V PHOTOGRAPHIC LENSES: HOW TO CHOOSE, AND HOW TO USE. An Elementary and Practical Guide to the selection and use of Photographic Objectives, With many original illustrations. BY JOHN A. HODGES. Author of Practical Enlarging,'' ''Elementary Photography,'" " The Lantern-Slide Manual,'' etc., etc. Percy Lund & Co., The Country Press, Bradford ; AND Memorial Hall, Ludgate Circus, London, 1895. THE "LUND LIBRARY OF PHOTOGRAPHY.'' Boards, is. 6d. ; cloth, 2s. od. THE STEREOSCOPE AND STEREOSCOPIC PHOTOGRAPHY. Translated from the French of F. DROUIN by MATTHEW SURFACE. THE HALF-TONE PROCESS, a Practical Manual of Photo-Engraving in Half-tone on Zinc and Copper. By JULIUS VERFASSER. PHOTOGRAPHIC LENSES: HOW TO CHOOSE AND HOW TO USE. An Elementary and Praaical Guide to the seledlion and use of Photographic ObjecStives. By JOHN A. HODGES. IN THE PRESS. SENSITIZED PAPERS: how made and used. All about making, printing, toning, fixing, developing, and everything else of interest in connexion with Sensitized Papers. By dr. H. C. STIEFEL. many others to follow CONTENTS. Section I. — Optical Principles and Definition of Terms, etc, CHAPTER I. Optical Principles . . . . . . , . . . 7 CHAPTER H. Definition of Terms .. .. .. .. ..13 CHAPTER HI. Definition of Terms, Continued . . . . • • 17 CHAPTER IV. Various Defects in Lenses .. .. .. ..19 CHAPTER V, The Diaphragm, or Stop, and its Functions . . 26 Section II. — A Description of Photographic Lenses. CHAPTER VI. Single Lenses . . . . . . . . . . • • 35 CHAPTER VII. Upon the Properties and Use of Single Lenses.. 40 CHAPTER VIIL The Rapid Rectilinear, or Non-Distorting Doublet 44 CHAPTER IX. Other Forms op the Doublet, including Wide- Angle Lenses , . , . 47 CHAPTER X. Portrait and Universal Lenses .. .. ..51 CHAPTER XI. New Types of Lenses Constructed of Jena Glass 57 CHAPTER XH. On Certain Obsolete Lenses . . . . . . . . 63 Section III. — The Choice and Use of Lenses, etc. CHAPTER XHL Upon the Choice of a Lens .. .. .. ..69 CHAPTER XIV. The Care of Lenses . . . . . . . . . . 80 CHAPTER XV. Upon Focussing . . . . . . . . . . . . 85 CHAPTER XVI. Upon Angle of View . . . . . . . . . . 92 CHAPTER XVII. Distortion : and Its Avoidance by the Use of THE Swing Back . . . . . . . . . . loi CHAPTER XVIII. Combination Lenses, Casket Lenses, and the Use OF Back Combinations, etc. . . . . . . 108 CHAPTER XIX. How TO Test a Lens .. .. .. .. ..114 CHAPTER XX. Lenses of Foreign Construction .. .. .. 125 CHAPTER XXI. On Purchasing Second-Hand Lenses.. .. .. 131 PREFACE. HE study of photographic optics has always been one of intense interest to the writer, who, during a long acquaintanceship with the art, has had opportunities of examining and using nearly every known form of lens. This book is the outcome of the experience so gained. It is in no sense a rival to existing treatises upon the subjecft of photographic optics, nor is it addressed to advanced students. Its purpose is essentially elementary ; its aim being to explain in simple language, and as far as possible without reference to technical terms, the properties and uses of photographic lenses. Elementary treatises upon photography, from lack of space, necessarily treat the subjedl with more or less brevity, and consequently a wide-spread demand exists among beginners for further information. The book is addressed to my friends **the beginners," in the hope that by a reference to its pages they may solve some of the doubts and difficulties not dealt with in detail in ordinary instrudlion books. THE AUTHOR. SECTION I. OPTICAL PRINCIPLES AND DEFINITION OF TERMS, ETC. CHAPTER I. OPTICAL PRINCIPLES. INTRODUCTORY LIGHT COMPOSITION OF LIGHT- REFRACTION PROPERTIES OF LIGHT CHROMATIC ABERRATION VISUAL AND ACTINIC FOCUS ACHRO- MATISM ETC. Although the aim and intention of the writer is to treat his subjecft from an essentially elementary and practical point of view, eschewing theory as far as possible, yet for the sake of lucidity it will be necessary, in the first place, to give a brief but simple explanation of the optical principles involved in the transmission of an image by a photographic lens ; and, secondly, to define the meaning of certain technical terms to which hereafter it will be impossible to avoid reference. Those who may be induced to study the theory of the subjedl are referred to such works as Lewis Wright's Light," Monkhoven's Photographic Optics," Abney's Treatise on Photography," Leaper's First Principles of Photography," and Traill Taylor's "Optics of Photography," all of which treat the subje(5l essentially from a photo- graphic point of view. B 8 PHOTOGRAPHIC LENSES. Light. — It is unnecessary for our present purpose to attempt to dive at all deeply into the theory of light. It will be quite sufficient to define some of its known properties. We may accept, therefore, the common theory that light, like sound, travels with an undulatory, wave-like motion in straight lines through space. The surfaces of all objecfls receive this light, and either reflecfl or absorb it ; and they appear to our vision more or less bright according to the amount of light which they refle(5l. CompositioTi of Light. — The nature and composition of light may be easily examined in the following way. Darken a room possessing a window upon which the sun's rays fall by means of an opaque shutter in such a way that all white light is excluded. If the aspedl be suitable, the ordinary photographer's dark-room will answer well. Now bore a small hole in the shutter so that a beam of light may enter and fall upon a sheet of white cardboard. The rays will appear to be colourless or white. If, however, we place a prism close to the aperture and cause the rays to pass through it, they immediately become analysed or split up into their component parts, and the wonderful phenomenon called the specSlrum is produced (fig. i). Refra(ftion. — Now let us refer to the diagram and endeavour to ascertain what has happened to PHOTOGRAPHIC LENSES. 9 the beam of white light in producing the beautiful result shown. The ray of light, it will be seen, follows the law we have already referred to, and travels in a straight line, but in passing through the prism becomes deflecfled or bent. This is due to what is called refradlion, a phenomena which always occurs when a ray of light passes from a medium like air to one that is denser, like glass. Fig. I. Properties of Light. — If we place a piece of sensitized albumenized paper in the image of the spedlrum, we shall find that it does not darken equally all over the surface upon which the various coloured rays fall ; the discolouration being confined pradlically to the region of the violet and blue rays, scarcely extending, if at all, to the yellow and red rays, which visually form the brightest portion of the specftrum. Now, if for the sensitive paper we 10 PHOTOGRAPHIC LENSES. substitute a thermometer, we shall find that whilst the violet and blue rays have no effecfl upon the mercury, it will rapidly rise when brought under the influence of the orange and red rays. These experiments, therefore, teach us that light possesses three distindl attributes or properties : i,e,^ visual properties, which enable us to see and recognise objecfts by their form and colour ; chemical properties; and, lastly, heating properties. The chief chemical acflion occurs, as we have seen, in and beyond the violet and blue rays ; the yellow rays are the most luminous; the heating properties being greatest in the red. Chromatic Aberration. — A photographic lens may be regarded as a modified prism, or rather series of prisms, its funcfhion being to focus or bring to a point the rays of light proceeding from objecfls in front of which it may be placed. It must not, however, be inferred that the funcftion of the lens is to produce the image. That it does not do so, the facft that a pin-hole aperture