get a better 
 distribution of the sharpness of definition at the fur- 
 ther distances. Any user of a camera with a meniscus 
 lens can find its focal length closely enough for prac- 
 tical purposes, if the catalog does not state the focal 
 length in describing the camera, by measuring from 
 the plane of the film to the optical centre of the 
 lens, which, with sufficient accuracy for most pur- 
 poses, may be assumed to be at the front surface. 
 
 Stops. Fixed-focus cameras costing more than 
 a dollar are generally provided with three stops, 
 which take the form of holes pierced in a metal 
 plate which can be moved so as to bring any chosen 
 aperture into place in front of the centre of the lens. 
 Each smaller opening usually has half the area of the 
 next larger, and therefore admits only half as much 
 light during an exposure of the same length. When 
 the speed of the shutter is unalterable (that of most 
 snap-shot shutters, if unmarked, may be taken as 
 3s second), the only way one can regulate the amount 
 of light reaching the film is by changing the stop. 
 If the largest stop is /:i6, and very few cameras 
 
AND USE A LENS. 19 
 
 with meniscus lenses are furnished with any larger 
 aperture the other stops are usually f:22 and f:^2 
 and require respectively twice and four times the ex- 
 posure of /:i6. They can be used for snapshots, in 
 the latitudes of the United States, only for subjects 
 of exceptional brilliancy, such as snow scenes or sea 
 views, because exposures on average landscapes re- 
 quire all the light which /:i6 will admit when the sun 
 is shining between the hours of 9 A.M. and 3 P.M. 
 
 Shutter Speeds. The regulation of exposure by 
 changing the size of the stop or diaphragm is sup- 
 plemented by altering the time the shutter is allowed 
 to remain open. The lowest priced cameras with 
 shutters have a rotary shutter arranged for snapshots 
 (or " instantaneous ") and " time;" the more elaborate 
 possess one giving " time," " bulb," and " instan- 
 taneous;" still more expensive ones provide speeds 
 marked in fractions of a second from I second to yitf 
 second, or even up to vfa or ^J^ second, if one is will- 
 ing to pay the price. However, one would find a 
 fast shutter of little use with a meniscus lens, so 
 manufacturers furnish only the TIB sort with this 
 lens. The " bulb " exposure is arranged so that the 
 shutter will remain open as long as pressure is main- 
 tained on the rubber bulb or on the flexible metal 
 release which has now almost driven the bulb and 
 tube from the market. The " time " exposure is 
 arranged so that it takes one pressure to open the 
 shutter and a second pressure to close it. A mo- 
 ment's thought will show that by combining different 
 lengths of exposure with different stops, one can 
 secure almost any desired amount of light action 
 greater than that represented by ?V second at /:i6, 
 
20 HOW TO CHOOSE 
 
 but nothing less than A- second at /:32. The light 
 action of the latter exposure is too short for practical 
 instantaneous use with a meniscus lens, except occa- 
 sionally in full sunlight on snow or sea. 
 
 What Meniscus Lenses Will Do. The greater 
 portion of the pictures interesting to the amateur in 
 the earlier stages of his photographic career can be 
 taken successfully with a meniscus achromatic lens. 
 The beginner is usually most interested in photo- 
 graphing friends or scenes incident to vacations or 
 holidays. Such pictures are generally made in the 
 summer time, between 9 A.M. and 3 P.M., well 
 within the range for summer snapshots already indi- 
 cated. In weaker sunlight, good results can be suc- 
 cessfully obtained by using a tripod and giving a 
 quick bulb exposure with the smallest stop. In fact, 
 if the tripod is utilized as much as possible, there are 
 few stationary subjects which cannot be well taken 
 with a meniscus lens. The smaller the stop, the more 
 clear and brilliant the rendering of fine detail in the 
 picture. The smaller the stop, the longer the ex- 
 posure. These two statements mark the limitations 
 of the lens, and show that its work is practically con- 
 fined to still subjects. If the subject is not in motion, 
 it can usually be successfully taken with a meniscus 
 lens, as far as the single factor of exposure is con- 
 cerned. There are optical defects inherent in the 
 meniscus lens, however, which render it unsatis- 
 factory for use on special classes of subjects. The 
 subject limitations of the meniscus lens will be more 
 apparent after we have considered the 
 
 Optical Defects of Single Lenses. A single men- 
 iscus achromatic lens cannot be so made that it will 
 
AND USE A LENS. 21 
 
 render straight lines as such at the margin of the 
 picture, or render fine detail critically sharp all over 
 the film, or represent a flat surface as equally sharp 
 all over. For general amateur photography these 
 defects matter only in architectural subjects. The 
 single lens is not suitable for taking pictures of build- 
 ings. It is too slow for making snapshots under poor 
 conditions of lighting. It cannot, in other words, be 
 expected to do the same work as an expensive lens. 
 Low price, in lenses, means small aperture or work- 
 ing speed, with consequent limitation to stationary 
 objects and tripod exposures if it is to be used on 
 subjects covering the whole field of photography. 
 
 Special Types of Meniscus Lenses. For artistic 
 photography, some workers prefer a lens which will 
 not give a perfectly sharp image, and so opticians 
 have produced the so-called " soft-focus " meniscus 
 lenses to satisfy pictorial workers. The original lens 
 of this sort, the " Smith Semi- Achromatic," was in- 
 tended to give the same quality of soft definition as 
 the pinhole, but to possess a speed of /:6, thus allow- 
 ing snapshot exposures instead of the prolonged time 
 exposures of a minute or more needed for a pinhole. 
 These lenses are partly achromatic, that is, they are 
 constructed to bring the yellow visual rays and the 
 blue photographic rays to nearly the same indefinite 
 focus, though they give less halo or flare around the 
 margins of light-colored masses if used with a yellow 
 ray filter. They do not, however, require a correc- 
 tion of focus, as does a lens made with a single piece 
 of glass, but give on the plate the same effect seen on 
 the ground glass. Such lenses naturally cannot be 
 used successfully with any camera which is not pro- 
 
22 HOW TO CHOOSE 
 
 vided with a ground-glass screen, for the image is so 
 soft and harmonious that it needs the most careful 
 focusing. By employing a smaller stop, the user can 
 secure somewhat greater sharpness of definition, but 
 never critically fine detail. As the softness depends 
 largely on the optical defect known as spherical 
 aberration, it is easier to secure the precise effect one 
 desires if the final adjustment of focus is made with 
 the stop to be used for the exposure, for a lens having 
 spherical aberration changes its focus slightly for 
 each stop. The makers of these lenses now manu- 
 facture also a series of double lenses of the same type, 
 which are much faster and can be made to give vari- 
 ous degrees of correction of chromatic aberration and 
 spherical aberration by stopping down. 
 
 The Use of Soft-focus Lenses. It is somewhat 
 difficult for one who has used only fully corrected 
 lenses to pass to the soft-focus type, and at once 
 master its peculiarities. A lens of this class produces 
 pictures which are not in sharp focus but have a 
 softness which tones down the hard outlines and 
 minute details of objects without losing the drawing 
 or entirely suppressing the details necessary to render 
 textures. There is no one plane where the picture is 
 sharpest. There is a region of focus rather than one 
 plane, and it is possible to move the lens some little 
 distance backwards and forwards without affecting the 
 definition to a marked degree, yet there is just one 
 point for each stop where the effect is most pleasing. 
 The lighting conditions govern to some extent the 
 stop which may be used, for there is more halo around 
 light-colored objects in a brilliant than in dull light- 
 ing. At any stop, it is important to use the " out- 
 
AND USE A LENS. 23 
 
 side focus " for firmness of drawing, for the " inside 
 focus " tends to produce too indefinite an effect. In 
 other words, the lens should be moved away from the 
 plate until the image goes out of focus and then be 
 brought back until the desired rendering of detail is 
 obtained. Lenses of the single series have from 50 
 to 60 per cent correction, and therefore show more 
 halo around the lights than the doublets of 75 per 
 cent correction, so that they must be stopped down 
 more to get rid of the flare or used in a duller light. 
 
 Soft-focus lenses are perhaps best fitted for home 
 portraiture, but the full aperture has to be handled 
 carefully, or there will be a band of halo spreading 
 from the lights into the shadows; for example, along 
 a white collar where it comes near a dark suit. This 
 effect may be minimized by using a double-coated 
 orthochromatic plate with a light yellow filter (2x) and 
 stopping down to /:8. The doublets give the same 
 effect at about /:5-6; in other words, they are prac- 
 tically twice as fast. As, however, the softening of 
 skin texture obtained by using a large opening is 
 often desirable, to say nothing of the saving in time, 
 it is advisable to use light backgrounds and light 
 draperies and to employ rather a flat lighting. If 
 the contrast between the lighted side and the back- 
 ground is small, the halo becomes negligible. The 
 shadow side should be so well lighted that the color 
 of the flesh is quite visible in the deepest shadow as 
 well as in the highest light. Underexposure exagger- 
 ates the flare. The spherical aberration blends the 
 finer details without losing the form, so that minor 
 skin blemishes disappear, and there is no need of re- 
 touching. Direct prints give the same impression of 
 
24 HOW TO CHOOSE 
 
 skin texture that one receives in looking at a person, 
 for one sees the face as a whole, not as multitudinous 
 lines, freckles, moles, pores, etc. If used with dis- 
 cretion, lenses of this type give results of much 
 artistic value, but diffusion of focus alone cannot 
 make a poor composition into a picture, and over- 
 done " fuzziness " is a blemish rather than a merit. 
 
 Other Soft-focus Lenses. Several other varieties 
 of soft-focus lenses were introduced after the Smith. 
 The Spencer Port-Land is a single meniscus lens pos- 
 sessing a distinctive character of its own. It gives 
 great softness without losing the drawing, and works 
 nominally at about /:4-5, though few workers can 
 utilize its image at any stop larger than about /:5-6, 
 on account of its giving a number of overlapping 
 images. Its softness is different from that of any 
 other lens. Then there is the Wollensak Verito, a 
 doublet intended chiefly for portraiture and work- 
 ing at /:4. The best results with this lens are per- 
 haps obtained by stopping down to /:5-6 or /:6. At 
 /:i6, the image is not very different from that of an 
 ordinary lens. Many workers look upon this prop- 
 erty as an advantage, as the Verito will allow them 
 to make soft or sharp pictures at will by simply 
 changing the stop. Yet another make in considerable 
 vogue is the Struss, which has been used with much 
 success by several New York pictorial workers in the 
 short time during which it has been on the market. 
 The great advantage of the soft-focus objectives as a 
 class is that they allow the individual to vary the 
 quality of the definition to suit different subjects and 
 to express his own personality in the picture. 
 
 Curvilinear Distortion. As we have mentioned 
 
AND USE A LENS. 25 
 
 before, a single lens cannot render straight lines as 
 such near the margin of the picture. This defect is 
 partly dependent on the varying thickness of the lens 
 from edge to centre, and partly on the position of the 
 diaphragm. If the stop is placed in front of the lens, 
 straight lines are bowed out from the centre of the 
 picture, producing what is known as barrel-shaped 
 distortion; when the stop is behind the lens, the lines 
 bow in, and cushion-shaped distortion is the result. 
 Consequently we cannot use a single lens for archi- 
 tectural photography, for copying, or for any purpose 
 where it is necessary to reproduce straight lines, 
 especially near the edge of the picture. This defect 
 has a very simple and obvious remedy. 
 
 Rapid Rectilinear Lenses. When two single 
 meniscus lenses are mounted at opposite ends of a 
 lens tube with the diaphragm or stop between them, 
 the barrel-shaped distortion of the rear lens is cor- 
 rected by the cushion-shaped distortion of the front 
 lens. The result is that straight lines are rendered 
 as such, and the lens is therefore called rectilinear. 
 The speed, or ratio between focal length and diameter 
 of stop, is also greatly increased by this arrangement, 
 being in fact almost exactly doubled. Most cameras 
 of moderate price are fitted with rapid rectilinear 
 lenses (often abbreviated R.R.). The defects of 
 spherical aberration and curvature of the field, as 
 well as astigmatism, are not eliminated, so the chief 
 advantages over the single lens remain those already 
 stated, namely, rectilinearity and greater speed. 
 Further corrections cannot be made with only two 
 varieties of glass, crown and flint. 
 
 To illustrate the formation of a rectilinear, let us 
 
26 HOW TO CHOOSE 
 
 suppose that a single lens of 16 inches focal length 
 and a speed of /:i6 is being used on a 5 X 7 plate. 
 The great focal length is necessary, since the field of 
 a meniscus lens is considerably curved and only the 
 centre of it can be used if sharp definition is to be 
 obtained at this large aperture. The diameter of the 
 stop is I inch. If another lens exactly like the first 
 is mounted in the front of the lens-tube, the focus of 
 the combined lens is now approximately half that of 
 either element, namely 8 inches, and for simplicity 
 of statement we may assume that the i-inch stop now 
 has the value /:8. This number is found by divid- 
 ing the focal length, 8, by the diameter of the stop, i. 
 The speed of the lens is now four times as great as 
 that of the single lens, as shown by the ratio between 
 the squares of the numbers: 16 X 16 is to 8 X 8 as 
 256 to 64, or as 4 to i. The reason for taking the 
 squares is that openings pass light in the ratio of 
 their areas, and with circular apertures the areas are 
 in proportion to the squares of their diameters. 
 The / numbers, then, are based on this ratio and ex- 
 press the relation between focal-length of lens and 
 diameter of opening. Any lens marked /:8, for ex- 
 ample, no matter what its focal length, is practically 
 as fast as any other lens also marked /:8. There may 
 be slight variations due to difference in color and 
 thickness of the glass, but they are unimportant as 
 compared with the permissible latitude of exposure. 
 In the example given above, we have seen how a 
 definite opening of I inch may be either /:i6 or /:8, 
 according to the focal length of the lens or lenses in 
 place in the lens-tube. 
 Utility of the Rapid Rectilinear Lens. A camera 
 
AND USE A LENS. 27 
 
 fitted with double or triple extension of the bellows 
 and a symmetrical or a convertible rectilinear lens 
 will be found a most useful general-purpose instru- 
 ment. The symmetrical rectilinear is composed of 
 two halves alike in construction and usually of the 
 same focal length, while the halves of the convertible 
 lens are generally of different focal lengths. The 
 complete lens is used for all ordinary work. The 
 halves of the lens may be used for landscape or 
 other work containing no important straight lines, 
 portraits, and distant objects. Lenses of this char- 
 acter are offered under a great many fancy names in 
 camera catalogues; but they are all rectilinears and 
 possess the faults already enumerated of spherical 
 aberration at the margins, curvature of field, and 
 astigmatism. Readers are urged to beware of putting 
 extra money into a camera because the rapid rectilin- 
 ear lens is described as having unusual corrections, 
 for which an extra price is charged. There is made 
 in the United States only one such lens, the Recti- 
 graphic, of which this is true, and its superiority re- 
 sults from the use of three instead of two elementary 
 lenses in each of the single combinations. Aside from 
 this lens, if an extra price is to be paid, it is better to 
 invest in one of the low-priced anastigmat lenses. 
 Rapid rectilinear lenses supplied on foreign cameras 
 are generally styled aplanats. The typical rapid rec- 
 tilinear lens works at /:8, but it does not cover its 
 plate with critical sharpness over the whole surface 
 unless it is stopped down to at least /:io. Rapid 
 rectilinear lenses are occasionally diaphragmed so as 
 to work at larger apertures than /:8. They can be 
 thus used for special work, such as portraiture, but 
 
28 HOW TO CHOOSE 
 
 cannot give good definition, as the larger opening 
 introduces more errors. 
 
 Choosing a Rapid Rectilinear Lens. When one is 
 buying a camera and it is offered in two models at a 
 slight difference in cost, the lower-priced being fitted 
 with a meniscus, and the higher-priced with a rapid 
 rectilinear, the choice depends on whether one is con- 
 tent to accept the disadvantages of the meniscus for 
 the sake of the average-focus feature. If not, the 
 rapid rectilinear should unhesitatingly be chosen. A 
 good rectilinear is an excellent general-purpose lens 
 for amateur photography. The makers can be relied 
 upon when one is buying a complete camera, whether 
 film or plate. The case is a little different when a 
 lens is to be chosen from several listed with a par- 
 ticular camera. 
 
 Cameras fitted with a single-extension bellows, such 
 as most roll-film and many film-pack and plate cam- 
 eras, gain nothing by having a convertible lens. The 
 symmetrical form of rapid rectilinear is usable on 
 such instruments only as a whole. When a double 
 extension of bellows is provided the rear lens alone 
 can be used. The more complete instruments of 
 folding or view style, with triple extension of bellows, 
 should preferably be fitted with a convertible lens. 
 For instance, in the 5X7 size a typical outfit has a 
 focus of 7J inches for the doublet, 12 inches for the 
 rear, and 18 inches for the front element. The bel- 
 lows extension is 20 inches. 
 