in a darkened room will projecfl an image of exterior objecfls is in itself sufficient evidence. The light rays which are refledled from all bodies are really the true image bearers or formers, the funcftion of a lens being to condense the image-bearing rays and bring them to a focus at a particular point, the position of which will vary according to the degree of curvature given to the lens. If we refer PHOTOGRAPHIC LENSES. II again to fig. i we shall see that the different coloured rays are unequally refracfted, that is to say, the violet rays are much more bent than the red. Supposing we have a lens ground from a single piece of glass, for instance, a bi-convex spedlacle lens, and proceed to take a photograph with it, upon developing the plate we shall find that the image is sadly wanting in definition, and is, technically speaking, **out of focus." Yet the image appeared quite sharp and distindl upon the focussing screen. The cause of this defecfl is Fig. 2. explained in what has already been said. Fig. 2 will, perhaps, make the matter clearer. The lines A A represent the red and yellow rays which form a luminous and visible image on the focussing screen, and which we can focus apparently sharply. But we have already learnt by our experiment with sensitive paper exposed to the spe(5lrum that these rays have little or no adlinic effedl, the paper darkening chiefly in the region of the violet and blue rays. The latter, it will be seen by referring again to fig. i, are shorter than the red and yellow 12 PHOTOGRAPHIC LENSES. rays, and therefore come to a focus in front of the yellow and red. The violet and blue are the chemical rays which we wish to utilise in producing our photograph, and are shown coming to a focus at the point V, fig. 2. In order, therefore, to produce a sharp photograph with such a lens, we must, after focussing the visual image, bring the plate into the plane of the chemical rays. We shall then succeed (although the image upon the focussing screen will then appear less sharp than when the glass was in the plane of the visual, or red rays) in producing a sharp negative. The early forms of photographic lenses were subject to this defecSl:, which is termed acSlinic aberration, or a want of coincidence of the chemical and visual foci, and such lenses were called uncorredled." Achromatism. — In pradlical photography the necessary adjustment after focussing, which varied in amount according to the distance of the objedl focussed from the camera, was found to be an intolerable nuisance, and after a while it was discovered that by construcfling lenses of two kinds of glass of different densities, generally of flint and crown, the adlinic and visual rays could be so controlled or direcfled that they came to a common focus, and produced an image free from colour. All modern lenses are so constru(5led, and are said to be achromatic." CHAPTER II. DEFINITION OF TERMS. OPTICAL PRINCIPLES AND DEFINITION OF TERMSy CONTINUED — FOCAL LENGTH BACK FOCUS EQUIV- ALENT FOCUS — CONJUGATE FOCI PRACTICAL IMPOR- TANCE TO THE PHOTOGRAPHER THE OPTICAL CENTRE. Focal Length. — When a distant objecfl, for instance, the sun, is sharply focussed upon the screen, the distance between the ground glass and the lens is popularly spoken of as its focus," or focal length. In the case of a doublet, the measurement is often taken from the back lens of the combination, and the distance is then called the **back focus." Such methods and expressions, though sufficiently accurate for rough work, are unscientific and misleading where any degree of accuracy is essential. For the purposes of scientific comparison the ^*back focus" is usually ignored, and what is called the equivalent focus" ascer- tained. This may be defined as the distance from the screen at which a single lens would have to be placed in order to produce an image of the same size as that produced by the lens the 14 PHOTOGRAPHIC LENSES. equivalent focus of which is to be found. The simplest method of ascertaining the equivalent focus of a lens is the following, though it is not quite exadl. Focus any given obje(fl — for instance, the cover of this book — so that its image appears upon the screen of exa(5lly the same size as the original. Then carefully measure the distance between the book and the ground glass, and divide by four. The result will give approximately the equivalent focus of the lens. Some lens makers of repute, notably Messrs. Taylor, Taylor and Hobson, now engrave the equivalent foci of their lenses upon their mounts, and the pracftice is one which deserves to be universally adopted. Conjugate Foci. — The distance between the ground glass and the lens when any particular objedl is sharply defined varies according to the degree of separation between that objecfl and the lens; i,e,^ having ascertained the solar focus, or, as it is called, the principal focus, by getting a sharp image of the sun, we find on attempting to focus objecfts situated in planes nearer to the lens that the separation between the lens and screen becomes greater, or the focus longer, the distance increasing in proportion as the objecfl focussed approaches the lens. These distances are termed conjugate foci." The diagram (fig. 3) may help to make the matter clearer. A is a lens, B B parallel rays proceeding from an objecSl situate at infinity (^.^., PHOTOGRAPHIC LENSES. 15 the sun's image), which come to a focus at C and form the solar or principal focus. This is an arbitrary or fixed distance, and not subjedl to variation. The image of an objedl nearer to the lens — for example, at D — comes to a focus at the point E behind the principal focus, C. These are the conjugate foci of that particular lens, the distance between the objecSl, D, and the lens. A, being spoken of as the major conjugate, and the distance between the sharp image and the lens, the minor conjugate. In an elementary handbook Fig. 3. it is unnecessary to go further into the laws of conjugate foci. Convenient tables for estimating the same have been published, and Mr. Traill Taylor deals lucidly with the subjecft in The Optics of Photography." The pracSlical application of the law of conjugate foci and its importance to the photo- grapher is demonstrated when copying or enlarging, and in the use of a hand-camera. For example, a photographer has to copy or enlarge a certain photograph to twice its original size, and desires l6 PHOTOGRAPHIC LENSES. to know at what distance from his lens the original should be placed. This, of course, he can do in a roundabout way, occupying more or less time, by trial and error, but knowing the true equivalent focus of his lens {q. v,) he can at once, by a simple arithmetical calculation, find the distance at which the subje(ft to be copied should be placed from the lens, and the degree of extension to which it will be necessary to rack the camera out in order to obtain a sharp focus. The rule is simple. Add one to the number of times the photograph is to be enlarged or reduced, and multiply by the equivalent focus of the lens. This gives the distance of the objecfl: from the lens. To find the distance of the ground glass from the lens, divide the equivalent focal length of the latter by the number of times it is proposed to enlarge the original, and add it to the equivalent focal length. In regard to hand-camera work, every pra(5lical photographer when he marks the varying points at which certain objecfts at different distances are in sharp focus upon his camera, and to which he adjusts the lens for each exposure, is unconsciously applying the principles of conjugate foci. CHAPTER III. DEFINITION OF TERMS. CONTINUED. DEFINITION, DEPTH OF FOCUS MORE DEPTH IN SHORT THAN LONG FOCUS LENSES — ADVANTAGE OF THE FORMER FOR HAND-CAMERA WORK ILLUSTRA- TIONS PROVING THE ADVANTAGE IN DEPTH OF DEFINITION OF SHORT FOCUS LENSES EFFECT OF STOPPING DOWN ON DEPTH OF FOCUS. Definition. — The defining power of a lens, or its capacity of giving a sharp image on the focussing screen, is affedled by various considera- tions : i.e.^ the type of lens, the care bestowed upon its construdlion, its aperture, and the manner in which it is employed, etc. It is essential from a technical, and certainly from a pracftical point of view that the definition of a photograph should be such as to bear critical examination without detecfting any wooUiness or diffusion in that part of the image which has been sharply focussed, putting aside for the moment the desirability of introducing diffusion for picftorial purposes. Before condemning any lens for faulty definition, it should be ascertained whether the ground glass screen and the dark slide are in accurate register, for it is l8 PHOTOGRAPHIC LENSES. essential that each when in position should occupy the same plane. Depth of Definition. — Stridtly speaking, and from a theoretical point of view, a lens has no depth" of focus, a focus being, as we have already seen, a point. In investigating the principles of conjugate foci, we found that this point varied according to the distance of the objecfl focussed from the lens ; therefore it is evident that when we focus a landscape having objecSls situated in different planes, we shall not be able to get all their images equally sharp upon the screen at the same time. Pradlically, however, it will be found that lenses possess more or less what is popularly termed depth of focus," the amount depending upon two facftors, first the focal length of the lens, and secondly its aperture. The shorter the focus of the lens, the greater the depth of definition, hence one advantage in using short focus lenses for hand-camera work. Depth of focus is increased, or in other words, objedls situated at varying distances are more sharply rendered, by reducing the aperture of the lens, or, as it is familiarly called, stopping down." This, however, will be at the expense of rapidity, for the more the aperture is reduced the longer will be the exposure necessary. Stops have other funclions which will be considered hereafter (see Chapter V., **The Diaphragm or Stop)." CHAPTER IV. VARIOUS DEFECTS IN LENSES. SPHERICAL ABERRATION ITS CAUSE EFFECT UPON THE IMAGE — CURVATURE OF FIELD CAUSE OF CURVED PLATES OR FILMS LENSES MOST AFFECTED CURVATURE DISTINCT FROM SPHERICAL ABERRA- TION DISTORTION CAUSE OF LENSES PRODUCING SAME — ANOTHER CAUSE EFFECT OF THE DIAPHRAGM BARREL SHAPED DISTORTION — PIN-CUSHION DIS- TORTION HOW ELIMINATED BY THE OPTICIAN. Spherical Aberration. — We have already referred to chromatic aberration, but there exists, more or less pronounced, in all lenses an aberration which is produced by their form, and called spherical." It may be defined as the property inherent in lenses of unequally refracfting rays of light passing through different parts of their surfaces, and thereby failing to bring them to a common focus. To make this clear we must refer to fig. 4. The rays represented by the dotted lines A A which pass through the margin of the lens are more deflecfted, and consequently brought to a focus at a point (B) nearer the lens than are the axial rays, C C, passing through its centre, 20 PHOTOGRAPHIC LENSES. which find their foci at the point D. Spherical aberration is of two kinds — positive and negative — and the fault is correcffced by the optician by adjusting the curves of the lens and employing suitable glass in its constru(ftion. The fault shows itself in confusion of image, and by an inability to secure a sharp focus. It may be lessened by inserting smaller diaphragms, although at the expense of rapidity. It is more often met with in a pronounced form in single lenses of large angular aperture (when it is always more or less present) Fig. 4. and in imperfecftly construcfled portrait lenses than in the rapid doublet. Curvature of Field. — If the central portion of an image produced by a lens with its full aperture be sharply focussed on the ground glass, the margins of the pidlure will be more or less indistincfl: and out of focus. If it is sought to remedy this by racking the lens nearer the screen, the marginal portions of the picSlure become sharper, but at the expense of the central definition, which will then be found to be as much out of focus as the margins PHOTOGRAPHIC LENSES. 21 were in the first instance. This defecfl is called curvature of field," and is due to the fa(5l that the image produced by a lens is not formed on a plane or flat surface, but on one that follows the curves of the lens — namely, a portion of a sphere. In fig. 5 A is a lens, the line B representing the central rays coming to a sharp focus at the point C, the lateral rays D D' coming to a locus at E E' the line F F' showing the curved field of the lens. The defecfl would be less noticeable if we were to use curved glass or paper supports for the sensitive emulsion. Such supports have been successfully used, but the pradlical difficulties of so working 22 PHOTOGRAPHIC LENSES. were found to be too great. Curvature of field is reduced by stopping down the lens. It exists in certain types of lenses, notably in single lenses of cheap make, to a greater extent than in others ; but in certain lenses recently introduced, and construcfted of the new optical glass, it has been almost eliminated. A single lens suffering from this defeat may have its field flattened by placing the diaphragm at a greater distance from the lens, Fig. 6. but this treatment will lessen the angle covered, and may induce flare" {q. v,) Curvature of the field must not be confounded with spherical aberration, which causes a general diffusion of focus, no portion of the image appearing sharp. Distortion. — Distortion, as seen in photo- graphs, may be the result of some inherent property in the lens which causes it to distort any straight line occurring in the objecfh photographed. PHOTOGRAPHIC LENSES. 23 Such distortion is produced by the single lens, some portrait lenses, and imperfecSlly construdled doublets. It is also produced with any lens when the sensitive plate is put in any other position than the vertical, or is not kept parallel with the building or objedl to be copied. The latter condition of things is remedied by a proper use of the swing back, in the chapter devoted to which the matter is fully explained. Fig. 7. The distortion produced by a single lens, i.e.^ curvilinear distortion," is clearly shown in figs. 6, 7 and 8. Fig. 6 is a diagram built up of a series of re(ftangles taken with a non-distorting doublet, and, consequently, truthfully reproduced. Fig. 7 represents the same diagram taken with a single achromatic lens, and it will be observed that all the lines except the central ones are curved or distorted, the effecft being most noticeable at the c 24 PHOTOGRAPHIC LENSES. margins. We have already seen that rays passing through the margin of the lens suffer most refradlion, the bending being less as the centre is approached, those passing through the axis emerging without refracftion, and it is this bending of the image- bearing rays that produces distortion. The position of the stop or diaphragm influences the nature of the distortion, it being barrel shaped when the stop is in its usual position in front of the lens, and of an opposite nature, usually referred to as pin-cushion," when the stop is placed behind the lens (fig. 8). Distortion in a single lens is reduced by placing the diaphragm quite close to the lens, but curvature of the field is then introduced (see curvature of field). The optician eliminates distortion by placing a lens on each side of the diaphragm, the distortion produced by one lens being neutralised by the other. All Fig. 8. PHOTOGRAPHIC LENSES. 