 Value of the Rapid Rectilinear Lens. For general 
 amateur work, a camera fitted with a rapid rectilinear 
 lens having a focal length about equal to the diagonal 
 of the film or plate is very suitable. It has sufficient 
 
AND USE A LENS. 29 
 
 speed for snapshots in sunlight at all seasons of the 
 year, and at reasonably early and late hours. When 
 it is fitted to a film camera the focusing scale has, of 
 course, to be relied on ; and this may at first trouble 
 the beginner. A few suggestions about focusing may 
 be of assistance here. The common amateur sizes 
 are 3J X 5i (postcard) and 4X5 and smaller, as far 
 as hand camera snapshots are concerned. The focal 
 lengths of the lenses usually fitted to these sizes by 
 makers are 6f and 6j inches. These lenses will give 
 a good general focus when set at 25 feet, with stop 
 /:8 in use. When set at 15 feet and stopped down to 
 /:i6, they focus everything 8 feet or farther from the 
 camera. The only real difficulty in focusing comes 
 when objects rather close, such as figure-studies or 
 portraits outdoors, are to be attempted, and ordi- 
 narily one can pace off the distance, or otherwise 
 measure it, for accurate estimation of the distance is 
 absolutely essential. 
 
 The Shutter. Of more importance than the lens 
 itself, in one way, is the exposure-shutter to which 
 it is fitted. The minimum of efficiency with a rapid 
 rectilinear lens is obtained when it is fitted to the 
 TIB shutter. Even then, however, it is of vastly 
 greater efficiency than the meniscus, for the simple 
 reason that the speed of the meniscus is such that, 
 for snapshot work, we are always verging on the 
 ragged edge of underexposure, whereas with the rapid 
 rectilinear, a snapshot exposure of approximately -5 
 second at /:8 gives full exposure not only for sum- 
 mer work in various lights, but also on an increased 
 field of subjects. When, however, the shutter has 
 time, bulb, and regulated speeds marked I, |, J, *V, 
 
30 HOW TO CHOOSE 
 
 s l v, and T iff second, the rectilinear is capable of doing 
 far better work. If The American Photography Ex- 
 posure Tables are consulted, the user can soon count 
 on getting a good picture almost every time. Con- 
 sidering /:8 as the ordinary aperture for snapshot 
 work, the range of instantaneous speeds just given 
 will cover all common requirements. In fact, we 
 have seen many satisfactory pictures made with a 
 rapid rectilinear lens set in an even simpler shutter, 
 having T, B, and three speeds supposed to run from 
 ^V to T&T7 second. We are of the opinion that the 
 most generally useful shutter for the rapid rectilinear 
 lens would be one having accurate speeds of TO > sV> ds 
 and jitf second. 
 
 Limitations of the Rapid Rectilinear Lens. Al- 
 though for most work the astigmatism and curva- 
 ture of field of the rapid rectilinear make little 
 difference, there are some pictures in which the poor 
 marginal definition is a menace to success. The 
 fault, strange as it may seem, is most likely to occur 
 when the worker is focusing on the ground glass. 
 The reason is that he tries to get the middle of the 
 picture critically sharp. This is easy, for the lens is 
 well adapted to give a considerable fineness of defi- 
 nition in the centre of its field. We well remember 
 the trouble which used to occur with a 6| X 8j 
 camera, fitted with an n-inch rapid rectilinear lens. 
 At /:8, focusing near the centre, the sharpness was 
 confined to a 5-inch circle and the rest of the plate 
 was very blurry. Through the help of a friend, it 
 was soon learned that the curvature of the field 
 would have to be split. Instead of focusing on the 
 centre, we marked on the ground-glass points about 
 
AND USE A LENS. 31 
 
 midway between the centre and the margins, focus- 
 ing on them and afterwards stopping down a little 
 if necessary. This method should invariably be 
 adopted when a rapid rectilinear is used on a camera 
 which allows focusing on the ground glass, for the 
 results will thus be as good as the qualities of the lens 
 will permit. The smaller the stop used, the less the 
 effects of the still uncorrected errors of the lens will 
 show. 
 
 Curvature of Field. The rapid rectilinear lens, 
 introduced about 1865, represents practically the 
 limit of correction obtainable with the old glasses 
 known as crown and flint. As we have already 
 stated, it is still afflicted with serious optical defects, 
 absolutely irremovable with these materials. Obvi- 
 ously, as the glass dry plate must be flat to be pre- 
 pared on any commercially possible scale, the image 
 produced by the lens should be in sharp focus over the 
 whole of a large flat surface. This is distinctly not 
 the case with the simple lens, the achromatic com- 
 bination, or the rapid rectilinear. With each of these 
 forms of lens, the image is brought to a focus on a 
 saucer-shaped field which, of course, cannot coincide 
 with a flat glass plate, though the use of a small stop 
 to a large degree obviates this defect, together with 
 many others. In the modern anastigmat this defect 
 is almost entirely overcome. 
 
 Astigmatism. The fifth and last of the important 
 aberrations of the lens, the most difficult to explain 
 and the hardest to remove, is known as astigmatism. 
 It is not only an affliction of the artificial lens made 
 in the workshop, but is unfortunately known by name 
 and effects to thousands of wearers of eyeglasses, who 
 
32 HOW TO CHOOSE 
 
 probably have little conception of just how their 
 affliction is caused. It is an optical defect by reason 
 of which a lens cannot bring to a sharp focus at one 
 time lines which run in different directions on a 
 plane surface. The oculist's test chart contains a 
 figure made of radial lines looking like the spokes 
 of a wheel. The reader whose eyes are affected with 
 astigmatism will see most of these lines sharp, but 
 one or more blurred. So the photographic lens which 
 is not corrected for astigmatism will render a series 
 of lines covering the whole surface of the plate sharp 
 in some portions of the image, and blurred in others. 
 By racking the lens back and forth any given line 
 can be made sharp, but some others, running in a 
 different direction on the same part of the plate, or 
 in the same direction on a different portion of the 
 plate, will be blurred. The defect is due to the in- 
 ability of the marginal portions of a lens made of 
 ordinary crown and flint glass to bring to a focus in 
 the same plane the images of lines radial to the lens 
 and tangential to it. When one is sharp, the other 
 will be blurred. 
 
 Mathematicians proved that astigmatism could be 
 removed from lenses by the production of glass of 
 certain definite qualities and a German chemist had 
 faith that such glasses could be produced. The 
 chemist, Schott, and the scientist, Abbe, joined hands 
 in 1884 and founded the famous glass-works at Jena, 
 which, with financial aid from the Prussian govern- 
 ment, finally produced not only the single sort of 
 dense barium crown glass of high refractive index 
 which was necessary for the first anastigmat, but an 
 enormous series of new types of glass of the most 
 
AND USE A LENS. 33 
 
 varied properties, which meets almost every need 
 of the optician, and has allowed the production of 
 lenses of qualities quite undreamed of a generation 
 ago. In curing astigmatism, the optician has also 
 been able to mold the other qualities of the lens al- 
 most at his will. 
 
 Advantages of the Anastigmat. Most anastig- 
 mats are not only free from astigmatism but possess 
 also very complete corrections for chromatic aber- 
 ration, spherical aberrations of different classes, and 
 curvature of the field. As most of these faults are 
 well removed in the rapid rectilinear lens at a work- 
 ing aperture of /:i6 or smaller, the chief advantage of 
 the anastigmats is speed. They do not require stop- 
 ping down to secure crisp, brilliant definition from 
 corner to corner of the plate. They do not need any 
 special precautions in focusing the image of a flat 
 object, for the image is almost as flat as the plate, 
 instead of being saucer-shaped, as with the simpler 
 lenses. They require, it is true, somewhat more care 
 than the R. R. in focusing. A very slight move- 
 ment of the focusing screw is sufficient to produce all 
 the difference between definition no better than that 
 of a common lens and the most beautiful, minute 
 sharpness. The margins of the picture are just as 
 crisp and clear as the middle, even if the sharpest focus 
 is secured at the centre. In other words, the anas- 
 tigmat at full aperture gives a sharper picture than 
 the R. R. will give unless the latter is stopped down 
 to /:i6 or smaller. This means that the practical, 
 everyday, available speed of the anastigmat is from 
 4 to 7 times that of the rapid rectilinear, depending on 
 the full aperture of the former. Even the lower-priced 
 
34 HOW TO CHOOSE 
 
 anastigmats working at /:8 or /:77 show this real, 
 practical increase of speed when compared with rapid 
 rectilinears working at the same nominal aperture. 
 The anastigmat need never be stopped down save to 
 secure increased depth of sharp field. This brings us 
 to a consideration of the next step in the price of an 
 outfit furnished with a choice of rapid rectilinear and 
 anastigmat lenses. 
 
 Rapid Rectilinear or Anastigmat. Reference to 
 almost any catalogue of cameras will show that an 
 anastigmat working at/:8 or/:7.7 is supplied for from 
 $5 to $10 more than the rapid rectilinear lens equip- 
 ment. More can be paid if one wishes a shutter 
 with a wider range of speeds or a lens working at 
 /:6.8, /:6.3, or faster. However as the vast bulk of 
 amateur work is done with apertures of /:8 or smaller, 
 and we may consider /:8 the average aperture for 
 general snapshot work, it is worth while appraising 
 the slower anastigmats. Should a particular camera 
 be desired, if the makers do^not offer this type of 
 anastigmat lens, the apparatus may be bought fitted 
 with the rapid rectilinear, and the anastigmat lens in 
 cells may be purchased separately and screwed into the 
 shutter in place of the glasses of the rapid rectilinear. 
 One American lens maker offers a set ready for sub- 
 stitution in the shutter of any 3^ X 5? kodak or 
 camera for $10. The same maker offers an /:6.8 lens 
 and shutter for $20 in postcard size. Several other 
 manufacturers offer similar opportunities to fit rapid 
 rectilinear outfits with anastigmat equipment. The 
 reader can easily see that price need no longer be a 
 serious obstacle to his owning a high-grade lens. 
 
 Price, notwithstanding, remains an important con- 
 
AND USE A LENS. 35 
 
 sideration. The novice finds it difficult to under- 
 stand why some anastigmats can be furnished with 
 a camera for only $5 increase in price while others 
 cost $20 to $50 more. There are several reasons for 
 the great range of prices. Manufacturing cost is the 
 most important. A lens with simple construction 
 and shallow curves can be ground and polished in a 
 very small fraction of the time required for deeper 
 curves. The cost of mounting and testing varies 
 tremendously for different types of construction. In 
 every instance, the faster the lens, the more difficult 
 it is to secure perfection, so the higher must be the 
 price. The construction is usually more complicated, 
 because it is difficult to correct a large aperture with 
 a simple form of construction. These causes may 
 make an /:4-5 lens, for instance, cost more to manu- 
 facture than one of a similar type working at /:6.3. 
 Speed, that desirable quality which allows very short 
 exposures, must be paid for. 
 
 Some formulas allow much more complete cor- 
 rection of certain optical errors than other apparently 
 similar formulas. The older anastigmats generally 
 retained some spherical aberration, particularly the 
 variety known as coma, which slightly softened the 
 image. Getting rid of coma in the latest types has 
 been a long and costly struggle, but most makers 
 have now produced lenses which are as finely cor- 
 rected as it is possible for objectives to be. In fact, 
 it is doubtful whether some of the present lenses can 
 be surpassed, unless some method of economically 
 grinding curved surfaces more complex than those of 
 the sphere can be devised. 
 
 Although anastigmats vary in excellence, all are 
 
36 HOW TO CHOOSE 
 
 better than rapid rectilinears and few workers are 
 likely to demand of even the cheaper ones more than 
 they are capable of doing. It is only when one is 
 doing special work, such as copying maps or plans, 
 etc., that a minute amount of coma would interfere 
 with the result. In general amateur and professional 
 photography, the anastigmat will answer all purposes 
 even if it is not perfectly corrected. To choose one 
 is a problem which can be solved only by knowing 
 with what apparatus the lens is to be used and what 
 classes of work are to be attempted. We advise 
 using some anastigmat, if possible; but the exact sort 
 to get is something which each user must decide for 
 himself. 
 
 The Types of Anastigmats. In the course of 
 time, partly to enable new manufacturers to produce 
 lenses which should not conflict with previous patents, 
 and partly because opticians have ceaselessly tried 
 to improve upon prior achievements, the number of 
 types of anastigmats has steadily increased. It may 
 be useful for us to review some of these, especially 
 those which have been placed upon the American 
 market, for there are a number of anastigmats which 
 are not sold in this country. 
 
 In the first place, it may be of interest to tell just 
 how the first anastigmat became possible. Before 
 the Jena glass factory was founded, there were but 
 two kinds of optical glass, crown and flint, and from 
 this normal glass-pair, as it is called, opticians were 
 able to make only a chromatically corrected lens 
 without astigmatic correction. The collecting lens in 
 this type both refracts and disperses less than the 
 dispersing lens. To get the anastigmatic correction, 
 
AND USE A LENS. 37 
 
 it was necessary to make the collecting lens of a more 
 highly refractive glass than the dispersing lens, while 
 the ratio of the dispersions was similar to that of the 
 older glasses. Abbe and Schott made such glasses, 
 and the combination is called an anomalous glass- 
 pair. The achromatic lenses made from these are 
 called new achromats to distinguish them from the 
 old achromats. The new achromat can be corrected 
 or over-corrected for astigmatism, but not for spheri- 
 cal aberration, while the case is reversed with the old 
 achromat. The combination of an old achromat and 
 a new achromat allows the correction of both errors, 
 as well as distortion, and the remaining aberration, 
 curvature of field, is disposed of by properly separat- 
 ing the two groups of lenses. 
 
 The Protar. The first anastigmat was made on 
 this basis in 1890, and was calculated by Rudolph of 
 Jena for the Zeiss works. It was an unsymmetrical 
 doublet, with central stop, and is still marketed as 
 the Series V Protar, used for wide-angle work. The 
 front combination is the same as that of a rapid 
 rectilinear lens, while the rear half is a new achromat, 
 the fourth glass being of the heaviest barite crown, 
 the glass which makes anastigmats possible. 
 
 Anastigmatic^Correction of the Single Lens. The 
 next step in the progress of the anastigmat was the 
 production of a single lens, which was achieved by the 
 combination of three glasses in the form of half of a 
 rapid rectilinear in which the rear glass has been 
 replaced by a new achromat. Rudolph immediately 
 developed this further into a four-glass combination, 
 which was composed of a new achromat and an old 
 achromat cemented together. This lens, developed 
 
38 HOW TO CHOOSE 
 
 in 1895, is the basis of the Series VII Protars, working 
 at / 112.5. The corrections for the various aber- 
 rations made possible by this formula are very satis- 
 factory, so that these single lenses are excellent 
 anastigmats of great brilliancy and crispness of defi- 
 nition. They are so slow that they find little favor 
 today except in the form of sets. 
 
 Convertible Sets. The Series VII Protar is the 
 basis of the convertible Protar sets. Each set con- 
 sists of a number of similar objectives with front 
 stops, which may be screwed into the two ends of the 
 lens tube, so that they can be used either singly or in 
 pairs. A set of two elements of different focal lengths 
 gives three focal lengths, three elements give six 
 focal lengths, and four elements give ten focal lengths. 
 Such a combination of like or similar halves auto- 
 matically corrects distortion, coma, and some other 
 lens errors. The double objective is therefore better 
 corrected than the single lens, and has a much larger 
 aperture. With such sets it is possible to photograph 
 a given object from any point in different sizes. 
 In combining the single lenses, it is advisable not 
 to exceed the ratio of two to three in focal length be- 
 tween the elements, the [lens of greater focal length 
 being always placed in front of the diaphragm. When 
 the single element is used, it should always be placed 
 in the rear end of the tube with the stop in front. 
 When two Series VII Protars are permanently com- 
 bined, we get a doublet known as the Series Vila, 
 working at the aperture of /:6.3 instead of the/:i2.5 
 of the single lens. 
 
 The Protar has never been surpassed as a sym- 
 metrical anastigmat doublet, with the possibility of 
 
AND USE A LENS. 39 
 
 using the halves singly. Its nearest competitors in 
 this respect are the three-glass cemented elements of 
 the Dagor and Collinear types. The halves of a large 
 number of symmetrical anastigmats, especially those 
 having elements composed of two separate lenses, 
 cannot be used singly, because they would have to 
 be stopped down too sharply in order to obtain a 
 moderately useful image. 
 
 The Double Anastigmat. While Rudolph was 
 working out his Protars, Von Hoegh of the Goerz 
 firm elaborated the three-glass element differently, 
 by combining two identical elements of Rudolph's 
 first type to form the double anastigmat, now known 
 as the Dagor. In his opinion, his doublet lens was 
 better corrected than Rudolph's Protar, but the 
 elements of the Dagor are not as well corrected as 
 those of the Protar. The effective aperture is /:6.8 
 for the shorter foci and less for the longer. This lens 
 is probably the most successful anastigmat ever 
 constructed. It has been made by hundreds of thou- 
 sands, and as the patents have expired, it is now con- 
 structed under various names by practically all lens 
 makers. Its maximum stop value of /:6.8 is nomi- 
 nally 50 per cent faster than that of the rapid rectilin- 
 ear lens. Actually it is at least five times as fast as 
 this lens, for the rapid rectilinear must be stopped to 
 /:i6 to approach it in defining power. At its full 
 working aperture the Dagor has almost perfect flat- 
 ness of field, with critical definition to the extreme 
 corners of the plate. Half of the lens is a good land- 
 scape lens. The complete doublet used with medium 
 or small stops is almost free from optical errors over 
 its entire circle of illumination, and therefore can be 
 
40 HOW TO CHOOSE 
 
 used on plates larger than those for which it is listed, 
 as a wide-angle lens. 
 