25 non-distorting doublets are construdled on this principle. Another form of apparent distortion, unfor- tunately not uncommon, is due to exaggerated perspecSlive, caused by using lenses of short focus, and, consequently, selecfting a point of sight too near the subje(5l. The evils arising from this mode of procedure, and their remedy, are fully discussed elsewhere. CHAPTER V. THE DIAPHRAGM, OR STOP, AND ITS FUNCTIONS. THE IRIS DIAPHRAGM, ITS ADVANTAGES — DIAPHRAGMS WEARING BRIGHT FLARE USE OF SMALL STOPS A POPULAR FALLACY EXPLAINED FUNCTIONS OF THE DIAPHRAGM — EFFECT OF STOPPING DOWN " DEPTH OF FOCUS AMOUNT OF DEPTH" DEPENDENT ON FOCAL LENGTH AND APERTURE — "FIXED FOCUS " GREATEST DEPTH IN SHORT FOCUS LENSES ANOTHER POPULAR FALLACY AN EXAMPLE RAPIDITY — LOSS OF LIGHT BY REFLECTION VALUEOF STOPS f/x SYSTEM HOW TO MEASURE THE DIAMETER OF STOPS COMPARING RAPIDITY OF LENSES THE UNIFORM SYSTEM A DIAPHRAGM NOT THE EQUIVALENT OF A STOP. All modern lenses are furnished with either Waterhouse or revolving diaphragms, or with the Iris diaphragm. The latter is a great improvement, and for several reasons is to be preferred. It is far more easy to adjust, and being permanent!}^ attached to the lens mount cannot be mislaid or lost, both of which contingencies frequently occur when Waterhouse diaphragms are employed. A PHOTOGRAPHIC LENSES. 27 third and special advantage possessed by the Iris is the absence of the slot in the lens tube, through which, in the case of the Waterhouse or revolving diaphragms, light often passes and produces a slight veiling or fogging of the image It has been suggested that the fri(5lion caused by the rubbing together of the plates or leaves composing the Iris diaphragm would cause the metal to wear bright, and so induce flare. There is pracflically very little fear of any such danger occurring. The only light that could produce such a result would be that refle(5led from the posterior, or back combination of the lens, and the amount would in the majority of cases be too small to produce any really injurious result. These remarks apply more particularly to symmetrical doublets ; in the case of a portrait lens, which is more susceptible to flare, the danger might be greater. The statement is frequently to be met with in text books on photography that the use of small stops produces flatness and want of contrast. Although theory may point to this conclusion, pradlice does not confirm it. Provided the exposure is sufficiently increased the use of small stops cannot affedl the gradation or density of the negative. Pi(5lorially we may diminish, or even destroy, the effedl of atmosphere in a photograph by unduly stopping down the lens, and thereby 28 PHOTOGRAPHIC LENSES. bringing both near and distant planes into equally sharp focus, but provided we give a sufficient increase of exposure, we cannot lessen the contrast in the picflure by any amount of stopping down. That can only be modified by exposure and development. Now what are the fundlions of a diaphragm or stop ? Its pradlical effedl upon the image, as will be apparent to the merest tyro, is to increase its sharpness ; but the more we decrease the size of the stop, the more light we cut off from the plate, consequently the lens becomes slower and we have to increase the exposure. A stop or diaphragm improves the definition of a single lens by correcfting its excessive spherical aberration. In the case of an aplanatic doublet, however, spherical aberration is pracflically eliminated in the construction of the lens, and stopping down in this case merely serves to extend the area of sharp definition and to increase the depth of focus. Depth of Focus. — Scientifically considered, a focus is a point, therefore ^* depth of focus" cannot exist. Pradlically, however, the matter may be differently regarded, and thus photographers are in the habit of speaking of the depth of focus,** or depth of definition," possessed by certain lenses. If we focus one plane of the subjedl: sharply, using the largest diaphragm or full opening of the lens, all other objecfts which PHOTOGRAPHIC LENSES. 29 are either nearer to, or farther from, the objedl focussed will appear more or less wanting in sharpness. The amount of depth" of focus possessed by any lens, whatever be its construcflion, depends solely upon two things : first, its foca^ length ; and, secondly, its aperture. The shorter the focal length of a lens, the greater its so-called depth of focus. This is one of the most important fadlors to be considered in selecfling a lens for rapid hand-camera work, and pracSlically limits the photographer to the use of small plates and short focus lenses. The term fixed focus," which is sometimes employed in describing hand-camera lenses, in the sense in which it is intended to be understood is equally unscientific and misleading ; because, as we have already seen, the focus, or distance of the ground glass from the lens, will vary according to whether the objedl is near to, or far from, the lens at the moment of focussing. There is a point, however, beyond which everything will be in picflorially good definition, but the longer the focus of the lens used, the further will the point beyond which everything is in sharp focus be removed from the camera. Mathematically speaking, the amount of depth possessed by a lens varies inversely as the square of its focus. For example, a lens of 6 inches focus will have four times the depth of one of 12 inches focus. This, of course, 30 PHOTOGRAPHIC LENSES. entirely dissipates the notion frequently entertained by photographers that some lenses have more depth than others of a different type but of the same focal length and aperture. It is the focal length and the aperture, apart altogether from construdlion, that determines the amount of depth possessed by any lens. For example, a rapid recftilinear lens and a wide-angle rectilinear lens, each of five inches focus, when stopped to //i6 possess identically the same amount of depth. The advantage of the recflilinear is found in the fadl that it will work, if necessary, with a larger stop, and, consequently, with increased rapidity, but as the rapidity is increased so will the amount of depth be diminished. The table on opposite page, taken from one of the American photographic journals, shows at a glance the distance in feet beyond which everything will be sharply defined when using lenses of focal lengths varying from two to fifteen inches, and with apertures of from //5 to//6o. Rapidity. — The rapidity of a lens is deter- mined by its aperture in relation to its focal length. It is also influenced by the number of reflecfling surfaces contained in the lens. The whole of the light passing through a doublet does not reach the plate, some of it being reflecfted back and lost. It is for this reason that with similar sized diaphragms single lenses are quicker than PHOTOGRAPHIC LENSES. 31 doublets. For pradlical purposes, however, the amount of light thus lost is so small that it may be disregarded. The rapidity of a lens is also afFedled by the charadler and quality of the glass used in its construdlion, but, pradlically, this may also be left out of consideration. For convenience in estimating and comparing the relative rapidities of different lenses, the size, or diameter, of the Equiv. focus f/5 f/10 15 f/20 f/30 f/60 3 inches 15 8 5 4 3 I* 4 >. 27 14 9 7 5 2* 5 » 46 21 14 II 7i 4 6 „ 60 30 20 i5i 10^ 5i 7 „ 82 42 27 21 7i 8 „ 107 54 36 27 19 10 9 „ 137 68 45 34 23 12 10 ,. 167 84 56 42 30 15 11 „ 202 lOI 67 51 37 18 12 „ 241 121 80 61 41 21 opening in the diaphragms, instead of being given in lineal measurement, is expressed in terms showing the relation of the diameter of the aperture to the focal length of the lens. This is called the f/x system, and the diaphragms of all modern lenses of good construdlion are so marked. To find the value of the stop in this way the 32 PHOTOGRAPHIC LENSES. equivalent focus of the lens must first be ascer- tained by the method given in Chapter II. The diameter of the opening in the stop having been accurately measured, dividing the focal length by the diameter of the stop will then give the fjx, or intensity ratio of that stop. For example, supposing the equivalent focus of the lens be eight inches, and the diameter of the stop one inch : 8 i = 8, giving //8 as the intensity ratio of that stop. Therefore, all lenses, irrespecflive of focal length, when used with similar apertures, work with identically the same amount of rapidity. For example, an extra rapid euryscope stopped down to //i6 is then no more rapid than a wide- angle redlilinear working at the same aperture. In order to simplify the correcft estimation of exposure with stops of different sizes, the Photo- graphic Society of Great Britain a few years ago introduced the uniform system" of numbering, which, used in conjuncftion with the fix numbers, indicates the relative increase of exposure required with each sized stop. The table opposite shows first, the intensity ratio, or apertures of the stops, ranging from //4 to 7/64 ; and, secondly, the uniform system, or relative exposure ratio. The openings in the stops are so calculated that each one requires double the exposure of the one preceding it. PHOTOGRAPHIC LENSES. 33 U. S. No. Intensity Ratio. U. S. No. 32. 64 128 . 256. Intensity Ratio. 4 8 16 2 I //22-6 //32 It is right here to state that although we have followed the ordinary custom of photographers in referring to diaphragms and stops" as being synonymous terms, it is not, stridlly speaking, correcfl so to do; **stop," in the language of the optician having a different meaning to ^'diaphragm," the former being placed in contacft with the lens, and the latter at some distance from it. SECTION II. A DESCRIPTION OF PHOTOGRAPHIC LENSES. CHAPTER VI. SINGLE LENSES. SINGLE LENSES — EARLY TYPES — FORMS OF SIMPLE LENSES — CONVERGENT LENSES DIVERGENT DITTO MENISCUS LENS PLANO-CONVEX GRUBB's " AP- LANATIC" SINGLE LENS J. H. DALLMEYER's LAND- SCAPE LENS T. R. DALLMEYER's RECTILINEAR LANDSCAPE LENS. A single lens construdled for photographic purposes, although apparently composed of one piece of glass, really consists of two, the inner surfaces being cemented together with Canada balsam. This is done in order to render the lens achromatic ( see '^achromatism,'" ) The earliest lenses of this type, such as those first used by Daguerre, were not achromatised, and it was consequently necessary to make an adjustment after focussing in order to bring the plate in the plane of the chemical rays. All single or landscape lenses of modern construdlion are properly achromatised, and the image produced by them when sharply focussed needs no readjustment. The simple lenses of which all compound lenses are construcSled may be divided into two 36 PHOTOGRAPHIC LENSES. classes : convergent lenses, which condense and bring to a focus light passing through them ; and divergent lenses, which disperse it. In fig. 9, i, 2 and 3 are examples of the former, i being a bi-convex, 2 a plano-convex, and 3 a converging meniscus. 4, 5 and 6 are divergent lenses ; 4, a bi-concave, 5, a plano-concave, and 6, a diverging meniscus. For the purposes of photography these lenses are construdled of glass of different densities, usually various kinds of flint and crown, and then combined in order to render them achromatic. 7, 1234 56 Fig. 9. 8 and 9 in fig. 10 are examples of how this may be effedled. 7 is a meniscus combination formed of a bi-convex cemented to a bi-concave ; 8, a plano- convex formed of a bi-convex cemented to a plano-concave ; 9 being a divergent lens consisting of a plano-convex combined with a bi-concave. The earliest type of single lens took the form shown at 7 in fig. 10, being what is termed a meniscus, that is to say, convex on one external surface and concave on the other. It was also PHOTOGRAPHIC LENSES. 37 sometimes constru(5led as shown at 8 in the same diagram, where one surface is convex and the other plane, such lenses being styled plano-convex. Both of these, slightly modified, are in use at the present day. The very cheap French single lenses are generally of the plano-convex type, the better class being meniscus in form. The latter are generally to be preferred, the former requiring a smaller stop and covering a more limited field. 789 Fig. 10. Grubb was the first optician who, by altering the construcftion of the single lens, succeeded in producing an improved instrument. He called his lens, which was meniscus in form (fig. 11), the patent aplanatic," but although it undoubtedly gave a sharper image with a larger aperture than that previously found in single lenses, yet it could not, stricftly speaking, be called an aplanatic lens, inasmuch as with an aperture larger than //13 38 PHOTOGRAPHIC LENSES. marked spherical aberration was present. He also increased the portability of the mount by making the portion carrying the diaphragm telescope into that carrying the lens. J. H. Dahmeyer in 1865 introduced a further modification of the meniscus lens by constru(5ling it of three instead of two elements, a concavo- convex flint lens being placed between two crown meniscus lenses. This enabled a very much larger angle of view to be covered, and also enhanced the defining power of the lens. On this model many modern wide-angle single lenses of the present day are construcfted. n n (I u u Fig. II. PHOTOGRAPHIC LENSES. T. R. Dallmeyer in 1887 brought out a lens which possessed decidedly novel features. This he called a *'re(5lilinear landscape lens," it being, as its name implies, a single lens possessing the property of preserving the redlilinearity of any straight lines that may exist in the objedl photographed. The component parts of this lens are somewhat similar to the wide-angle landscape lens already referred to, but they are differently placed, the position of one of the crown glass elements being reversed, so that an air space is left between it and the other elements of the lens. It possesses the property of giving a very softly defined image with a comparatively large aperture, and when so used is a valuable instrument in the hands of those who objecft to wiry definition in their photographs, although when stopped down any desired amount of definition can be obtained. D CHAPTER VII. UPON THE PROPERTIES AND USE OF SINGLE LENSES. PROPERTIES OF SINGLE LENSES ECONOMY SINGLE LENSES versus DOUBLETS DISTORTION PRODUCED BY SINGLE LENSES CONDITIONS INFLUENCING DIS- TORTION SINGLE LENSES FOR ARCHITECTURAL SUB- JECTS SLOWNESS OF SINGLE LENSES SINGLE LENSES WITH LARGE APERTURES WHY UNSATIS- FACTORY ABSENCE OF REFLECTING SURFACES BRILLIANCY OF IMAGE PRODUCED BY SINGLE LENS SINGLE LENSES WITH LARGE APERTURES GIVE SOFT IMAGES — SUITABILITY OF SINGLE LENSES FOR SPECIAL PURPOSES. A single achromatic lens, even when of good -quality, can be manufacftured and sold at a much lower price than any form of doublet, which is a matter of importance to a great many photographers. The question, therefore, is frequently asked, will not a single lens answer my purpose equally as well as the more expensive doublet ? " The answer to be given must always depend upon the kind of work which is in contemplation. In the first place, .all single lenses produce a barrel shaped distortion PHOTOGRAPHIC LENSES. 