 Other Three-glass Cemented Systems. Other 
 anastigmat elements have been used by various 
 makers. One, calculated by Von Hoegh, has long 
 been made by Watson of London, under the name 
 Convertible Lens, while the Zeiss Convertible Series IV 
 is of the same type. Another invention of this type 
 was produced simultaneously by Voigtlander of 
 Brunswick and Steinheil of Munich, and introduced 
 under the respective names of Collinear and Ortho- 
 stigmat. These lenses not only work at /77 to /:6.8, 
 but can also be satisfactorily corrected in smaller sizes 
 to work at /:54. As new glasses have been pro- 
 duced at Jena, it has been possible still further to 
 improve this class of lenses, until one of them, the 
 Euryplan Series Va, works satisfactorily at the high 
 speed of /4-5, about three times as fast as the rapid 
 rectilinear at its largest opening. 
 
 Eight- and Ten-lens Double Objectives. The 
 Rietzschel Linear is similar to Rudolph's four-glass 
 element, differing in the arrangement of its dis- 
 persing and collecting surfaces. In practice this and 
 other eight-glass lenses do not surpass the six-glass 
 symmetrical anastigmat, while the manufacturing 
 difficulties, especially that of centering the various 
 glasses in assembling the lens, naturally increase. 
 For this reason opticians have not looked with favor 
 on increasing the number of glasses, and while ten- 
 lens systems have been constructed, the only one now 
 on the market is the Turner-Reich. Increasing the 
 number of glasses may increase the efficiency of the 
 single element, but produces no improvement in the 
 
AND USE A LENS. 41 
 
 doublet in proportion to the increased expense of 
 production. It is possible that there remains for 
 this type of lens a great increase of working aperture, 
 but this field has not yet been thoroughly investigated. 
 
 Triplets with Air Spaces. In 1894, H. Dennis 
 Taylor, scientific director for the famous English tele- 
 scope manufacturer, Cooke, invented a lens consist- 
 ing of three single glasses separated by air spaces, 
 which effectively eliminated coma. This is now 
 manufactured in two types, one working at f:g to 
 /:6.8, the faster series working at /:5-6 to /:4-5- The 
 Cooke lenses, and similar ones made by Voigtlander 
 under the names Triple Anastigmat and Portrait 
 Anastigmat, are not convertible, but where this 
 feature is not required it is hard to find a more satis- 
 factory objective, as the corrections of the different 
 series are well carried out, and the lenses are of very 
 compact construction and light weight, especially 
 when mounted in aluminum barrels. As the Cooke 
 lenses are completely free from coma, they have been 
 found of great value for photo-engraving purposes, 
 and a majority of all lenses used in the United 
 States for this purpose are of this type. 
 
 Heliar, Tessar, and Dynar. Other makers soon 
 produced types derived from the Cooke by the in- 
 troduction of another kind of glass and of a collecting 
 cemented surface. The Voigtlander Heliar was cal- 
 culated in 1902 by Harting, and in this lens the zonal 
 errors are so minimized that the lens can be con- 
 structed with the full aperture of /:4-5, up to a focal 
 length of 24 inches. The Heliar is a five-glass system, 
 containing two doublets with a single lens between, 
 all separated by air spaces. The lens is entirely free 
 
42 HOW TO CHOOSE 
 
 from coma over the whole field. The Zeiss Tessar, 
 calculated by Rudolph in 1902, contains four glasses, 
 the rear cemented combination consisting of two 
 glasses. The first Tessar worked at /:6.3, but it is 
 now made for general use to work at /:4-5, and for 
 cinematography to work at /:3-5- Harting states 
 that the Tessar and the Heliar are as completely 
 corrected as it is possible to make lenses, and con- 
 siders them unlikely ever to be surpassed. They 
 have no excessive curvatures and the spherical errors 
 are completely removed over the whole field. The 
 ic Tessar covers a larger area in proportion to focal 
 length than any other lens yet made. Harting also 
 calculated for Voigtlander another five-glass combi- 
 nation, the Dynar, which works at /:6, and is in- 
 tended for general hand-camera use. It shows the 
 same complete corrections as the Heliar. 
 
 Four-glass Air-space Types. The replacement 
 of the central glass by an air space in the single com- 
 bination of the Orthostigmat or Collinear type re- 
 sulted in producing lenses of somewhat less compli- 
 cated construction, but of very good efficiency. 
 Such lenses have been produced by various makers 
 in speeds up to /:4-5- Among them we may mention 
 the Ross Homocentric, and the Goerz Celor and 
 Syntor. These lenses have good anastigmatic cor- 
 rections. There are many other lenses of this type 
 on the market, but some of them, while well cor- 
 rected for astigmatism, show a great amount of coma. 
 For some purposes this is no disadvantage, but in 
 many cases it is found necessary to stop them down 
 strongly to overcome this defect. 
 
 Miscellaneous Types. In addition to the many 
 
AND USE A LENS. 43 
 
 groups of lenses already enumerated, there have been 
 marketed numerous others, from the Aldis, with two 
 cemented glasses in front and a single one behind the 
 diaphragm, a lens which gives a most remarkable 
 sharpness of definition, to the air-space lenses contain- 
 ing many separate glasses, and cemented ones contain- 
 ing as many as ten elementary lenses. Some of these 
 lenses are of the highest quality, while others are in- 
 ferior. In buying lenses, one has to rely partly on 
 the reputation of the maker and partly on the results 
 of test exposures. The principal point of dispute be- 
 tween lens makers today is as to the relative efficiency 
 of cemented and air-space lenses. 
 
 Why is there a Difference Between Anastigmats? 
 At this point the reader may ask, " What is the neces- 
 sity for so many types of anastigmats and which is 
 actually the best?" The only answer that can be 
 given is that there is no best anastigmat, because 
 with the number of variable elements at the com- 
 mand of the lens calculator, it is not possible in any 
 lens to remove all the aberrations for light of all 
 wave lengths, and for all zones of the lens. A mathe- 
 matical optician must choose which aberrations he 
 will completely remove over the whole field, and for 
 all colors, and calculate his lens accordingly. If he 
 is making a lens for the three-color worker, he must 
 produce three sharp images of three different colors 
 which will be coincident in size, and lenses of this 
 type are called apochromatic. If he is making a lens 
 for the process-engraver, he must endeavor to remove 
 coma absolutely, probably at the expense of speed. 
 For the hand camera worker, speed is the greatest 
 consideration, but a moderately large field must be 
 
44 HOW TO CHOOSE 
 
 covered. For cinematography everything can be 
 sacrificed to the production of enormous aperture, 
 even down to /:i.9 Again, competition in price 
 exists, and simplicity of manufacture must therefore 
 be taken into account in low-priced lenses, necessi- 
 tating fewer glasses, fewer cemented surfaces and 
 shallow curves, even at some expense in corrections. 
 
 The Performance of Modern Unsymmetrical An- 
 astigmats. The performance of lenses seems to 
 have reached its greatest possible height, under 
 present conditions, in lenses containing air-spaces, 
 such as the Cooke, Heliar, Ross, Tessar, and others. 
 In these lenses there is such a high degree of freedom 
 from aberration with large aperture and wide angle 
 of view, that no practical necessity exists for im- 
 provement. Some may, however, prefer symmetri- 
 cal lenses with cemented halves, because with them 
 we have the choice of a long or short focus. Such 
 lenses will give absolute sharpness at the edge of the 
 plate at full aperture, and if stopped down will give 
 complete sharpness of definition over the whole field 
 of illumination, so that the modern universal ob- 
 jectives, as we may name systems working at from 
 /:6 to /:7, are really wide angle lenses which define 
 just as well as an astigmatic wide angle, but at a 
 much larger opening. In this connection we may re- 
 mark that if extremely wide angles are required the 
 Hypergon double anastigmat of Goerz, which consists 
 only of meniscus lenses with a diaphragm between, 
 works, when focused at /:22 and exposed at /:3i, 
 over the enormous angle of 140 degrees, an angle 
 which constructive optics can never hope to increase. 
 
 The Limit of Effective Aperture. While anastig- 
 
AND USE A LENS. 45 
 
 mats for cinematographic purposes can easily be made 
 to give perfect anastigmatism at/:3.5, and satisfactory 
 performance down to/: 1. 9, for hand camera work or 
 any purpose of general photography, no aperture 
 larger than /4.5 is useful to the average pho- 
 tographer. At any larger opening, while greater 
 rapidity of exposure can be obtained, it is not pos- 
 sible to obtain sufficient depth. A lens of this speed 
 must be focused with the utmost care, and prac- 
 tically the only finder which is satisfactory is the 
 camera itself. For some purposes focusing on a 
 ground glass with a tripod would be satisfactory, but 
 for speed work a lens of this aperture should always 
 be used on a reflecting camera, for only with an in- 
 strument of this type is it possible easily and cer- 
 tainly to obtain critical focus. In general, the lens 
 working at /4-5 cannot give as great an extent of 
 sharp field as one working at/:8, for even when a faster 
 lens is stopped down to the same aperture, it is sur- 
 passed by the slower lens, because in sacrificing speed 
 it is possible to correct astigmatism over a larger 
 field. As to sharpness in the centre of the field, both 
 anastigmats and astigmats may have good definition, 
 depending on the perfection of the corrections for 
 spherical aberrations. But certain types of anastig- 
 mats, especially the Tessar, Heliar, and Dynar, give 
 a central sharpness which is sought in vain among 
 the astigmatic lenses. Lenses working at /:3-5 are 
 used to a slight extent by press photographers and 
 others who believe they must possess the utmost 
 possibility of speed. 
 
 Air-Spaces. Some makers who produce only 
 cemented lenses intimate that any air-space lens 
 
46 HOW TO CHOOSE 
 
 must be inferior because of the internal loss of light 
 by reflection. Their opponents retort that thick 
 cemented lenses lose as much light by absorption as 
 the air-space lenses do by reflection. Dr. Hans Hart- 
 ing says, " The question of the relation of actual 
 brilliancy in the case of cemented and uncemented 
 lenses of the same relative opening is very often 
 raised. Absorption depends on the kind of glass; 
 as in the same melt various portions of glass may 
 vary with respect to absorption, the loss of light may 
 vary somewhat even in two lenses of the same con- 
 struction and focal length. For several years I had 
 the opportunity of comparing almost uninterruptedly 
 for their photographic brilliancy a six-lens symmetrical 
 objective, consisting of two halves of three cemented 
 lenses, and an objective consisting of three single 
 lenses. Both anastigmats of medium focal lengths 
 have the same relative aperture, either /:6.8 or /7-7. 
 I was never able to distinguish a photographically 
 measurable difference in the actual brilliancy of these 
 two kinds of objectives representing the extremes of 
 construction. It is true that the loss of light in the 
 simple landscape lens is less than in the Zeiss Double 
 Pro tar consisting of eight lenses. But even this 
 difference is unimportant in picture taking, in com- 
 parison with the far greater deviations from the 
 proper exposure in sunlight which even the most ex- 
 perienced photographer cannot avoid, but which are 
 made absolutely imperceptible in the process of 
 development." The truth seems to be that well- 
 designed air-space objectives pass as much light as 
 cemented ones of the same effective aperture. Under 
 unfavorable circumstances, too many air-spaces cause 
 
AND USE A LENS. 47 
 
 general fogging of the image through scattered light 
 inside the camera, but this fault is mainly to be 
 ascribed to the fact that the circle of illumination is 
 much larger than the plate covered, so that light is 
 reflected from the inside of the bellows. The obvi- 
 ous remedy for this is to employ a lens hood, which 
 will eliminate the light before it enters the camera. 
 If bright lights appear in the picture, as in night 
 photography, the air-space lens may show false 
 images called ghosts, or spots of light called flare 
 spots, but the only lens which would not show these 
 under such trying conditions is a single lens contain- 
 ing only one piece of glass. These images caused by 
 internal reflection are often so much less bright than 
 the original that they do not appear in the photo- 
 graph, though they may be visible on the ground 
 glass. General fog ascribed to air-spaces is often 
 due to lack of sufficient sodium sulphite in the 
 developer, and in this case will disappear if the 
 developer is so compounded that each fluid ounce 
 contains ten grains of anhydrous sulphite. 
 
 The choice between cemented and air-space lenses, 
 then, depends chiefly on the value of convertibility, 
 adaptability to wide angle work, and smaller factors. 
 If one is using only a film camera with single exten- 
 sion of bellows, the air-space type of lens is perfectly 
 satisfactory. If one's camera has double or triple 
 bellows extension, the symmetrical or convertible 
 anastigmat may be preferable, as it enables one to do 
 more kinds of work. This then brings us to 
 
 The Choice of an Anastigmat. Speed. The first 
 consideration is that of speed. Anastigmat lenses are 
 offered of all speeds from /:8 to/:i.9, the latter for 
 
48 HOW TO CHOOSE 
 
 motion-picture work only. The higher speeds are 
 only really available for general work in lenses of 
 moderate focal length, for the depth of field is so 
 small in the larger sizes that they can be used only on 
 objects which do not move and which are nearly in 
 one plane. This confines their use almost wholly to 
 portraiture and copying. For general photography, 
 the greatest speed ever useful is/4.5. Lenses of this 
 speed may be used very successfully on hand cameras 
 in miniature sizes. Above 3J X 4i they should be 
 used only on reflecting cameras or those provided 
 with a focusing screen, and for general photography 
 5 X 7 is the useful limit of size with this speed. 
 With a 6| X 8J plate, /:5.6 is about the limit, while 
 /:6 to /:6.3 marks the practical speed for 8 X 10. 
 Of course, in portraiture, photo-engraving, etc., /:4-5 
 lenses up to a focal length of 24 inches are often 
 used. For the photography of rapidly moving ob- 
 jects and for general work in bad lights the /4.5 
 anastigmats are indispensable, but the lack of depth 
 of field and consequent difficulty of focusing render 
 the use of the full aperture inadvisable except in 
 case of necessity, and so lenses working at /:6.3 and 
 /:6.8, usually spoken of as " universal " lenses, are to 
 be preferred for general photographic work where ex- 
 cessive speed is not needed. 
 
 Focus. Other things being equal, the most 
 pleasing angle of view for probably 90 per cent of all 
 amateur photographs is that obtained when the 
 equivalent focus of the lens is equal to the diagonal of 
 the plate. Most makers list anastigmats of shorter 
 focus than this for each plate, for instance 6 inches 
 instead of 6| inches for 4 X 5, 7 inches instead of 83 
 
AND USE A LENS. 49 
 
 inches for 5 X 7, and so on. In such a case it is often 
 better to select the next size larger. A greater focal 
 length is of advantage from a pictorial standpoint, 
 hence the popularity of symmetrical and convertible 
 anastigmats, the halves of which can be used as 
 single view lenses, if the camera has a long bellows. 
 If, however, one intends to do only the usual types of 
 vacation pictures, snapshots of the children at play, 
 and an occasional out-door figure study or indoor 
 portrait, a lens of focal length equal to the diagonal 
 will be the best compromise, and the speed should 
 depend on the camera and the shutter. With a re- 
 flecting camera of any size, the use of a lens working 
 at/4-5 is decidedly to be recommended because this 
 gives the utmost possibility for speed work. When a 
 faster lens is stopped down to the same aperture as a 
 slower one of the same focal length, their performance 
 is much the same, though the slower one will prob- 
 ably cover a larger field. The greatest disadvantage 
 of large aperture (that is speed) is the diminution 
 of the depth of sharp field. The nearer the object 
 focused, the shallower the depth of field, hence the 
 practical necessity of using a reflecting camera for an 
 /:4.5 lens of moderate focal length. If, however, 
 much portraiture in the home is to be done with any 
 type of camera, the advantages of the /:4-5 lens are 
 so great that it should unhesitatingly be chosen in 
 focal lengths up to about 9 inches, at any rate. As 
 a portrait lens alone, this speed is not too great in 
 lenses of even longer focus in fact, for this special 
 use, there is a great advantage in using as large a lens 
 as can be worked in the available space. For in- 
 stance, a 1 6-inch lens at 16 feet will give better per- 
 
50 HOW TO CHOOSE 
 
 spective than an 8-inch lens at 8 feet, though the two 
 images will be of the same size. 
 
 Types. The choice between the different types 
 of lenses is very largely a matter of taste; for the 
 rest, as already hinted, the use of a symmetrical or a 
 convertible lens on a single-extension camera is no 
 better than a tying-up of valuable properties which 
 cannot be used. The user may, however, some day 
 get another camera on which he can use the single 
 combinations, so there is something to be said in 
 favor of planning for the future. Otherwise, there 
 are lenses of thoroughly satisfactory correction in 
 both classes. 
 