41 of any straight lines existing in the objecfl photo- graphed, whether it be a building, a diagram, or a plan (see distortion). This is most pronounced at the margin of the field, and becomes less and less noticeable as the centre is approached, hence it follows that in some cases by utilising only the centre of the field, the amount of distortion may be so slight as to be hardly perceptible. But in such cases the angle included will necessarily be small, whereas when photographing architecftural subjecSls an angle more or less large is generally imperative in order to include the whole of the subjecft. As a pracftical illustration of what is meant, the back combination of a half-plate recftilinear (which will be found to have a focus about double that of the lens when used in its complete form) will reproduce an architecSlural subjedl without perceptible distortion, although if the same lens be used on the full size plate that it will cover (probably a 10x8 or 12 x 10) then will curvilinear distortion, more or less pronounced according to the position of the diaphragm, be at once apparent. The second disadvantage of the single lens is its slowness when compared with the rapid re(5lilinear. It is true that single lenses are now issued by some opticians with apertures as large as //8, but the amount of spherical aberration (q, V.) then present is so great as to preclude the 42 PHOTOGRAPHIC LENSES. possibility of obtaining an optically sharp image in any portion of the field. For special purposes, such as the producftion of large diredl portraits, a single lens so used may possess valuable qualities from an artistic and picftorial point of view, and these will be found fully dealt with in the chapter devoted to portraiture. For ordinary purposes an aperture of at least //ii, and preferably //i6, must be employed before anything like critical sharpness over a moderate angle can be obtained. On the other hand, on account of their being but two reflecfting surfaces, instead of four, the single lens produces an image of greater brilliancy than the doublet, there being less refledled light. In regard to the latter point, although some experts strongly insist upon its importance, the writer, after many comparative tests, is inclined to think that the superior brilliancy of photographs taken with a single lens has, from a pra(fl:ical point of view, been somewhat over-rated ; indeed, under ordinary conditions, it would puzzle a skilled photographer to discriminate between landscape photographs taken respedlively with single and doublet lenses. When used with fairly large apertures — -//16 to //20 — the single lens gives beautifully soft definition, with an absence of that wiry sharpness in the plane focussed which is generally found with lenses of the doublet type ; PHOTOGRAPHIC LENSES. 43 whilst with smaller stops even microscopic definition may be obtained. To briefly recapitulate what has been said, the single lens will answer as well, or better than any other form of lens for pure landscape, or landscape and figure subje(5ls,for architedlural work under the special conditions referred to, and also for portraiture where extreme rapidity of exposure is not imperative. On the other hand, if such work as copying diagrams, plans, etc., or the photographing of architecfture, either exterior and interior, is to be attempted, then, for the reasons given, the single lens will be found less satisfacftory than a non-distorting doublet. CHAPTER VIII. THE RAPID RECTILINEAR, OR NON-DISTORTING DOUBLET. steinheil's rapid aplanat " — dallmeyer's RAPID rectilinear" CONSTRUCTION OF SAME JENA GLASS EURYSCOPES ADVANTAGES OF THE RAPID RECTILINEAR — AN ALL-ROUND " LENS. It is not proposed, nor would it serve any useful purpose in a pradlical handbook, to attempt to trace the history of the evolution of the doublet. From its first inception, about the year 1840, down to the present time, it has undergone many modifications and alterations. Prior to 1866, when Steinheil introduced his rapid aplanat working at //7, all doublet lenses required comparatively small apertures in order to obtain sufficient definition and flatness of field for pradlical purposes. Although Steinheil is generally — and rightly — regarded as the first introducer of a lens of the rapid type, Dallmeyer, working independently, about the same time produced his celebrated "rapid recftilinear." The Steinheil lens may be taken as the type or model upon which the con- strudlion of all modern rapid lenses is based, with PHOTOGRAPHIC LENSES. 45 the exception of some recently introduced lenses constru(5led of the celebrated Jena glass. Different makers, for reasons which can be appreciated, call their lenses by various names, such as rapid symmetrical, rapid redliilinear, panorthoscopic, ortho-panadlinic, etc., but subjedl: to certain slight modifications in the nature of the glass used and the curves employed, the general principle of construdlion is in all cases the same. Fig. 12, Fig, 12. which is a sedlional diagram, clearly shows the construdlion of such a lens. It will be seen to consist of two achromatic single lenses mounted in a tube with their concave surfaces facing each other. It was corredtly designated by Steinheil an aplanatic " lens, in that it will give a sharply defined image with its full aperture, which varies between //7 and//8. The introdudlion of the new glass from the Jena fa(5lory has enabled opticians to 46 PHOTOGRAPHIC LENSES. construcft very perfecSl lenses of this type having apertures so large as //5, and in some cases even //4. Such lenses are generally styled euryscopes, extra rapid recftilinears, or universals. Where only one lens can be afforded, a rapid recftilinear is undoubtedly the most useful. It has, indeed, a wider sphere of usefulness than any other lens. It is sufficiently quick, with a rapid plate, to give an excellent portrait in the studio, and though its rapidity is much less than that of the portrait lens, its depth of focus and covering power are far greater. It answers well for architecftural work where only a moderate angle is required, and it produces no distortion. For so-called instantaneous work it is unequalled, good pidlorial definition being obtainable with < omparatively large apertures. For rapid ex- posures it should not be strained to the full extent of its covering powers. It should be used on a smaller plate than it is intended to cover, the illumination rapidly falling off towards the corners of the plate. CHAPTER IX. OTHER FORMS OF THE DOUBLET, INCLUDING WIDE-ANGLE LENSES. EARLY TYPES HISTORICAL REFERENCES ROSS'S ACTINIC DOUBLETS WIDE, MEDIUM, AND NARROW ANGLE DALLMEYER'S WIDE-ANGLE RECTILINEAR ADVANTAGES OF WIDE-ANGLE LENSES STEINHEIL's WIDE-ANGLE APLANAT THE PORTABLE SYMMETRICAL A FAVOURITE LENS ENLARGING ITS APERTURE — ALTERATION OF FOCUS BY INTERCHANGING COMBINATION — RISK OF SO DOING. Avoiding reference to objecflives, which having •ceased to be construdled commercially, now possess only an historical interest, we may pass by Sutton's panoramic lens, Harrison's globe lens, Steinheil's periscope, and others, taking the reader down to 1864, when the late Thomas Ross introduced his adlinic doublets. They were issued in three series, giving respecftively wide, medium, and narrow angles. The latter was intended for rapid work, and is now superseded by the rapid symmetrical. The former gave an extremely wide angle (100^ in some cases), and is still copied by some makers of foreign lenses. The 48 PHOTOGRAPHIC LENSES. medium-angle doublet was also very much esteemed. It worked with an aperture of about //15, and gave a soft and evenly defined image. These lenses are occasionally to be met with second-hand, and either the wide or medium angle may with confidence be purchased if obtainable at a moderate price, some old photographers preferring them to more modern instruments. We would not say they are better, but they are, at all events, quite equal to many of recent const rucftion. Fig. 13. In 1866 Dallmeyer introduced his wide-angle re(flilinear (fig. 13), one special feature of which is the smaller diameter of the posterior, or back combination, a modification probably introduced to reduce the danger from flare " {q» v.) These lenses work at a maximum aperture of f/16 in the larger sizes, the smaller ones being slightly more rapid. For all purposes where an extreme angle of view is desirable they are exceedingly useful lenses, and, like all good things, they have been copied by nearly every maker, English and foreign. PHOTOGRAPHIC LENSES. 