 Using an Anastigmat. First Perplexities. When 
 one has been using a rectilinear, particularly when it 
 has been employed chiefly for tripod work with 
 medium stops, the first impression of an anastigmat 
 at full opening is that it has very little depth and 
 needs stopping down. The image, where it is in per- 
 fect focus, is so much more clean-cut and brilliant than 
 anything ever seen before that the out-of-focus parts 
 look distressingly soft and fuzzy. The second thing 
 noticed is that a very minute movement of the lens 
 is sufficient to throw the image out. If the lens has 
 been put on an old camera, with a front which is 
 shaky from wear, one side of the picture may be in 
 perfect focus and the other blurred. A perfect lens 
 must have a perfect camera. The front must be 
 rigid and hold the lens with its axis perpendicular 
 to the film, or the definition will be poorer than that 
 obtained with the rapid rectilinear. When using the 
 focusing scale, extreme care must be taken to set 
 the pointer accurately to the required distance. An 
 
AND USE A LENS. 51 
 
 error of a few hundredths of an inch may be suffi- 
 cient to make the difference between crisp, spark- 
 ling detail and a general softness which ruins the 
 effect. Finally, one discovers that when a flat sur- 
 face is focused, the detail is as good in the corners 
 as at the centre and one does not have to split the 
 focus, as the field is perfectly flat. This brings us to 
 the conclusion that the lens will need stopping down 
 only when it is necessary to increase the depth of the 
 sharp field, not to secure covering power. 
 
 Securing the Anastigmat Advantages. Other 
 things being equal, an anastigmat in a poor, cheap 
 shutter is not able to do its best work. Here we have 
 a usable aperture of say, /:6.3, and a nominal T ta 
 second which may be really ^. In bright summer 
 sunlight it is necessary to stop down in order to avoid 
 over-exposure, or conversely, we cannot take a sharp 
 picture of athletic sports, because we lack the needful 
 iffa second actual exposure which a better shutter 
 gives. Again, the low-priced shutter has no speed 
 between -fa and second. One is too fast for a suc- 
 cessful snapshot at full aperture in poor light, the 
 other too slow for the camera to be held in the hand 
 without causing blur. The costlier shutter, how- 
 ever, has an accurate T V second which is just what 
 is needed. In short, when you get your anastigmat, 
 have it mounted in the best shutter you can afford, 
 if you are not purchasing a camera fitted with a 
 focal-plane shutter. It is well to remark that a high- 
 grade shutter with really accurate speeds may prove 
 a difficult tool to become acquainted with after using 
 a cheap and inaccurate one, leading to serious under- 
 exposure if used without realizing the fact that the 
 
52 HOW TO CHOOSE 
 
 speeds may be twice as great as those of the same 
 markings on the cheap shutter. Occasionally it is 
 an advantage to have the lens in a shutter instead of 
 a barrel even on a reflecting camera, as the focal- 
 plane shutter of the latter is not so well suited to 
 give slow snapshots and time exposures as the be- 
 tween-lens shutter. Extra speed in the lens itself is 
 not of much use if it cannot be made available by a 
 wide range of shutter speeds. 
 
 Focusing. Anastigmats being perfectly achro- 
 matic for two and usually for three colors, the image 
 seen on the ground glass is exactly what one gets 
 on the plate, if the latter is in register with the 
 former. Sometimes a fine lens gives poor results, 
 and the trouble is found to be due to the fact that the 
 holders are not bringing the plate within a sensible 
 distance of the same plane as the ground side of 
 the glass. In such cases, the holders must be tested 
 by means of a straight-edge and a wedge, and the 
 position of the ground glass altered to correspond. 
 Users of reflecting cameras may require special 
 spectacle lenses fitted inside the hood in order to 
 focus accurately. Incorrect register of the ground 
 glass is not unknown, even in costly reflecting instru- 
 ments. But when the register has been proved cor- 
 rect, one often has difficulty in learning to focus. 
 The tendency is to move the focusing screw too 
 rapidly. The difference between a microscopically 
 sharp focus and a blur may be one of a touch of the 
 pinion-head. Care must be taken not to focus too 
 far back, thus sharpening the distance at the ex- 
 pense of the foreground and making the latter un- 
 pleasantly blurry by contrast with the extreme, 
 
AND USE A LENS. 53 
 
 needle-point definition of the background. Stopping 
 down, however, will be needed less often than one 
 would at first expect, as, owing to the perfect flatness 
 of field, one can focus at any point of the picture, 
 and then carefully divide the sharpness between far 
 and near objects, favoring the foreground as much as 
 possible, before putting in a smaller stop. Generally, 
 if the plane of sharpest focus is one-third of the dis- 
 tance from camera to farthest object, a medium stop 
 will give sufficient depth; but if the plane of sharp 
 focus were placed at two-thirds of this distance, the 
 smallest stop would often hardly suffice. 
 
 Scale Focusing. When the camera is sold fitted 
 with an anastigmat, the correctness of its setting 
 should be tried by making an exposure at the exact 
 measured distance for each mark on the focusing 
 scale. In making these tests be sure that the front 
 is solidly locked or drawn out to the stop, or the 
 pointer set exactly on the mark. Should the scale prove 
 to be incorrectly placed, adjust it by careful trial. 
 
 Hyperfocal Distances. The greatest possible 
 depth of sharp field is obtained when a lens is set to a 
 definite distance for each stop. The distance is 
 found by the formula given on page 17. Suppose a 
 6-inch lens works at /:4-5- Its hyperfocal distance 
 for this stop is 66 feet. In the same manner, the 
 distances for the other stops may be worked out and 
 tabulated, a trial made at these measured distances, 
 and a supplementary scale made for the camera. 
 Such a scale is far more useful than one marked with 
 conventional distances. To use it, set the pointer to 
 the distance corresponding to the stop in use, and 
 the depth of sharp field will extend from half the 
 
54 HOW TO CHOOSE 
 
 hyperfocal distance to infinity. Thus, with the lens 
 already mentioned, the 66-foot setting will give a 
 sharp image of everything more than 33 feet from 
 the lens. The same lens at /:8 (which we consider 
 the normal stop for hand-camera snapshots) may be 
 set at 38 feet, and will give good definition from 19 
 feet to the horizon. It is by applying this rule to 
 the /:i6 meniscus of short focus that the manu- 
 facturers produce the so-called fixed-focus lens. 
 
 Other Points About Anastigmats. We might 
 write many pages on the use of anastigmat lenses 
 without covering all the points which might arise. 
 The owner of one, however, will learn more from 
 using it than from reading directions. The chief ad- 
 vantages of the anastigmat are that its corrections 
 have been carried out so that it condenses to a very 
 small circle of light in the image, each bundle of rays 
 originating in a point of the object, whether the ray 
 passes centrally or obliquely through the lens. This 
 circle of confusion, as it is called, should have a 
 diameter of about T^TF of an inch in a rapid rectilinear, 
 and only from ^^ to -5^ in an anastigmat. This 
 means in practice that an anastigmat will give a 
 critically sharp image over the entire plate at a large 
 aperture. 
 
 Care of Lenses. A fine instrument requires care- 
 ful handling. When not in use the lens should be 
 kept in a stout box, with a cap over each end, in a 
 cool place. Dust may be removed from the com- 
 binations with a clean camel's hair brush, holding the 
 lens up and brushing it very gently from underneath 
 so that the dust will fall off. If a greasy film collects 
 on the glasses, they may be unscrewed and very 
 
AND USE A LENS. 55 
 
 gently wiped with the Japanese tissue paper sold by 
 optical and physicians' supply houses, under the 
 name of lens paper. Rubbing must be avoided. 
 Glass surfaces are relatively very soft. If one rubs 
 the surface with a dusty cloth, the polish may be de- 
 stroyed or the curvatures altered, ruining the lens. 
 No other substance than lens paper should ever be 
 allowed to touch the surfaces of the glasses. The 
 greatest care must be exercised in taking apart and 
 putting together air-space anastigmats. With some 
 of these an error of adjustment of ^iv of an inch 
 will cause considerable deterioration of the quality 
 of the image. A lens may require recementing after 
 years of use, or repolishing, if the softer varieties of 
 Jena glass assume a rainbow-hued appearance. In any 
 such case, send it to its maker for the necessary repairs. 
 
 Special Fields of Photography. So far, we have 
 discussed only the ordinary kinds of photography, 
 but many prospective purchasers of a lens may wish 
 to use it for such special fields as portraiture, copying, 
 or enlarging. The " how to use " part is so different 
 that it requires a little consideration. 
 
 Portraiture. Although good portraits can be 
 made with a simple outfit, there are certain diffi- 
 culties which can be overcome only by knowing all 
 the tricks and having a camera provided with all 
 possible movements, inasmuch as the use of a lens 
 very near to a subject which has considerable por- 
 tions projecting forward and back of the main plane 
 focused on may lead to poor results. Short exposures 
 are desirable, which means using a large aperture 
 and thereby losing depth of field. The deficiency 
 must therefore be made up not only by tilting the 
 
56 HOW TO CHOOSE 
 
 camera to point down at the sitter, but also by uti- 
 lizing the vertical and horizontal swings to equalize 
 definition between nearer and farther parts. For 
 instance, if a sharp focus is secured on the sitter's 
 eye, with the ground glass vertical, it will be found 
 impossible to secure sharpness in a hand resting on 
 the lap, unless the lens is stopped down or the vertical 
 swing is used to bring the ground glass away from 
 the vertical plane to the place where the sharp image 
 of the hand really lies. Similarly, both shoulders can 
 be focused, in a three-quarter pose, only by using the 
 horizontal swing. For these reasons a good view 
 camera will be found the best purchase for an ama- 
 teur wishing to do much portraiture. Since vertical 
 lines seldom appear in a portrait, the ground glass 
 may be placed in any position where it will receive a 
 sharp image. The nearer any object is to the lens, 
 the farther its image lies from the lens. 
 
 Copying and Enlarging. The last statement gov- 
 erns the use of the lens for copying and enlarging. 
 The ordinary camera with a single extension of bel- 
 lows can be used to focus objects only up to about 
 6 feet. If a larger image is desired, it must be obtained 
 through the use of a supplementary lens (" portrait 
 attachment " or " copying lens "), or by using a 
 camera with double or triple extension of the bellows. 
 For instance, to copy a photograph " same size " with 
 a 7-inch lens (about the focus of that supplied on a 
 33 film camera) the lens must be 14 inches from the 
 ground glass and 14 inches away from the " copy. 1 ' 
 To secure an enlargement of 4 diameters with the 
 same lens, that is, to enlarge from 3i X s| to 13 X 22, 
 the lens would have to be 8| inches from the negative 
 
AND USE A LENS. 57 
 
 and 35 inches from the bromide paper on the easel. 
 As the bellows is too short for such use, an enlarging 
 apparatus would have to include a box behind the 
 kodak of such length as to secure the necessary 8| 
 inches' extension or more, if smaller-scale enlarge- 
 ments were desired. Suffice it to say, it is useless to 
 purchase the most universal anastigmat unless it is 
 fitted to a camera having adequate draw of bellows. 
 As a matter of fact, unless one confines photographic 
 activity to outdoor hand-camera work, it will be 
 found that two or more cameras of different sizes and 
 capabilities are needed to cover every department of 
 photography successfully. 
 
 Wide-angle Lenses. A lens capable of producing 
 a negative embracing an angle of view greater than 
 70 is called a wide-angle lens. Up to this angle, 
 although it is much greater than that seen by the 
 eye at once, the perspective is not too extreme, but 
 above this angle, objects near the edge of the picture 
 are much distorted, and such lenses should be used 
 only where cramped situations render their use oblig- 
 atory. A 6-inch rapid rectilinear, for instance, will 
 work fairly well on a 4 X 5 plate, including 56 along 
 the diagonal of the plate. If the same lens were 
 used on a 5 X 7 plate the angle included along the 
 diagonal would be 71, and the lens in this instance 
 would be a wide-angle lens for that plate. Ordinary 
 rectilinears, however, are not well adapted to such 
 use; a special form, made of heavier glasses and with 
 deeper curvatures, working at/:i6 or slower, can be 
 bought. Cells to fit ordinary shutters and lenses in 
 barrel-mounts are still obtainable and are useful ad- 
 ditions to an outfit, particularly for copying or for 
 
5 8 HOW TO CHOOSE 
 
 taking interiors or other close subjects when the regu- 
 lar lens cannot be placed far enough from the subject 
 to include enough of it. For instance, suppose one 
 has a 5 X 7 camera with single extension of bellows 
 and fitted with an 8j-inch rapid rectilinear lens. A 
 set of wide angle cells, listed at $6, will furnish, for 
 the same shutter, a lens having an equivalent focus 
 of about 5J inches. Such a lens would include 78 
 on the diagonal, allowing its use for small interiors. 
 It could also be used to make a copy half the size of 
 the original with the short bellows furnished on the 
 camera, whereas the regular rapid rectilinear would not 
 make one larger than size with the same extension. 
 
 Limitations of the Old Rapid Rectilinears. The 
 special wide-angle types of rectilinears, of course, have 
 all the defects of their class, with the added disad- 
 vantage of a small maximum opening for focusing. 
 Still, if they are carefully used, they are capable of 
 doing remarkably fine work with very small stops. 
 It is necessary to split the curvature of field when 
 focusing, and in interior work, to make sure that the 
 proper depth is secured by focusing wide open on 
 some subject only a third of the whole depth of the 
 subject from the lens. For instance, if the interior 
 measures 30 feet from lens to most distant wall, a 
 point 10 feet from the lens should be chosen for the 
 sharpest focus at full aperture and the lens then be 
 stopped down until everything is sharp enough. 
 
 Modern Wide-Angles. Some of the earlier an- 
 astigmats were wide-angles, notably the Zeiss Series 
 V. Several other special forms of anastigmat wide- 
 angle lenses have been marketed, but the symmetrical 
 cemented double anastigmats, as well as some of the 
 
AND USE A LENS. 59 
 
 unsymmetrical objectives, answer the purpose so per- 
 fectly that they are generally chosen. For instance, 
 a 6-inch lens which is intended for a 4 X 5 plate will 
 cover 5X7 at/:i6 and 6| X 8J at/:22 or/32. In 
 planning for two cameras of different sizes, therefore, 
 it is not inadvisable to select for the smaller instru- 
 ment a lens which will answer for use as a wide-angle 
 on the larger camera. 
 
 Teleobjectives. When it is necessary to obtain 
 a large direct picture of a distant object, in order to 
 avoid lenses of extreme focal length, with corre- 
 spondingly long bellows, lens systems are used which 
 contain a negative or dispersing lens in combination 
 with a collecting lens. Telephoto attachments may 
 be had for either rapid rectilinear or anastigmat 
 lenses, and give various magnifications by varying 
 the separations of the lenses. They greatly increase 
 the focal length of the lens, and therefore reduce the 
 effective aperture or / value, so that exposures must 
 be very long. 
 
 Limitations of Telephotography. It is difficult to 
 obtain good pictures with the long exposures needed 
 at high magnifications. The camera itself must be 
 solidly built to obviate vibration, and the air must be 
 as still as possible. There are, on the whole, very 
 few days in the year which will allow successful tele- 
 photographic work. The greater the enlargement, 
 the more noticeable is the effect of air currents in 
 blurring the detail. The lower amplifications, 4^ to 
 6 times, are more likely to prove usable. 
 
 Fixed-magnification Teleobjectives. Practical con- 
 siderations, such as have been mentioned, led to the 
 abandonment of the variable magnification and the 
 
60 HOW TO CHOOSE 
 
 construction of teleobjectives suitable for reflecting 
 and hand cameras and having a fixed magnification 
 of about 2 or 2\ diameters. In other words, these 
 lenses give a large image with short bellows extension. 
 One of them, the Ross Telecentric, gives almost as 
 fine definition as an anastigmat and has a working 
 aperture of /:54. The chief advantage, as com- 
 pared with a long-focus regular lens having the same 
 equivalent focus, is the rigidity of the short bellows. 
 The 17-inch Telecentric /:54, for instance, has a 
 back focus, or distance from rear lens-cell to ground 
 glass, of only 9 T V inches. It could thus be ad- 
 vantageously fitted to a 4X5 reflecting camera 
 which could hardly accommodate a 1 7-inch Tessar, for 
 instance, even if the photographer could afford to 
 pay for the latter. Other manufacturers brought 
 out various forms of the telephoto objective, among 
 which may be mentioned the Cooke Telar, the Dall- 
 meyer Adon, and the Zeiss Magnar. 
 
 Trying a Lens. Most workers in buying lenses 
 are interested in those of the highest possible cor- 
 rections and desire to know just how they can make 
 a wise selection from the many varieties offered. 
 Since the prices of anastigmat lenses have come down, 
 the difficulty of making a choice has increased. One 
 worker is willing to pay any sum if he can be shown 
 that the lens is really better than some other of equal 
 or lower cost. Another worker is strictly limited by 
 his pocketbook, but doubts whether the price he can 
 afford will actually buy a genuine anastigmat of good 
 corrections and capable of doing well all the sorts of 
 photographic work in which he is interested. The 
 following paragraphs give, for the benefit of both 
 
AND USE A LENS. 61 
 
 classes, some methods of testing lenses, based on an 
 article published in The British Journal of Photog- 
 raphy. By trying the lenses, usually sent with the 
 privilege of a ten days' trial by the makers, the reader 
 can at slight trouble determine just how good they are. 
 
 Testing a Lens. There are two distinct things 
 which have to be ascertained about a lens before 
 taking it into use its capabilities and its qualities. 
 Under the heading of capabilities are three principal 
 points: The intensity, or, as some call it, the speed at 
 full aperture ; the relation of the circle of illumination 
 to focal length or angle of view; and whether the 
 image is sufficiently rectilinear for the work the lens is 
 to be used for. Want of rectilinearity is not a defect 
 for some work: it may even be an advantage, so we 
 place this point under capabilities instead of qualities. 
 