49 Steinheil subsequently issued a wide-angle aplanat, the construdlion of which was similar to his rapid aplanat. The diameter of this lens was exceedingly small, and the combinations were very thick and placed quite close together, merely allowing sufficient space tor the insertion of a diaphragm. Similar lenses were afterwards manufa(5lured by Ross under the name of portable symmetrical (fig. 14). The diameter of the combinations, even in the larger sizes, being Fig. 14. small, the entire series, varying in focal length from 3 to 21 inches, are all made to fit into mounts of similar size, and screw into the same flange. This type of lens has always been a favourite with the writer, for it gives wonderful definition over a large angle. As issued by Ross, it works at a maximum aperture of//i6. The makers, presum- ably, set up a high standard of definition, for the writer has found that the smaller sizes will cover a more limited angle with great sharpness with an 50 PHOTOGRAPHIC LENSES. aperture so large as //ii, and used in this way, one of these lenses of five inch focus does him excellent service as a hand-camera lens. Another advantage of this type of lens is that by interchanging the combinations, various focal lengths can be obtained. For instance, supposing the reader to be possessed of two such lenses of four and six inches focus respedlively, by using the front combination of the six inch and the back combination of the four inch a lens of intermediate focal length — namely, five inches — can be obtained, a point of great utility in cases of emergency. It is, of course, not generally advisable to tamper with a lens, for the optician may be presumed to send it out in a form that will give the user the best results ; but in this case no harm is done to either obje(5live, and the advantage of an additional focal length is secured, with pradliically little or no loss of optical quality, except that perhaps the available field will be slightly less than would be obtained with a lens of similar focus used intadl. The portable symmetrical is one of the most useful of all modern objecSlives where great rapidity is not essential, and it is now made by nearly all the leading English opticians. CHAPTER X. PORTRAIT AND UNIVERSAL LENSES. THE PORTRAIT LENS ITS USEFULNESS NOT AN "all-round" lens CONSTRUCTION SPECIAL FEATURES RAPIDITY ANGLE PETZVAL's ORIGINAL PORTRAIT LENS ARTISTIC PORTRAITURE DALL- MEYER's portrait LENS DIFFUSION OF FOCUS HOW OBTAINED PROPERTIES OF THE PORTRAIT LENS ITS UNSUITABILITY FOR WORK OTHER THAN POR- TRAITURE NOT A GROUP LENS UNIVERSAL LENSES THEIR CONSTRUCTION INTENSITY CAPABILITIES TABLE SHOWING INTENSITIES OF VARIOUS FORMS OF LENSES. Regarded from an optical point of view, the portrait lens (fig. 15) may truly be called a marvel of the optician's skill, for without it studio work in the pre-gelatine days would have been pradlically impossible, except in the best of light and under the most favourable conditions. It is, however, outside its special sphere, perhaps the least useful all-round lens extant. Before, however, referring to its defe(5ls, it will be well to notice in what respecfts it differs in construcftion from other lenses. 52 PHOTOGRAPHIC LENSES. It consists of two combinations : the front a plano-convex achromatic lens, cemented ; the back combination consisting of a bi-convex lens and a concavo-convex lens, not cemented, but so mounted as to leave an air space between the lenses. The amount of separation between the front and back A Fig. 15- combination is greater than in any other objecflive, and the diameter of the lens is very large in proportion to its focal length. Its special feature, therefore, is rapidity, and in order to obtain the same in a high degree it has been found necessary to sacrifice other important qualities. The avail- PHOTOGRAPHIC LENSES. 53 able angle of view given by a portrait lens is extremely small, and its field exceedingly curved, rendering it pracftically useless for any purpose other than portraiture. The foregoing is a brief description of the original Petzval portrait lens introduced in 1841, and it is the model upon which the cheaper U SJ Fig. i6, foreign portrait lenses are still made. A great objedlion from an artistic point of view, in the opinion of many photographers, was the wiry, needle-point definition produced in the plane focussed by the portrait lens, the lack of definition in other planes appearing, by contrast, all the more pronounced. J. H, Dallmeyer direcfled his 54 PHOTOGRAPHIC LENSES. attention to this matter, and as the result of his investigations produced a lens in which any desired degree of diffusion could be obtained by slightly unscrewing the cell containing the back combina- tion, although when used intadl the acme of sharpness was obtainable. Fig. i6 shows the constru(5lion of Dallmeyer's lens. Although the front lens is similar in form to that previously figured, the back combination is totally dissimilar^ for not only are the lenses different in form, but their position is also reversed, the flint and crown changing places as it were. To produce the diffusion eSe6i, the cell containing the flint lens is unscrewed one or any part of a complete revolution, the degree of diffusion being dependent upon the amount of separation. The exacft effecft once obtained may always be repeated by noting the position of the index or pointer. It follows from what has been said about the properties of a portrait lens, that it should be used only for portraiture. It is not suitable for groups, indeed it is often a tax upon its powers to attempt to photograph a group of even two or three persons, unless a small stop be employed, which at once deprives the lens of its special feature^ namely, rapidity. Therefore, under such conditions a lens of the rapid or, preferably, universal type should be used. The term universal" has been applied to PHOTOGRAPHIC LENSES. 55 lenses which are really a modification of the portrait lens, and they are so named because they will serve either to take portraits and groups in the studio, or, with smaller stops, a landscape or building out of doors. Fig. 17 indicates its construdlion. The back combination in the lens shown is cemented, although some makers prefer to use a modification of the portrait back combination. Fig. 17. It usually works with an aperture of from //5 to //6, and has an available working angle midway between the portrait lens and the rapid re(5lilinear. Although not equal to the portrait lens in rapidity, it is superior to it in all other respedls. For groups in the studio and out of doors it is unsurpassed, and when stopped down it becomes a very good instrument for general out-door work, where only a moderate angle of view is required. E PHOTOGRAPHIC LENSES. Before dismissing this portion of the subjecfl, a brief summary of the comparative intensities of different forms of lenses in common use may be useful. The following are the intensities in tabulated form : — Portrait lens //4* Universal lens, recSlilinear portrait //5 Euryscope, extra rapid recftilinear //6 Rapid redlilinear //8 Single landscape lens (certain types) ... //lo Medium-angle recflilinear (including port- ables) fjii Wide-angle re(5lilinears //i6 *In certain special forms this is reduced to 7/3. CHAPTER XI. NEW TYPES OF LENSES CONSTRUCTED OF JENA GLASS. ABBE-SCHOTT GLASS RECENT IMPROVEMENTS EFFECTED BY ITS ADOPTION ROSS'S CONCENTRIC LENS ITS SPECIAL PROPERTIES ZEISS'S ANASTIG- MATS THEIR ADVANTAGES GOERZ's DOUBLE ANASTIGMATIC RIVAL CLAIMS COMPARATIVE AD- VANTAGES OF THE NEW CONSTRUCTION REDUCTION OF SPHERICAL ABERRATION AND ASTIGMATISM IMPROVED FLATNESS OF FIELD CONDITIONS UNDER WHICH