 Focal Length. To arrive at the intensity, or, as 
 it is often called, angular aperture, we must first 
 ascertain with some degree of accuracy the actual 
 or equivalent focal length of the lens. There are 
 many ways of doing this, perhaps the simplest being 
 to focus a distant^object, to mark the camera ex- 
 tension on the baseboard, and then to focus any 
 object, such as a printed card, so that the image on 
 the ground glass is exactly the size of the original. 
 The difference between the original extension as 
 marked and the extension required for a full-sized 
 image is the equivalent focal length of the lens. 
 
 Focal Aperture. The intensity is got by dividing 
 the diameter of the largest aperture of the diaphragm 
 into the focal length. This, however, although not 
 difficult of accomplishment, is not quite as easy as it 
 sounds. As in obtaining the focal length, it is not 
 
62 HOW TO CHOOSE 
 
 enough to measure the distance between the lens and 
 plate with a rule, so, in measuring the aperture, it 
 is not correct to measure the diaphragm opening with 
 a pair of dividers. In many lenses there is a con- 
 siderable convergence of the rays before they reach 
 the diaphragm, so that it is quite possible for a lens 
 having an actual aperture of /:6.3 to seem to have 
 one no larger than f:j. To ascertain the amount of 
 this convergence, we must focus a distant object 
 upon the screen, and after replacing the ground glass 
 by a card which has had a fairly large pinhole per- 
 forated in the centre, take the camera into a darkened 
 room and hold a candle flame close to the pinhole. 
 If a piece of ground glass be placed in contact with 
 the front cell or hood of the lens, we shall see an 
 illuminated circle, .and this circle is the true working 
 aperture of whatever diaphragm aperture happens 
 to be in position. In a general way, we should start 
 with the largest aperture; we may then proceed to 
 check all the others. As it is somewhat difficult to 
 measure the disc of light while holding the ground 
 glass in position, it is a good plan to rule a pencil line 
 upon it, and mark off upon that the various aper- 
 tures. A better way is to place a piece of gaslight 
 paper in the same position, and after developing, 
 measure the diameter of the blackened circle. This 
 gives a permanent record. 
 
 Circles of Illumination. It is not quite so easy 
 to determine the size of the circle of illumination with 
 a lens unless one has access to a very large camera. 
 With a 4 X 5 lens, a 10 X 12 camera will answer, but 
 for lenses of over 8 or 9 inches focal length, some 
 special device will be necessary. An optician's test- 
 
AND USE A LENS. 63 
 
 ing-bench is, of course, the most convenient thing 
 for this purpose, but is hardly likely to be available. 
 The easiest way is to fasten the lens to a fairly large 
 front board and to fix this in a window the rest of 
 which can be covered with a dark curtain. A child's 
 hoop of about 30 inches in diameter, covered with 
 tracing paper, will serve as a focusing screen, and if 
 a scale of inches, starting with zero, in the centre, is 
 marked on the paper, it will at once show the limits 
 of the field. Here, again, a sheet of bromide or gas- 
 light paper on a movable easel is preferable. The 
 lens should be focused on infinity for this test. To 
 ascertain the covering power for any size of plate, it 
 is a good plan to draw a circle the size of the field of 
 the lens, and to lay an actual plate upon it. It is 
 then easy to see how much rise and fall is possible 
 without any further measurement or calculation. 
 
 The test for rectilinearity should be made on the 
 class of subject it is desired to take, as it is a fact 
 that lenses which are rectilinear at some distances 
 may not be so at others. We once possessed a lens 
 w r hich was absolutely correct on a test chart at a few 
 feet, but which gave a perceptible curvature of the 
 field in a church interior. The test should also be 
 made on the size of the plate to be used, as some 
 single lenses give practically no distortion over a 
 moderate angle. The test should take the form of 
 an actual exposure, as it is much easier to examine a 
 negative than an image upon the ground glass. 
 
 Qualities of the Lens. Before attempting to 
 make any tests as to the quality of a lens, the camera 
 upon which it is to be fixed should be carefully ex- 
 amined. The larger the aperture and the more 
 
64 HOW TO CHOOSE 
 
 highly corrected the lens is supposed to be, the more 
 necessity exists for this. Most present-day cameras 
 are very lightly built, and a slight bending of the 
 struts or straining of the hinges will cause a lack of 
 parallelism between the front and back which will 
 seriously impair the accuracy of the result. 
 
 The principal defects which have to be looked for 
 are spherical and chromatic aberrations, astigmatism, 
 curvature of field, and flare. Besides these, the 
 quality of the glass must be examined for specks. 
 Faults in mounting, such as bad centering and strain 
 through too tightly fitting the glasses into the cells, 
 may also be present. 
 
 Spherical Aberration. Spherical aberration means 
 incapacity to give sharp definition at a large aper- 
 ture. It arises from the fact that unless properly 
 corrected, the rays passing through the margin of a 
 lens have a slightly different focal length from those 
 passing through the centre. This causes a softness 
 of outline which is most noticeable around the highest 
 lights of the subject, such as the white collar in a 
 portrait or a patch of white wall in a view. It may 
 be recognized by focusing a printed page from a 
 good magazine, on which the ink is black and the 
 paper smooth. If it is impossible to secure a sharp 
 image in the center of the field without stopping 
 down the lens, spherical aberration is present. Ex- 
 cept in very bad cases, a sharp image can be secured 
 when the aperture is reduced to /:i6. Stopping down 
 an R. R. lens may not remove astigmatism, however. 
 A more delicate test is to use the light of a small lamp 
 reflected upon a mercury bulb (an ordinary thermom- 
 eter will do as the test object) in a darkened room, 
 
AND USE A LENS. 65 
 
 as the blurring is shown more clearly. Another way 
 which also demonstrates the cause as well as proves 
 the existence of this aberration is to make two stops 
 of black paper. One is a disc about two-thirds the 
 diameter of the full aperture of the lens, the other a 
 disc which will fit inside the lens hood, and having a 
 central perforation of a diameter a little larger than 
 the solid disc. The perforated disc is placed in the 
 hood, and any object sharply focused, the exact 
 position of the screen being marked. This disc is 
 then removed, and the other disc fixed on the centre 
 of the front lens with a touch of gum or rubber solu- 
 tion. If spherical aberration is present, the image 
 will no longer be sharp, but it can be made so by re- 
 focusing. If poor definition is due to other causes, 
 such as bad centering or badly annealed glass, there 
 will be little difference in the definition when the 
 discs are changed. It should be remembered that 
 for some classes of work spherical aberration is ad- 
 vantageous, and special means for introducing and 
 regulating it are provided in the Patent Portrait 
 lenses of Dallmeyer, and more recently in the Cooke 
 Anastigmat Portrait objectives. In either of these, 
 by simply turning one of the lens cells, a range from 
 absolute sharpness to decided fuzziness may be ob- 
 tained. The Wollensak Velostigmat Series II has a 
 similar adjustment in the larger sizes. 
 
 Astigmatism. Astigmatism has been already de- 
 fined. It differs from spherical aberration inasmuch 
 as it cannot be wholly removed by any amount of 
 stopping down. It is usually present to a large ex- 
 tent in portrait lenses of the Petzval type, and to a 
 lesser one in a rapid rectilinear. The handiest test 
 
66 HOW TO CHOOSE 
 
 for it is a cross made of two strips of black paper 
 about three inches long and a quarter of an inch wide, 
 pasted upon a white card. If this be focused upon 
 the center of the ground glass, the vertical and hori- 
 zontal lines will appear equally sharp, but upon ro- 
 tating the camera so as to bring the image near the 
 edge of the plate, it may appear unsharp. This may 
 be due to curvature of the field, in which case, a 
 sharp image can be obtained by refocusing. If astig- 
 matism be present, it will be found that only one of 
 the lines can be focused at a time; when the verti- 
 cal one is sharp, the horizontal one is fuzzy, and vice 
 versa. If the mercury bulb or " artificial star " be 
 used for this test, the bright dot focused in the centre 
 of the field is drawn out, on rotating the camera, into 
 a short line, which takes a horizontal or a vertical 
 position as the camera back is racked out or in. 
 
 Chromatic Aberration. Chromatic aberration is 
 a serious defect for ordinary work. A lens afflicted 
 with it will give a sharp image upon the ground glass; 
 but, upon taking a negative, the image of the object 
 appears unsharp, while other objects at a greater or 
 less distance appear sharper than they did upon the 
 screen. This is due to the fact that the yellow rays, 
 which are the most luminous, are those which are 
 mainly used in focusing and produce what is known 
 as the visual image, while the blue and violet rays, 
 which are more chemically active, come to focus in 
 another place. The simplest test for a lack of achro- 
 matism is to fix up a long strip of sharply-defined 
 printed matter at an angle with the axis of the lens, 
 the centre of the strip being opposite the lens; in a 
 strip of two feet of length the inclination should make 
 
AND USE A LENS. 67 
 
 the nearest end of the strip about eight inches closer 
 to the camera than the further one. It is convenient 
 to place a couple of pins to mark the centre of the 
 print. A column of newspaper answers well for this 
 purpose. Then focus the line lying between the pins, 
 and make a negative. If the line focused is the 
 sharpest in the negative, the lens is free from chro- 
 matic error; if it be undercorrected, a more distant 
 line will be in the best focus; if over-corrected, a 
 nearer one. A properly achromatized lens brings 
 both blue and yellow rays to the same focal plane, 
 while an apochromatic one makes practically all the 
 rays of the spectrum coincident. It is obvious that 
 for this test the surfaces of the ground glass and the 
 sensitive plate must fall into exactly the same plane, 
 and for this reason it is recommended to focus upon 
 a piece of ground glass placed in the actual plate- 
 holder which is to be used. A trained observer can 
 form a good estimate of the chromatic correction by 
 a visual inspection with a telescope eyepiece, but 
 the novice will do well to trust nothing but a photo- 
 graphic test. 
 
 Curvature of Field. Curvature of field is easily 
 discovered by examination of the image of a flat wall 
 on which, if possible, some printed pages should be 
 fixed. Test charts are not good for this purpose, as, 
 being usually small, they require a longer camera 
 extension, and this naturally increases the covering 
 power of the lens. Consequently, a better perform- 
 ance can be obtained from a chart at two feet than 
 from a wall forty feet away. If a flat object be not 
 available any distant one, such as a weather-cock or 
 a chimney, will serve. This should first be focused 
 
68 HOW TO CHOOSE 
 
 upon the centre of the screen, and the camera rotated 
 upon the tripod until the image falls near the margin 
 of the screen. The amount of racking-in which is 
 necessary is a measure of the curvature of the field. 
 In the case of some anastigmats, it is not safe merely 
 to compare the image formed at the centre of the 
 field with that formed at the margin, as the field fre- 
 quently has a dip extending in a circular zone, after 
 which it recovers its original flatness. We have 
 used an expensive modern lens, which at the same 
 aperture gave better definition at the corners of a 
 6i X 8| plate than it did upon the 4X5 size. It is 
 very necessary to see that the front and back of the 
 camera are absolutely parallel, as any swing or in- 
 clination will vitiate this test. 
 
 Flare Spot. Flare spot occurs in two forms, one 
 being a decided disc which appears at a distance from 
 the centre of the plate corresponding to that of any 
 bright object, such as a gas flame or a window which 
 may be in the field, while the other, which usually 
 occurs in single lenses, takes the form of an ill-defined 
 central patch of fog. The former is easily discovered 
 by focusing a gas flame placed in front of a dark 
 background, when, upon rotating the camera, the 
 image of the flame and the disc will be seen to ap- 
 proach and recede from each other. Many useful 
 lenses suffer more or less from this defect, which 
 appears only under trying conditions. The other 
 form is more difficult to detect upon the ground 
 glass, but may be found by taking a bright sky 
 against a dark mass of foliage. If a patch more or 
 less circular in form is visible in the centre of the 
 subject, it is probably due to the flare. 
 
AND USE A LENS. 69 
 
 Mechanical Defects. It is now rare to find 
 lenses suffering from mechanical defects, except those 
 due to accident. If a lens is badly centered, that is 
 to say, if all the axes of all its components are not in 
 the same line, the definition will be affected. This 
 may be due to careless workmanship or to the position 
 of the lenses having been altered by a fall. It may 
 be detected by focusing a candle flame at a little 
 distance from the centre of the field, and then gently 
 rotating the lens in its flange. If the lenses are per- 
 fectly centered, the image will remain stationary; if, 
 not, it will move in a circular direction. If the cells 
 are separately rotated in the same way, it will be 
 seen in which the fault is present. Another test can 
 be made without a camera by holding the lens be- 
 tween the eye and a gas flame, so that a number of 
 small images of the flame are seen inside the lens. 
 In a properly centered lens these can be brought into 
 a straight line, but if one or more cannot be made to do 
 so, it shows that one at least of the glasses is displaced. 
 
 Defective glass sometimes gives rise to poor defi- 
 nition, and so does too tight a cell. These can be 
 detected only with the aid of polarized light, which 
 shows any strain, as in badly annealed or compressed 
 glass, as dark patches. Veins in the glass may be 
 seen by holding the lens up to a small gas or candle 
 flame, so that the surface is uniformly luminous. The 
 streaks will then show quite plainly. Care must be 
 taken not to confuse them with any streaks which 
 may have their origin in the eye of the observer. If 
 the lens is turned, a genuine vein will turn with it, 
 but a spectral one remains stationary. Air bubbles 
 of any size which any manufacturer will pass, are 
 
70 HOW TO CHOOSE 
 
 of no importance, and may be disregarded. Being 
 magnified by the lens, they are actually much smaller 
 than they appear. 
 
 Choice of Focal Length. Most cameras are fur- 
 nished with a lens having a focal length about equal 
 to the diagonal of the plate, or a little shorter. This 
 is unquestionably the best all-around length for 
 general amateur photography outdoors. There are, 
 however, a great many subjects which are included 
 in a much narrower angle of view than such a lens 
 gives. For example, a 6-inch anastigmat on a 4 X 5 
 plate includes about 56 on the diagonal or about 45 
 on the longer side or base. The real picture, as seen 
 by the eye, seldom includes much more than 30, so 
 that the print (or enlargement) may often include only 
 a small portion of the negative. It is far more con- 
 venient, with any camera except a reflex, to use the 
 lens with 45 angle, trimming the prints, than to use 
 a long-focus lens which includes 30 on the base. 
 The reason is that a short-focus lens has so much 
 greater depth of field that it can be used successfully 
 with a focusing scale, whereas the long-focus lens 
 (9-inch for 4X5) would need more careful focusing 
 and the use of a smaller stop to obtain similar depth. 
 We find that a 6-inch answers well for general work 
 if great care is taken to keep clear of the temptation 
 to work for too large an image. If the image is too 
 small, provided only it is sharply focused, it will 
 stand enlarging to at least four diameters without 
 looking at all blurred. One can often utilize a piece 
 of a 4X5 negative, measuring from i X i^ to 
 2^ X 3 inches, throwing it up to 10 X 12 or n X 14, 
 and still have a picture sharp enough for framing. 
 
AND USE A LENS. 71 
 
 Practically, then, a short-focus lens has a great ad- 
 vantage. The ideal way, naturally, is to have a 
 " battery " of lenses of different focal lengths, so that 
 when once the proper station-point for the lens has 
 been found, a selection may be made so as exactly 
 to fit the picture to the plate. 
 
 Focal Length and Perspective. Strictly speaking, 
 the perspective of the picture is determined by the 
 point at which the lens is placed. When the de- 
 cision is made to photograph a certain scene, the eye 
 generally concentrates on an angle of about 30. If, 
 now, the picture is made from the position from 
 which the view appears right to the eye, using a 
 45 lens, the result will almost invariably appear 
 unsatisfactory. The principal object seems too small 
 and too far away, and there is altogether too much of 
 the neighborhood included in the margins. As already 
 stated, the remedy is to enlarge the part which con- 
 tains what is wanted; but most workers are inclined 
 to go closer the next time, so as to secure a larger 
 image for contact printing. The result is that the 
 foreground objects appear too large and distant ob- 
 jects too small. If, on the contrary, one always used 
 a 9-inch lens for a 4 X 5 negative there would be a 
 constant incentive to move farther away from the 
 subjects, to make sure of getting them all in. To 
 some extent, this tendency would counterbalance 
 the smaller " depth " of the 9-inch lens; but when 
 larger sizes than 5X7 were used, it would be found, 
 in practice, that 30 lenses would require considerable 
 stopping down. 
 
 Readers whose interests are chiefly along the lines of 
 artistic portraiture and landscape work will, however, 
 
72 HOW TO CHOOSE 
 
 find it advisable to have at least one long-focus lens. 
 Outside of commercial interiors and exteriors in 
 narrow streets, it is safe to lay down as a principle 
 that the most generally useful lens for a given plate 
 is one of a focus equal to the diagonal; for pictorial 
 work, equal to the sum of the two sides ; and for special 
 work of a telephoto nature, one equal to twice the long 
 side, or base. For 4X5, these focal lengths would be 
 6J, 9, and 10 inches; for 5 X 7, 8 J, 12, and 14 inches. 
 
 Any one of these three focal lengths will give the 
 same perspective if all are used from the same station- 
 point; but the usable part of the picture will be con- 
 tained in different parts of the plate. We confess to 
 a liking for using long-focus lenses, composing the 
 picture on the ground glass with the lens which gives 
 the exact composition over the entire plate, and en- 
 larging the whole plate; yet we have had exactly as 
 good results by enlarging a part made with the 45 
 lens. The whole secret is knowing where to stop in 
 advancing towards the subject. 
 
 Perspective in Portraiture. We invariably advise 
 that no portrait should ever be taken with the lens 
 any nearer the head of the sitter than 8J feet. A 
 greater distance is advantageous, for if some part of 
 the figure is brought nearer the lens than the main 
 mass of the body, it will appear too large unless the 
 station-point of the lens is far away. Amateurs, how- 
 ever, generally try to get very close in order to make 
 a head of good size, and their pictures lack likeness 
 because the features are distorted by the too-close 
 viewpoint. If the size of the room will permit, the 
 half of a symmetrical lens (generally of the longer 
 focus recommended above, that is, twice that of the 
 
AND USE A LENS. 73 
 
 complete lens) will be found best for portrait work, as 
 the loss of speed is compensated for by the better per- 
 spective and increased depth of sharp field. Even if 
 the longer-focus lens is used at the same distance, it will 
 give the same perspective but a larger head. When 
 it is used from twice the distance to get the same size 
 head, its advantage is tremendous, for the perspective 
 is much more natural and pleasing and all parts of 
 the figure can be focused at once without excessive 
 use of the vertical and horizontal swings. Thus one 
 avoids, for example, having the nearer shoulder too 
 large and out of focus and the farther shoulder a 
 shapeless and detailless blur. 
 
 Equivalent Focal Length. The terms, focus, focal 
 length, and equivalent focus, as well as equivalent 
 focal length, are frequently met with in descriptions 
 of lenses. The correct term is equivalent focal length, 
 although the other three are often used. A simple 
 lens of the double convex type, with both curves 
 alike, has an easily measurable focal length, this being 
 the distance from the centre of the lens to the image, 
 when focus has been obtained on some object distant, 
 say, 200 times the focal length. With other types of 
 lenses, the optical centre does not coincide with the 
 physical centre. Consequently since the distance 
 from the centre of the lens to the centre of the 
 image cannot be accepted as the focal length of the 
 lens, we must find some easily measurable distance 
 to which the focal length is equivalent. It is geomet- 
 rically true that when two lenses are used under 
 identical conditions, if they produce images of the 
 same size their focal lengths are equivalent. If, then, 
 a simple lens and a complex lens yield images of 
 
74 HOW TO CHOOSE 
 
 equal sizes, when used under identical conditions, 
 their focal lengths must be equivalent. As the focal 
 length of the single lens is directly measurable, the 
 equivalent focal length of the complex lens is known 
 immediately. Consequently, if the single lens has a 
 focal length of 6 inches, the complex lens will have 
 an equivalent focal length of 6 inches. 
 
 A simpler conception of this may be had, if, instead 
 of a single lens, we visualize in its place an ordi- 
 nary pinhole, whose " focal length " may be con- 
 sidered as the distance from the pinhole to the centre 
 of the image. The same reasoning which applied to 
 the single lens, applies in the case of the pinhole, and 
 there is no ambiguity as to the path of the light rays, 
 to confuse the beginner, if the position of the optical 
 centre of the single lens is not entirely clear to him. 
 For practical purposes it may be said that the equi- 
 vocal focal length of a lens is the same as that of a 
 single lens or pinhole producing an image of equal 
 size under identical conditions. 
 
 Back Focus. In purchasing a lens for a reflecting 
 camera, it is sometimes important to know the back 
 focus, or distance from the rear lens cell to the ground 
 glass when the lens is focused for far objects, in 
 order that we may be sure it will clear the mirror. 
 Some anastigmats have a back focus considerably 
 shorter than the equivalent focal length and some 
 kinds have one considerably longer. One 6-inch lens 
 may have a back focus of about 4! inches and so 
 can be fitted to a sJ X 4! reflex, but scarcely to a 
 4X5; while another make may have a back focus 
 of about 7 inches for a 6-inch lens. 
 
 Circle of Illumination. Although a lens is often 
 
AND USE A LENS. 75 
 
 chosen because it can be used as a wide-angle ob- 
 jective on a larger camera, it may sometimes fail to 
 give clear negatives when used on the smaller camera. 
 The reason is that the large circle of illumination 
 covers the walls of the bellows, from which great 
 volumes of light are reflected to the plate, fogging it 
 all over. The trouble is most pronounced in focal- 
 plane folding cameras which are not fitted with the 
 accordion type of bellows, but present flat surfaces of 
 considerable area acting as mirrors to cast the light 
 on the plate. For this reason, a reflecting camera 
 often works better with a lens incapable of covering 
 sharply a plate more than one size larger than it is 
 rated for; and wiseacres who think every lens should 
 have great reserve covering power are often con- 
 founded by the superior work of what they deem an 
 inferior lens. Many complaints that air-space lenses 
 give foggy images are due to reflected light from the 
 bellows. The trouble can, of course, be diminished 
 by reblacking the camera interior with a really dead 
 black paint and supplying a deep lens hood to cut 
 off all scattered light, even if the hood cuts down the 
 circle of illumination. No harm will be done so long 
 as it does not interfere with covering the plate in use. 
 Focusing with the Swingback. Although depth 
 of field is easily secured by stopping down, users of 
 plate cameras provided with vertical and horizontal 
 swings have a tremendous advantage over the film- 
 camera devotee. The swings can be used in two 
 ways. The first, and most obvious, is to allow one 
 to keep the back always vertical, even if the camera 
 is pointed up or down. This movement is invaluable 
 in architectural work, for a vertical line can be ren- 
 
76 HOW TO CHOOSE 
 
 dered as such only if the surface receiving the image 
 is vertical. The second way, far more useful, is to 
 allow the back to be moved to bring an image into 
 focus without stopping down. Assuming a 6-inch 
 lens, the image of an object several hundred yards 
 away is formed 6 inches from the lens, but that of 
 an object 6 feet away will lie about 6f inches away. 
 If, now, the camera is tilted down towards the near 
 foreground object and the vertical swing is used to 
 set the top of the ground glass two-thirds of an inch 
 behind the point where the distant object appears on 
 it, both images can be focused with a moderately 
 large stop. On the contrary, if the back were kept 
 in its normal position, the use of a very small stop 
 would become necessary to secure a sharp image of 
 both at once. The horizontal swing is equally useful 
 in countless instances, particularly when near objects 
 occur at one side and not at the other, or to focus. a 
 diagonal line, as of a procession, throughout its depth. 
 In fact, so useful are the swings, rising and falling 
 front, and other adjustments of a good form of plate 
 camera, that one can get the utmost out of a lens, in 
 a difficult situation, only by knowing how to take 
 full advantage of every movement. The lack of ad- 
 justments is the most serious drawback of the reflex 
 camera, as well as of the roll-film camera. Still, the 
 use or non-use of these conveniences depends largely 
 on the class of work one attempts, and the simplest 
 box camera will handle a surprisingly large variety 
 of work if intelligently used. 
 
 Supplementary Lenses. The owner of only one 
 camera often finds that he would like to do some 
 kind of work for which his lens is not of quite the 
 
AND USE A LENS. 77 
 
 right focal length. For instance, to get a large 
 head in a portrait, the use of a lens having a focal 
 length of about 3^ feet, placed in front of the rapid 
 rectilinear lens like a lenscap, will allow the camera to 
 be brought to 3! feet from the head. This results 
 in nearly sharp focus and a good-sized head, as the 
 supplementary lens, called a " portrait attachment," 
 brings the image to a focus without requiring the 
 camera front to be moved from its loo-foot position. 
 In the same manner, lenses of greater or lesser focus 
 can be utilized to alter the focus of the lens and 
 obviate the need of a long bellows. Supplementary 
 lenses are often sold in sets, consisting of portrait, 
 wide-angle, copying, and telephoto lenses, with a 
 " duplicator " and a cheap glass ray filter added. By 
 purchasing such a set, the owner of a short-bellows 
 camera can get some of the effects which otherwise 
 would need a long-bellows camera. Spectacle lenses 
 can also be utilized. The rule is to place before the 
 lens a lens having a focus equal to the distance from 
 lens to subject. Simple lenses introduce aberrations 
 into an optical system, so a medium or a small stop 
 must often be used to secure satisfactory definition. 
 
 Utilizing a Curved Field. Although curvature of 
 field is a lens defect, it is possible at times without 
 stopping down, to avoid the natural poor quality of 
 definition of an uncorrected lens. For instance, when 
 making a negative of a group of people, who can be 
 arbitrarily arranged at different distances from the 
 lens, their images can be brought into equally sharp 
 focus on the plane of the plate, by arranging them in 
 an arc of a circle, the arrangement being dictated by 
 what one sees on the ground glass. 
 
HOW TO CHOOSE 
 
 Table of Hyper 
 
 F 
 
 It 
 
 *e3 c 
 
 N 
 
 w~g 
 
 A = f value of lens 
 
 ^ 
 
 II 
 
 w 
 
 1.9 
 
 2.2 
 
 3.5 
 
 4.2 
 
 4.5 
 
 5.4 
 
 5.6 
 
 6.3 
 
 6.8 
 
 7 
 
 7.7 
 
 2 
 
 77 
 
 70 
 
 61 
 
 38 
 
 32 
 
 30 
 
 25 
 
 24 
 
 21 
 
 20 
 
 19 
 
 17 
 
 3 
 
 77 
 
 158 
 
 136 
 
 86 
 
 71 
 
 6 7 
 
 56 
 
 54 
 
 48 
 
 44 
 
 43 
 
 39 
 
 3l 
 
 777 
 
 161 
 
 139 
 
 88 
 
 73 
 
 68 
 
 57 
 
 55 
 
 49 
 
 45 
 
 44 
 
 40 
 
 4 
 
 577 
 
 211 
 
 182 
 
 114 
 
 95 
 
 89 
 
 74 
 
 
 63 
 
 59 
 
 57 
 
 52 
 
 4s 
 
 707 
 
 
 
 145 
 
 121 
 
 113 
 
 94 
 
 90 
 
 80 
 
 74 
 
 72 
 
 66 
 
 5 
 
 577 
 
 
 
 149 
 
 124 
 
 116 
 
 96 
 
 93 
 
 83 
 
 77 
 
 74 
 
 68 
 
 5^ 
 
 577 
 
 
 
 180 
 
 I5O 
 
 140 
 
 117 
 
 
 IOO 
 
 93 
 
 ' 90 
 
 82 
 
 6 
 
 577 
 
 
 
 214 
 
 179 
 
 167 
 
 139 
 
 134 
 
 119 
 
 no 
 
 107 
 
 97 
 
 6^ 
 
 577 
 
 
 
 252 
 
 210 
 
 196 
 
 163 
 
 157 
 
 140 
 
 129 
 
 126 
 
 114 
 
 7 
 
 577 
 
 
 
 292 
 
 243 
 
 227 
 
 189 
 
 182 
 
 162 
 
 150 
 
 146 
 
 133 
 
 8 
 
 577 
 
 
 
 305 
 
 254 
 
 237 
 
 198 
 
 190 
 
 169 
 
 156 
 
 152 
 
 139 
 
 9 
 
 577 
 
 
 
 386 
 
 3 2I 
 
 300 
 
 250 
 
 241 
 
 214 
 
 199 
 
 193 
 
 175 
 
 10 
 
 T77 
 
 
 
 357 
 
 298 
 
 278 
 
 231 
 
 223 
 
 198 
 
 184 
 
 179 
 
 162 
 
 u 
 
 4j 
 
 
 
 288 
 
 240 
 
 224 
 
 187 
 
 1 80 
 
 1 60 
 
 148 
 
 144 
 
 131 
 
 12 
 
 
 
 
 343 
 
 286 
 
 267 
 
 222 
 
 214 
 
 190 
 
 176 
 
 171 
 
 156 
 
 13 
 
 77 
 
 
 
 
 252 
 
 235 
 
 196 
 
 189 
 
 168 
 
 155 
 
 
 137 
 
 14 
 
 7*7 
 
 
 
 
 292 
 
 272 
 
 227 
 
 219 
 
 194 
 
 1 80 
 
 175 
 
 159 
 
 15 
 
 
 
 
 
 
 335 
 
 313 
 
 260 
 
 251 
 
 223 
 
 207 
 
 201 
 
 183 
 
 16 
 
 
 
 
 
 
 356 
 
 296 
 
 286 
 
 254 
 
 235 
 
 229 
 
 208 
 
 17 
 
 77 
 
 
 
 
 430 
 
 401 
 
 334 
 
 323 
 
 287 
 
 266 
 
 258 
 
 235 
 
 18 
 
 7*7 
 
 
 
 
 482 
 
 450 
 
 375 
 
 362 
 
 321 
 
 298 
 
 289 
 
 263 
 
 19 
 
 7 1 ? 
 
 
 
 
 537 
 
 501 
 
 418 
 
 403 
 
 358 
 
 332 
 
 322 
 
 293 
 
 20 
 
 1 
 
 
 
 
 595 
 
 556 
 
 463 
 
 446 
 
 2Q7 
 
 368 
 
 
 
 21 
 
 77 
 
 
 
 
 656 
 
 O 
 
 613 
 
 T-^O 
 
 510 
 
 492 
 
 oy / 
 
 437 
 
 o ^ 
 
 405 
 
 394 
 
 358 
 
 22 
 
 7*7 
 
 
 
 
 720 
 
 672 
 
 560 
 
 540 
 
 480 
 
 444 
 
 432 
 
 393 
 
 2? 
 
 JL 
 
 
 
 
 787 
 
 73 C 
 
 612 
 
 
 r 2 C 
 
 486 
 
 
 
 o 
 24 
 
 X 
 
 
 
 
 II 
 
 857 
 
 1 OO 
 
 800 
 
 666 
 
 643 
 
 J ^ O 
 
 571 
 
 529 
 
 5H 
 
 467 
 
 
 
 This table shows the nearest point in sharp focus when objects 
 infinitely distant are focused on. It is calculated from the 
 
 F 2 
 formula hf= . , where hf = hyperfocal distance in 
 
 12 /\ A /\ U 
 
 feet, u = diameter in inches of the circle of confusion or greatest 
 allowable unsharpness of the image of a point, F = the equiva- 
 lent focal length of the lens in inches, and A = the / number. 
 This table is usually calculated for a single circle of confusion 
 
AND USE A LENS 
 
 79 
 
 focal Distances 
 
 A =f value of lens 
 
 8 
 
 9 
 
 11.3 
 
 12.5 
 
 16 
 
 18 
 
 22.6 
 
 25 
 
 32 
 
 36 
 
 45.2 
 
 50 
 
 64 
 
 17 
 
 15 
 
 12 
 
 II 
 
 8* 
 
 7* 
 
 6 
 
 si 
 
 4 
 
 3i 
 
 3 
 
 2| 
 
 2 
 
 38 
 
 33 
 
 27 
 
 24 
 
 19 
 
 17 
 
 13 
 
 12 
 
 9\ 
 
 8| 
 
 6| 
 
 6 
 
 41 
 
 38 
 
 34 
 
 27 
 
 25 
 
 19 
 
 17 
 
 14 
 
 12 
 
 91 
 
 8 
 
 6! 
 
 61 
 
 4^ 
 
 SO 
 
 44 
 
 35 
 
 32 
 
 25 
 
 22 
 
 18 
 
 16 
 
 13 
 
 ii 
 
 9 
 
 8 
 
 6; 
 
 63 
 
 56 
 
 45 
 
 41 
 
 32 
 
 28 
 
 22 
 
 20 
 
 16 
 
 14 
 
 ii 
 
 10 
 
 7} 
 
 65 
 
 58 
 
 46 
 
 42 
 
 33 
 
 29 
 
 23 
 
 21 
 
 1 6 
 
 IS 
 
 12 
 
 10 
 
 8| 
 
 79 
 
 70 
 
 56 
 
 50 
 
 39 
 
 35 
 
 28 
 
 25 
 
 20 
 
 18 
 
 14 
 
 13 
 
 10 
 
 94 
 
 83 
 
 66 
 
 60 
 
 47 
 
 42 
 
 33 
 
 30 
 
 23 
 
 21 
 
 17 
 
 15 
 
 12 
 
 no 
 
 98 
 
 78 
 
 70 
 
 55 
 
 49 
 
 39 
 
 35 
 
 28 
 
 25 
 
 20 
 
 18 
 
 14 
 
 128 
 
 H3 
 
 90 
 
 82 
 
 64 
 
 57 
 
 45 
 
 4i 
 
 32 
 
 28 
 
 23 
 
 20 
 
 16 
 
 133 
 
 119 
 
 94 
 
 85 
 
 67 
 
 59 
 
 47 
 
 43 
 
 33 
 
 30 
 
 24 
 
 21 
 
 17 
 
 169 
 
 150 
 
 119 
 
 1 08 
 
 84 
 
 75 
 
 60 
 
 54 
 
 42 
 
 38 
 
 30 
 
 27 
 
 21 
 
 156 
 
 139 
 
 no 
 
 IOO 
 
 78 
 
 69 
 
 55 
 
 So 
 
 39 
 
 35 
 
 28 
 
 25 
 
 2O 
 
 126 
 
 112 
 
 89 
 
 81 
 
 63 
 
 56 
 
 45 
 
 40 
 
 32 
 
 28 
 
 22 
 
 20 
 
 16 
 
 150 
 
 133 
 
 1 06 
 
 96 
 
 75 
 
 67 
 
 53 
 
 48 
 
 38 
 
 33 
 
 27 
 
 24 
 
 19 
 
 132 
 
 117 
 
 93 
 
 85 
 
 66 
 
 57 
 
 47 
 
 42 
 
 33 
 
 29 
 
 23 
 
 21 
 
 17 
 
 153 
 
 136 
 
 1 08 
 
 98 
 
 77 
 
 68 
 
 54 
 
 49 
 
 38 
 
 34 
 
 27 
 
 25 
 
 19 
 
 176 
 
 156 
 
 124 
 
 H3 
 
 88 
 
 78 
 
 62 
 
 56 
 
 44 
 
 39 
 
 31 
 
 28 
 
 22 
 
 200 
 
 I 7 8 
 
 141 
 
 128 
 
 IOO 
 
 89 
 
 7i 
 
 64 
 
 So 
 
 44 
 
 35 
 
 32 
 
 25 
 
 226 
 
 2OI 
 
 1 60 
 
 144 
 
 H3 
 
 IOO 
 
 80 
 
 72 
 
 56 
 
 So 
 
 40 
 
 36 
 
 28 
 
 253 
 
 225 
 
 179 
 
 162 
 
 127 
 
 H3 
 
 90 
 
 81 
 
 63 
 
 56 
 
 45 
 
 41 
 
 32 
 
 282 
 
 251 
 
 199 
 
 181 
 
 141 
 
 125 
 
 IOO 
 
 90 
 
 7i 
 
 63 
 
 So 
 
 45 
 
 35 
 
 312 
 
 2 7 8 
 
 221 
 
 200 
 
 156 
 
 139 
 
 no 
 
 IOO 
 
 78 
 
 69 
 
 55 
 
 50 
 
 39 
 
 344 
 
 306 
 
 244 
 
 220 
 
 172 
 
 153 
 
 122 
 
 no 
 
 86 
 
 77 
 
 61 
 
 55 
 
 43 
 
 378 
 
 336 
 
 267 
 
 242 
 
 189 
 
 168 
 
 J 34 
 
 121 
 
 95 
 
 84 
 
 67 
 
 61 
 
 47 
 
 413 
 
 367 
 
 292 
 
 265 
 
 207 
 
 184 
 
 I 4 6 
 
 132 
 
 103 
 
 92 
 
 73 
 
 66 
 
 52 
 
 450 
 
 4OO 
 
 318 
 
 288 
 
 225 
 
 200 
 
 159 
 
 144 
 
 H3 
 
 IOO 
 
 80 
 
 72 
 
 56 
 
 inch) for all focal lengths, but as the definition required 
 for motion pictures or small negatives for enlargement made 
 with short-focus lenses is much greater than that needed for 
 portraits, the value of u taken for each focal length is given. 
 If the lens is focused on objects at the distance given in the table, 
 the sharpness will be satisfactory between half that distance and 
 infinity! 
 
8o 
 
 HOW TO CHOOSE 
 
 Reducing and Enlarging Tables 
 
 All fig- 
 ures in 
 table]are 
 in ins. 
 
 Reductions 
 
 Samel * I .* I .* I .4 I .* I .* I .* I .* I ft I A I 
 size | size I size size | size I size size size | size size size I size 
 
 Focus 
 of lens 
 used 
 
 Enlargements 
 
 Same 2 3 4 5 fc 
 size times times times times 
 
 6 
 
 61 
 
 7 
 
 8 
 
 9 
 10 
 11 
 12 
 
 6 
 
 6 
 7 
 7 
 8 
 8 
 9 
 9 
 
 10 
 10 
 11 
 ii 
 12 
 
 12 
 
 13 
 
 13 
 14 
 14 
 16 
 16 
 18 
 18 
 20 
 
 22 
 
 24 
 
 24 
 
 12 
 
 6 
 
 6i 
 15 
 
 7j 
 
 8| 
 18 
 
 21 
 
 24 2 
 
 12 
 
 27 
 
 i3l 
 30 
 i5 
 33 
 
 36 
 
 18 
 
 12 
 
 4 
 14 
 
 16 3 
 
 si 
 
 18 
 
 6 
 20 
 
 6| 
 22 
 
 7i 
 
 24 
 
 8 
 
 26 
 
 28 3 
 
 9i 
 32 
 
 iof 
 36 
 
 12 
 
 40 
 
 i3i 
 
 44 
 i4l 
 48 
 16 
 
 15 
 3l 
 
 4f 
 20 
 
 5 
 22| 
 
 s! 
 
 25 
 
 6| 
 
 61 
 30 
 
 7i 
 
 35 
 
 40 
 
 45 
 
 50 
 
 55 
 
 60 
 
 15 
 
 18 
 
 3l 
 21 
 
 4i 
 24 
 
 4f 
 27 
 
 5! 
 30 
 
 6 
 33 
 
 6f 
 36 
 
 7* 
 39 
 
 7* 
 42 
 
 8f 
 48 
 
 9-! 
 
 54 
 60* 
 
 12 
 
 66 
 
 13* 
 72 
 
 6 
 
 times times times times times times times 
 
 21 
 
 28 
 
 4l 
 31* 
 
 si 
 
 35 
 
 5f 
 38| 
 
 42 
 
 7 
 
 49 
 
 8^ 
 56 
 
 63 
 70 
 77^ 
 
 84 
 
 14 
 
 24 
 
 3f 
 28 
 
 4 
 32 
 
 4* 
 36 
 
 5* 
 40 
 
 5f 
 
 44 
 
 6? 
 48 
 
 6f 
 52 
 
 7f 
 56 
 
 64 
 
 72 
 
 80 
 
 88 
 
 96 
 
 3U 
 
 36 
 
 40 
 
 45 
 
 49^ 
 
 54 
 
 7r 5 e 
 63 
 7l 
 72 
 
 81 
 
 90 
 
 99 
 
 108 
 
 13* 
 
 9 
 
 30 
 
 3l 
 35 
 
 3t 
 40 
 
 4t 
 45 
 
 5 
 50 
 
 5-i 
 
 55 
 
 60 9 
 
 6| 
 65 
 
 7i 
 70 
 
 80 
 
 90 
 
 100 
 
 110 
 
 120 
 
 10 
 
 ime 
 
 33^ 
 
 44 
 
 4 T 4 o 
 
 55 
 
 si 
 
 66 
 
 71* 
 
 72% 
 
 77 
 
 88 
 
 99 
 
 110 
 
 121 
 
 132 
 
 11 
 
 36 
 
 3T 8 r 
 42 
 3i 9 r 
 48 
 
 64 
 
 4H 
 
 60 
 
 5i 5 T 
 
 66 
 
 6 
 
 72 
 
 12 
 
 39 
 
 451 
 
 52 
 
 65 
 
 5lf 
 78 
 61 
 
 78 
 
 84 ' 
 
 7T 7 T 
 
 96 
 
 8 T 8 T 
 108 
 
 9T 9 T 
 
 120 
 
 iof 
 132 143 
 
 144 
 
 104 
 
 117 
 
 9l 
 
 130 
 
 156 
 
 13 
 
 Bold figures are distances of lens from easel in enlarging, or 
 from lens to photo being reduced in copying. Light figures are 
 distances from lens to negative being enlarged or camera exten- 
 sion in case reduced size copies are being made. The outer end 
 of lens (cap end) should face bromide paper in enlarging and in 
 reducing should face object being copied. Distances are meas- 
 
AND USE A LENS 81 
 
 ured from nodal points, not diaphragm of lens, and while measur- 
 ing these distances from diaphragm will give satisfactory results 
 in many cases when enlarging with large apertures or at great 
 distances final focusing should be done by inspection. Data 
 not given in the table may be calculated as follows: 
 
 Conjugate Foci. Let u = distance of object from lens, 
 v = distance of image from lens, F = focal length of lens. 
 
 or F (u + v) = uv, e.g., 3 (12 + 4) = 12 X 4- 
 
 If object is reduced n times upon the focusing screen, u is 
 n -\- i times the focal length of the lens, and v is the focal length 
 
 plus of the focal length. Thus 12 in. photographed down to 
 n 
 
 i in. with a 6-in. lens gives u = 13 X 6, and v = 6 + (^ X 6) 
 = 6i 
 
 Rule for Copying. To find distance from lens to original. 
 Multiply focal length of lens by the number of times of reduc- 
 tion, and add one focal length thereto. To find camera ex- 
 tension. Divide focal length by number of times of reduction, 
 and add one focal length thereto (see tables). 
 
 Rule for Enlarging. To find distance from negative to lens, 
 divide focal length by number of times of enlargement, and add 
 one focal length thereto. 
 
 To find distance from lens to paper, multiply focal length by 
 number of times of enlargement, and add one focal length thereto. 
 
 To Graduate Focusing Scale. Camera extensions for various 
 
 pF 
 distances are given by the formula _ , where p is focal length 
 
 of lens, and F the distance. Thus 20 ft. with 5-in. lens, 240 X 
 5 -- (240 - 5) = 5-1- 
 
 In practice it is most convenient to graduate focusing scale 
 by trial, focusing on a test object at known distances. 
 
 Combining Lenses. To find the focal length of two lenses 
 separated by a short distance, multiply the focal lengths together 
 and divide by focal lengths added together, less the distance 
 between the lenses. 
 
 Resultant focal length = 
 
 
82 
 
 HOW TO CHOOSE 
 
 To find the focal length of supplementary lens necessary to 
 reduce or increase the focal length of a given lens, multiply the 
 focal length to be altered by the focal length desired, and divide 
 the product by the original focal length less the final focal length. 
 
 /i equalling the original and F the final focal length. 
 
 To reduce the focal length use a positive lens. 
 
 To increase the focal length use a negative lens and calculate 
 it as minus in the formula. 
 
 The focal length of a "magnifier" to bring near objects 
 into focus should be equal to the distance of the object. 
 
 Diaphragm Numbers and Uniform System Numbers 
 
 /values 
 U. S. nos 
 
 3-i6 
 
 4 
 
 i 
 
 4-5 
 
 5.6 
 
 2 
 
 6-3 
 
 6.8 
 
 7 
 
 7-7 
 
 8 
 
 9 
 
 Relative exposures j 
 
 i 
 
 1 
 1.6 
 
 r 5 
 
 2 
 
 3-i 
 
 V 
 
 2.9 
 4-6 
 
 3 
 4-9 
 
 3-75 
 6 
 
 6.4 
 
 "i 
 
 There are today two important scales of diaphragm numbers 
 in use, but as both are expressed in the same notation on all 
 modern lenses and shutters except some made in the United 
 States, no confusion arises from this source except that some 
 diaphragm numbers are not found in all exposure tables. On 
 account of the latitude of exposure of all plates and films, using 
 the exposure for the nearest number given will rarely lead one 
 astray. The English system of marking lenses uses /:4 as a 
 unit. It is generally replaced on American cameras by the 
 Uniform system, in which the same unit is numbered i. In 
 the Stolze system, used on Zeiss and other German lenses, the 
 unit is/: Vio = /:3.i6. The following table compares the three 
 systems and gives the relative exposure times for the different 
 stops. Opposite the caption "/ values " are shown the/ numbers 
 in both the English and Stolze systems; beneath are listed the 
 equivalent apertures in the U. S. system. The figures showing 
 the relative exposures are computed for two systems, the unit 
 exposures being those for / 14 and /:3-i6 respectively. The ex- 
 
AND USE A LENS 83 
 
 posures which bear the relation of 2 to i to each other are printed 
 in boldface type. 
 
 On old lenses various other systems of markings may be found. 
 Zeiss lenses were once marked with numbers running in reverse 
 direction to that given above, / :ioo being marked I and /:4-5, 
 512. Later /:5O was adopted as the unit, 7:4.5 then being num- 
 bered 128. Old Goerz and Dallmeyer lenses may occasionally 
 be found with numbers based on /:62, which is numbered 384, 
 and the numbers run down to 3, equivalent to 7:5.5. Still other 
 systems may be found on French lenses, but these are so rarely 
 seen outside of that country that it would be useless to give 
 space to them here. For full details of the relation of stops to 
 exposure, the reader is referred to No. I of this series, The 
 Secret of Exposure. 
 
 11.3 
 
 12.5 
 
 16 
 
 18 
 
 22.6 
 
 25 
 
 32 
 
 36 
 
 45.2 
 
 50 
 
 64 
 
 7i 
 
 100 
 
 8 
 
 
 16 
 
 
 3 2 
 
 
 64 
 
 
 128 
 
 
 2^6 
 
 
 
 8 
 
 13 
 
 10 
 
 16 
 
 16 
 
 26 
 
 20 
 32 
 
 32 
 
 51 
 
 39 
 64 
 
 64 
 
 IO2 
 
 78 
 
 128 
 
 128 
 
 205 
 
 156 
 256 
 
 256 
 
 410 
 
 ffi 
 
 1024 
 
What Lens Shall I Buy? 
 
 THIS PERPLEXING QUESTION is answered in detail by our 
 booklet bearing the above title. It contains much information of value 
 to the buyer of a lens and fully describes the Bausch & Lomb-Zeiss 
 line of Tessars and Protars. 
 
 THE TESSAR is the arch enemy of failure it makes all days 
 bright. TESSAR Ic has three times the speed of the ordinary lens. 
 This is the lens for reflecting cameras. 
 
 TESSAR lib is 61% to 100% faster than ordinary lenses. It 
 is the standard high grade equipment for the compact hand camera. 
 
 PROTAR Vila is convertible, the most universal lens made. 
 The PROTAR gives you three or more lenses in one. With these 
 you can choose your image, size or angle of view without moving 
 your camera. 
 
 " What Lens Shall I Buy?" is sent on request. 
 Try out a lens on your camera through any dealer. 
 
 Bausch & Ipmb Optical (o. 
 
 ST. PAUL STREET, ROCHESTER, N. Y. 
 NEW YORK WASHING TON GHICAQO SAN FRANCISCO 
 
 Please mention Practical Photographs Ah^n writing Advertisers 
 
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 LENS 
 
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 Today the world over GOERZ, on any photographic, optical or 
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 GOERZ LENSES AND GOERZ CAMERAS now have such a repu- 
 tation. See your dealer or write to 
 
 C. P. GOERZ, AMERICAN OPTICAL COMPANY 
 
 317Y EAST 34TH STREET, NEW YORK CITY 
 
 Please mention Practical Photography when writing Advertisers 
 
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 AT A SAVING OF 30 TO 50 PER CENT. 
 
 The following listed bargains which we have on hand at the present time will give 
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 Goerz Vest Pocket Tenax Dagor Lens - - - 42.50 
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 Eastman Vest Pocket Kodak- ------ 3.75 
 
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 6|X8 Ic Tessar F:4.5 65.00 
 
 4X5 Premo Film No. 1 R R Lens - - - - 5.75 
 Eastman 4A Speed Kodak ------- 19.50 
 
 5X7 Speed Graphic -- 25.00 
 
 3A Folding Brownie R R Lens 7.50 
 
 All of the above-listed items are fully guaranteed to be in first-class condition, and 
 all are practically the very latest type. We would be pleased to send any of these on 
 trial for a period of five days, and if same should prove unsatisfactory, money will be 
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 we will be pleased to make you the very best allowance possible. 
 
 First-class DEVELOPING, PRINTING and ENLARGING at lowest prices 
 
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 It's Th' Little Things That Count 
 
 See This Little Book : Our adv. is in it, and WHY ? Because 
 it's a good seller. It covers its subject and costs only 25 cents. 
 
 We are selling lots of them and can send you one at the price. 
 
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 its convenient items. 
 
 See Our Bargain List : It's full of good things, at catchy prices. 
 
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 Please mention Practical Photography when writing Advertisers 
 
A New Visual Quality Lens 
 
 SERIES IV DOUBLET 
 
 (75 PER CENT CORRECTION) 
 
 EXPERT OPINIONS: 
 
 "Old boy, this is not a lens; it is a MAGIC chunk of glass." 
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 " This new doublet is the best thing I have ever seen hi the way 
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 Later Mr. Anderson wrote, "The more I use it, the better I like it." 
 
 THIS NEW LENS is offered as the best possible compro- 
 mise between the original Smith lens (Series I) and the fully- 
 corrected anastigmat for the general purposes of the pictorial 
 photographer. Workers who desire a fast lens capable of giv- 
 ing every effect from extreme breadth to moderate softness with 
 firm drawing of detail will find in this new series exactly what 
 they have been looking for. It is especially suitable for reflect- 
 ing cameras and home portraiture, on account of its great speed 
 of/: 4.5. With its 75 per cent correction, the full aperture 
 gives about the same quality as /: 6 of the older series, and 
 f\ 5.6 will be found equivalent to/: 8 of the same. Write for 
 Particulars of 
 
 THE VISUAL QUALITY LENS 
 
 Pinkham & Smith Company 
 
 Two Stores: 
 
 288-290 BOYLSTON ST. 13! BROMFIELD ST. 
 Boston, Mass. 
 
 Please mention Practical Photography when writing Advertisers 
 
SOONER OR LATER EVERY GRAFLEX OWNER 
 WILL POSSESS A STRUSS PICTORIAL LENS 
 
 C, For scientific work use an ana- 
 stigmat, but for PICTURES make 
 your outfits more complete by own- 
 ing a Struss Pictorial Lens. Unsur- 
 passed for Street Scenes requiring 
 quick action and speed, Q Send 
 twenty-five cents for a beautifully 
 illustrated, intensely interesting and 
 instructive BOOKLET FJ. This 
 amount will be refunded to each 
 lens purchaser. C. Made in the fol- 
 lowing focal lengths, in a highly 
 finished light aluminum mount, Iris 
 Diaphragm. 
 
 SINGLE LENS, F ; 5.5 
 9* 12* 15* 18* 21" Other sizes 
 $15 $20 $25 $30 $35 and speeds 
 
 FURTHER PARTICULARS FROM 
 
 KARL STRUSS, 
 5 W. 31st St., New York 
 
 NEW ROSS LENSES 
 
 Cover Perfectly with Brilliant Definition and Are Extremely Rapid 
 Adaptable to All Modern Cameras 
 
 ROSS' New "XPRES" Patent 
 
 F/4.5 FULL APERTURE F/4-5 
 
 Working at full aperture. For Studio Portraits and Groups' 1 and for all work requiring 
 extreme rapidity. Exceptionally Brilliant. Definition to margins. 
 
 i2-inch "XPRES" THE LENS for Cabinets and Whole-plate Groups. Admirable for 
 Reflex and Hand Cameras. 
 
 ROSS' New "TELECENTRIC" Patent 
 
 F/5-4 TWO SERIES F/6.8 
 
 Large Image with Short Camera Extension. Critical Definition at full aperture. Short- 
 ened Exposures Ideal for Sporting Events. Inexpensive Studio Lens. Covering to Margins. 
 
 ROSS' New " COMBINABLES " Patent 
 
 F/II SINGLES AND DOUBLETS F/S-5 
 
 The Singles are the most Rapid Single Anastigmats, perfectly corrected, giving Critical 
 Definition to the margins without distortion for Landscapes, Large Portraits and Groups. 
 The Universal Doublets are specially suitable for Portraits and Groups in the Studio and all 
 outdoor Photography, Interiors, Copying, etc. 
 
 For Full Particulars see Ross' New Price List, sent free on application 
 American Agents 
 
 GEORGE MURPHY, Inc. 
 
 57 East Ninth Street .... NEW YORK 
 
 Please mention Practical Photography when writing Advertisers 
 
aWOLLENSAKS 
 
 LET US 
 HELP YOU 
 
 DECIDE THAT ALL IMPORTANT 
 QUESTION 
 
 I 
 
 "WHAT LENS? 
 
 As manufacturers of the most 
 complete line with 
 
 B8 
 
 "A LENS FOR EVERY PURPOSE" 
 
 our co-operation, flavored with vast experience, will be very 
 helpful to you. And all the assistance we can give is yours 
 for the asking without any sort of obligation on your part. 
 
 LENSES 
 
 Just consider these : 
 
 Velostigmat Series I F : 6.3 
 Velostigmat Series II F : 4.5 
 Velostigmat Series III F : 9.5 
 Vinco-Anastigmat F : 6.3, F : 6.8 
 Versar Portrait and View F : 6 
 
 Vitax Portrait F: 3.8 
 Vesta Portrait F : 5 
 Verito Diffused Focus F : 4 
 VoltasF:8 
 Wide Angle F: 16 
 
 RAY FILTER 
 
 and Shutters : Optimo, Auto, Regno Victo, Ultro, Studio. 
 
 Now a postal request and our beautiful complete catalogue and 
 any special information desired, will reach you by first return mail. 
 The time to drop the postal is TODAY. 
 
 WOLLENSAK OPTICAL CO. 
 
 ROCHESTER, N. Y. 
 
 Please mention Practical Photography when writing Advertisers 
 
CONCERNING 
 
 PRACTICAL PHOTOGRAPHY 
 
 FROM THE PUBLISHER TO THE READER 
 
 A study of this number of Practical Photography will show you the aim of the series, which 
 is to give in each number all of the necessary information for a mastery of some small part 
 of the domain of photography. It is not the intention of the Editors to include a lot of use- 
 less theories, but each number will reflect the present state of our knowledge of some branch 
 of photography, plainly told in such a way that the knowledge can be immediately put into 
 practice in the workroom or the field. Each book will be illustrated as liberally as a com- 
 plete understanding of the subject demands, but no pictures will be introduced merely be- 
 cause of their beauty, or because of their artistic value. The books will be well made from 
 the standpoint of typography, paper, and illustrations, will be sewed to open flat, and will 
 handily fit the pocket. They will be issued in two bindings, paper and cloth. The paper 
 edition will cost twenty-five cents per issue, or $2.50 by subscription for twelve numbers. 
 The price of the cloth edition will be fifty cents a copy, or $5.00 for twelve numbers. Some 
 description of the first three numbers follows. 
 
 NO. i. "THE SECRET OF EXPOSURE."^ 
 
 If exposure is correct, all other steps in the making of a good negative are merely mechan- 
 ical. This book contains a complete system for obtaining correct exposure, on all occasions. 
 It discusses all the factors governing exposure, all the conditions under which it should be 
 made, and gives full and explicit directions for obtaining correct exposure under all circum- 
 stances. Complete exposure tables and speed lists of all plates on the American market 
 are given, and the book concludes with a clear discussion of the use of exposure meters. It 
 is the most complete and practical treatise on exposure ever written. 
 
 NO. 2. "BEGINNERS' TROUBLES." 
 
 Though this book would seem to be intended only for the novice in photography, it con- 
 tains an enormous number of practical hints of value to every practitioner. In a way it is 
 a manual of photographic work, for it gives useful pointers for success in all the departments 
 of photography for making a perfect negative and a satisfactory print from it. Its primary 
 purpose is to show what may be the causes of failure, and to help the reader to avoid these 
 causes. 
 
 NO. 3. "HOW TO CHOOSE AND USE A LENS." 
 
 Without being too theoretical, this book explains the reason for existence and the particu- 
 lar qualities of each type of lens now in use in photography. It tells exactly what each will 
 do, and how it may be used to the best advantage. It particularly describes the more mod- 
 ern varieties of lenses, and gives full information as to the differences between them. It tells 
 how to test a lens, and answers many questions which every user of a lens is sure to ask. 
 
 NO. 4. "HOW TO MAKE PRINTS IN COLOR." 
 
 These books are for sale and subscriptions will be received by dealers in photographic 
 supplies, newsstands, booksellers, or the publishers. 
 
 AMERICAN PHOTOGRAPHIC PUBLISHING CO. 
 221 COLUMBUS AVENUE, BOSTON, MASS. 
 
 Please mention Practical Photography when writing Advertisers 
 
THE SELECTION OF A LENS after you have con- 
 sidered the matter fully with due regard to the question 
 of Convertible Lenses versus all others, freedom from 
 air spaces, well balanced focal lengths, maximum working aper- 
 ture consistent with good depth of focus and moderate price 
 should lead you gently and firmly to the conviction that 
 
 3iirner~5ieieh 
 
 CONVERTIBLE ANASTIGMATS F:6.8 
 
 meet absolutely every requirement by coming nearer to the opti- 
 cian's idea of a perfect lens and the photographer's wish for a 
 universal lens. Find a lens if you can that will equal the 
 
 TURNER-REICH ANASTIGMAT 
 
 at every point. Don't hope to buy one second hand as they are 
 rarely sold by those who own them. 
 
 Send for complete catalogue of Cameras and Lenses. 
 
 Gundlach-Manhattan Optical Co. 
 
 7 83A CLINTON AVE., SO., ROCHESTER, N. Y. 
 Please mention Practical Photography when writing Advertisers 
 
EVERY reader of this book is sure to be seriously interested in photography. This 
 being the case, he should follow the progress of the art, and keep continually in touch 
 with its latest developments. The surest way for him to do this is to subscribe to one 
 or more of the photographic magazines, and we should be very happy, in case he is not ac- 
 quainted with them, to send him sample copies of our two monthlies, AMERICAN PHO- 
 TOGRAPHY and POPULAR PHOTOGRAPHY. Each of these magazines is edited 
 with the idea of giving each reader as much help and service as the Editors can possibly 
 furnish. The answering of perplexing questions, the criticism of readers' pictures, with an 
 idea of showing how they may be improved, the stimulation of effort by the awarding of 
 prizes in monthly contests, the furnishing of information as to how the camera may be made 
 a means of pecuniary help, are but a few of the manifold ways in which our Editors give 
 the readers service. A more particular description of the characteristics of the two maga- 
 zines follows, but the best way to learn about them is to send for a sample copy, which will 
 cost you nothing and subject you to no obligation. 
 
 AMERICAN PHOTOGRAPHY is today the standard American photographic magazine- 
 and has for years endeavored to follow its motto by " representing all that its name implies." 
 It covers the whole field of photography, for amateur, professional and commercial photog- 
 rapher, publishing each month numerous strong articles on the best current practice and the 
 latest advances in each of these departments. A strong motion picture department is one 
 of its specialties; and it continually endeavors to interest its readers in other departments 
 of photography which will broaden their activities and give them some interests beyond 
 the making of ordinary pictures, which may lose interest after a while. Its departments 
 include "Questions and Answers," "Practical Hints," "Our Portfolio" for picture criticism, 
 "The Round World Exchange Club" for the exchange of prints, "The Market Place," with 
 information as to ways of selling pictures, "Competitions," literary and pictorial, "Notes 
 and News" of clubs and manufacturing activities. The subscription price is $1.50 a year 
 in the United States. 
 
 POPULAR PHOTOGRAPHY is a magazine intended primarily for the less advanced 
 amateur. While the illustrations of AMERICAN PHOTOGRAPHY come largely from 
 exhibitions and show the achievements of the more distinguished photographers of the day, 
 POPULAR PHOTOGRAPHY is illustrated solely from the work of readers, of the kind 
 which every amateur wants to take, and with each print is given a complete description of 
 the exact method by which it was made and a criticism telling how it might have been im- 
 proved; thus forming a guide whereby every reader can make prints similar to those which 
 he admires in the pages of the magazine. In addition to these picture criticisms, the maga- 
 zine contains numerous short and pointed articles, most of which are written in response to 
 definite queries from readers for information on certain points. They thus reflect the needs 
 of the day, and have proved extremely popular. The whole keynote of the magazine is to 
 be brief and practical, and no long theoretical articles are published. It has consequently 
 proved very attractive to professional photographers as well as the amateurs for whom it is 
 primarily designed. A monthly competition and some other departments are run by this 
 magazine as well as by AMERICAN PHOTOGRAPHY. The subscription price is $1.00 
 a year in the United States. 
 
 AMERICAN PHOTOGRAPHIC PUBLISHING CO. 
 440 POPE BUILDING, BOSTON, MASS. 
 
 Please mention Practical Photography when writing Advertisers 
 
As right as 
 a full 
 jeweled 
 watch. 
 
 The Vest Pocket 
 Autographic Kodak 
 
 with Kodak Anastigmat lens, f. 7.7. 
 
 A vest pocket camera that will really go in the vest pocket com- 
 fortably. 
 
 A lens that gives microscopic definition and has speed to spare, a 
 ball bearing shutter that works silently, accurately, without jerk or jar. 
 
 " Autographic " of course. All the folding Kodaks now are. 
 
 In this camera Kodak simplicity and the utmost convenience are combined 
 with an optical quality that perfectly meets the requirements of those who 
 demand an instrument of the highest type. 
 
 Pictures, 1# x 2> inches. 
 
 Price, $10.1 
 
 If it isn't an Eastman, it isn't a Kodak. 
 
 Catalogue free at your dealer's, or by mail. 
 
 EASTMAN KODAK CO., ROCHESTER, N. Y., Th. Kodak a ty . 
 
 Please mention Practical Photography when writing Advertisers 
 
The construction 
 of the 
 
 Brownie 
 
 Enlarging 
 
 Camera 
 
 is so simple that its equally 
 simple operation becomes 
 a self-evident fact. 
 Insert your negative at the small end of the camera, 
 your Velox paper at the other, expose to daylight, 
 and develop and fix in the regular way. 
 
 No focusing No dark-room. 
 
 Signalize some of your vacation masterpieces in large prints. 
 THE PRICE. 
 
 V. P. Kodak Enlarging- Camera, for 3^ x 5M enlargements from 1& x 2} negatives, $1.75 
 No. 2 Brownie Post Card Enlarging Camera, for 3% x 6M enlargements from 2% x 
 
 3} negatives, ---- 1.75 
 
 No. 2 Brownie Enlarging Camera, for 5x7 enlargements from 2^ x 3X negatives, - 2.00 
 
 No. 3 Ditto, for 6M x 8K enlargements, from 3% x 4K negatives, 3.00 
 
 No. 4 Ditto, for 8 x 10 enlargements, from 4x6 negatives (will also take 3^ x 5^ 
 
 negatives), ......------ ... 4.00 
 
 EASTMAN KODAK COMPANY, 
 ROCHESTER, N. Y. 
 
 At your dealer's. 
 
 Please mention Practical Photography when writing Advertisers 
 
The Kodak Film Tank 
 
 has made the dark room a relic and 
 better negatives the rule. 
 
 The big link in the Kodak chain 
 of daylight all the way. 
 
 THE PRICE 
 
 Brownie Kodak Film Tank, for use with No. 1, No. 2 and No. 2 Folding: 
 
 Pocket Brownie cartridges, complete, $2.50 
 
 Vest Pocket Kodak Film Tank, for Vest Pocket cartridges, complete, 2.50 
 
 2M-inch Kodak Film Tank, for use with all Kodak or Brownie cart- 
 ridges having a film width of 2> Inches or less, complete, - - 3.60 
 
 3K-inch Kodak Film Tank, for use with all Kodak and Brownie cart- 
 ridges having a film width of 3> inches or less, complete, - - 5.00 
 
 5-inch Kodak Film Tank, for use with all Kodak and Brownie cart- 
 ridges having a film width of 6 inches or less, complete, 6.00 
 
 7-inch Kodak Film Tank, for use with No. 5 Cartridge Kodak or shorter 
 
 film cartridges, complete, - 7.60 
 
 EASTMAN KODAK COMPANY, 
 ROCHESTER, N. Y. 
 
 At your dealer's. 
 
 Please mention Practical Photography when writing Advertisers 
 
You want to keep your negatives, of course, 
 and you want to keep them where you can lay 
 your hands on them without delay particularly 
 those containing autographic records. 
 
 The Eastman 
 Film Negative Album 
 
 will preserve your negatives against injury or loss 
 and will provide the handiest kind of a reference 
 book wherein the answer to such questions as 
 "When did I take this?" "Where was this 
 taken ?" may be found on the instant. 
 
 THE PRICE 
 
 For 100 negatives, IH*2%, 10.75 
 
 For 100 negatives, 2% x4%, or smaller, .75 
 
 For 100 negatives, 3K x 4#. or 4x5, .... 1.00 
 
 For 100 negatives, 3% x 5^, or smaller, - 1.00 
 
 For 100 negatives, 5 x 7, or smaller, .... 1.50 
 
 EASTMAN KODAK COMPANY, 
 
 ROCHESTER, N. Y. 
 At your dealer 1 1. 
 
 Please mention Practical Photography when writing Advertisers 
 
Flowers are only one 
 of a number of subjects 
 that lend themselves 
 particularly to the 
 Kodak and the 
 
 Kodak 
 
 Portrait 
 
 Attachment 
 
 Made with Kodak and Kodak 
 Portrait Attachment. 
 
 If a vase of flowers struck. your fancy, you would not take 
 up a position ten fett away in order to admire it. 
 
 It's that way with the Kodak the Kodak can't see all the 
 beauty until it comes within close range. 
 
 The Kodak Portrait Attachment enables you to work as close 
 to your subject as two feet, eight inches with the Folding Pocket 
 Kodaks near enough so that little of beauty or interest may 
 escape it even though the subject be small in size. 
 
 It's just an extra lens which, when slipped on over the regular 
 lens equipment, brings the Kodak in focus at short range. 
 
 And it costs butffty cents. 
 EASTMAN KODAK COMPANY, 
 
 ROCHESTER, N. Y. 
 
 At your dealer* i. 
 
 Please mention Practical Photography when writing Advertisers 
 
We are as eager to have you 
 make good prints as you are 
 that's the reason for 
 
 VELOX 
 
 a photographic paper that Jits. 
 
 Use^the new Contrast Vtlox with flat negatives. 
 
 At your dealer'*. 
 
 NEPERA DIVISION, 
 
 EASTMAN KODAK CO., ROCHESTER, N. Y. 
 
 Please mention Practical Photography when writing Advertisers 
 
HOME USE 
 
 CIRCULATION DEPARTMENT 
 MAIN LIBRARY 
 
 This book is due on the last date stamped below. 
 1 -month loans may be renewed by calling 642-3405. 
 6-month loans may be recharged by bringing books 
 
 to Circulation Desk. 
 Renewals and recharges may be made 4 days prior 
 
 to due date. 
 
 ALL BOOKS ARE SUBJECT TO RECALL 7 DAYS 
 AFTER DATE CHECKED OUT. 
 
 LD21-A30m-7,'73 
 (R2275S10)476 A-32 
 
 General Library 
 
 University of California 
 
 Berkeley 
 

 YB II 127 
 
 
 V 
 
 328323 
 
 UNIVERSITY &F CALIFORNIA pBRARY