THE IMPROVEMENT IS MOST MANIFEST AN UNJUST COMPARISON. The introdu(ftion, a few years ago, of the Abbe-Schott glass from the facflory at Jena opened up a new field of investigation and calculation to mathematicians, and, as subsequent invention has proved, marks a new era in photographic optics. At first great doubts were entertained as to the durability of certain varieties of the new glass, but carefully applied tests and experiments have led to the rejecflion of all but the more reliable samples. At the present time this new glass is not only used in the constru(5lion of certain entirely new types of 58 PHOTOGRAPHIC LENSES. lenses, but is also largely employed for rapid re(5lilinears, and other older forms of lenses, greatly to the improvement of their optical qualities. It is a disputed point among opticians as to whom the credit is due of first making practical use of the new power placed in their hands. Whatever may have been the result of laboratory experi- ments abroad, Ross's Concentric " (fig. i8) was the first lens commercially manufacftured in Eng- land to attradl the attention of photographers there. This remarkable lens derives its name from its peculiar construcflion, the inner and outer surfaces are struck from a common centre, and are therefore concentric. It is, comparatively, a slow lens, for although issued with a maximum aperture of //i6, it requires to be stopped down to //22 in order to obtain sharpness. The special property of the concentric is its wonderful flatness of field. The anastigmatic lenses of Zeiss, although entirely different in const rudlion, aim at securing Fig. i8. PHOTOGRAPHIC LENSES. 59 the same result, but it is claimed that they do so with the additional advantage of working at a larger aperture. They are issued in several series, which may be divided into two classes, rapid and Fig. 19. wide angle, the former with apertures varying from fly tof/g, the latter from f/12 to //18. Fig. 19 is an example of the rapid type, and fig. 20 of the wide angle. Like the Concentric these lenses Fig. 20. have a much flatter field than is found in any of the older forms, with the additional advantage of working at a larger aperture. In 1892 the Double Anastigmatic lens of 6o PHOTOGRAPHIC LENSES. Goerz was introduced (fig. 21). This without doubt is the most wonderful objecSlive of modern times. It is composed of two triple cemented lenses, either of which may be used alone as a single lens. It is claimed for the double anastig- mat that it works with an aperture of //y, and, with that intensity, gives greater flatness of field and width of angle than any existing form of lens. Its introduction, together with the claims put for- ward by its inventor, have given rise to a great Fig deal of controversy between the latter and those interested in rival lenses, but exhaustive tests, applied independently, appear to show that the special properties claimed have not been unduly exaggerated. If doubt existed upon this point, the fadl that Messrs. Ross & Co., who had already been appointed sole licensees for the con- strudlion of the Zeiss objecftives in the United Kingdom, have also obtained the sole right to PHOTOGRAPHIC LENSES. 6i manufacfture Goerz's anastigmatic, would go far to dispel it. The pracflical advantages which these new lenses possess may be briefly summarised. In the first place the definition, aperture for aperture, is finer, spherical aberration and astigmatism being more perfe(fl:ly corredled. But the most striking chara(5leristic when compared with older lenses, is the wonderful flatness of the field. This is most noticeable when photographing a flat surface, as for instance when copying a diagram or plan^ or taking the front elevation of a building, broad- side on, as it were. In reproducing such a subje(fl with an ordinary rapid re(5lilinear, or a wide angle redlilinear lens, every photographer knows how rapidly the definition falls away towards the margins of the plate, unless a very small stop is employed. Here then the superiority of the new lens is most marked, for with comparatively large apertures,// 1 6 to//20, such a subjedl is reproduced with almost critical definition to the margin. When, however, the subjedl photographed is not in itself flat, but is more or less curved, marginal objecfts being nearer to the lens than those more centrally situated (a condition of things obtaining when photographing a street scene or an avenue of trees), the advantage of a lens perfe6ily cor- redled for flatness of field is less evident. Indeed, the writer has heard these new lenses spoken of in 62 PHOTOGRAPHIC LENSES. disparaging terms by certain photographers who have failed to grasp the elementary truth, that a lens perfecftly corredled for flatness of field is only seen at its best when the objecfi: photographed is itself flat. That which at present is an impossi- bility in photographic optics, must not be expe(5led of these wonderful lenses. CHAPTER XII. ON CERTAIN OBSOLETE LENSES. THE OTHOSCOPIC, OR OTHOGRAPHIC LENS NOT A NON-DISTORTING LENS HARRISON'S GLOBE LENS ITS DEFECTS STEINHEIL's PERISCOPE DALL- MEYER's TRIPLE ACHROMATIC ITS ADVANTAGES EFFECT OF REMOVING CENTRAL LENS — SUITABILITY OF THE TRIPLET FOR COPYING. It is necessary to devote a chapter to certain lenses which although no longer manufacflured still occasionally fall into the hands of the photo- grapher, either by gift, or by purchase from a pawnbroker and from other sources. Taking them in order of date we have first the orthographic, or orthoscopic lens (fig. 22), as it was indifferently called. It was first made by Voightlander on calculations supplied by Petzval, the inventor of the portrait lens. It was originally introduced as a non-distorting lens, but the claim could not be substantiated, inasmuch as it pro- duced distortion of vertical lines in a building though in an opposite direction to that produced by the single lens, and of the chara(fter shown in 64 PHOTOGRAPHIC LENSES. fig. 9. It works with an aperture of about //lo, covering, however, only a Hmited area. With smaller stops the angle is greatly increased, an aperture of 7/32 giving an angle of about 70^. On account of the distortion produced by it, its sphere of usefulness is curtailed, leaving it only available for landscape subjedls and groups in the open air. It is interesting to note that this — the earliest of all doublets, excepting the portrait lens — was fitted by Harrison, of New York, with an iris diaphragm. The globe lens of the last mentioned optician, introduced in i860, deserves mention, as it is occasionally to be met with second-hand. Com- pared with more modern wide angle lenses it is of Fig. 22. PHOTOGRAPHIC LENSES. 65 little but historical interest by reason of the fa(5l that it suffers greatly from flare " and spherical aberration, two faults which put it beyond the category of useful instruments. Steinheirs periscopic, brought out in 1865, was the fore-runner of the rapid aplanat. It con- sisted of two symmetrical uncorredled meniscus lenses with a stop midway between the combinations. It was non-achromatic, and the focussing screen required to be adjusted after focussing. In i860, Dallmeyer introduced his well-known triple achromatic lens, now commonly referred to as the triplet.*' It is perhaps not right to include this lens among those that are defuncfl, inasmuch as the firm of Dallmeyer still include it in their catalogue, though I believe it is now only made to order. Being fully aware of its many excellent, and in some respects unique properties, the writer can only wonder that the demand for it is not greater, and that it ever went out of fashion. Many photographers, however, are alive to its advantages, and high prices are still paid for second - hand instruments in good condition. Other triple systems had been introduced prior to that of Dallmeyer, but his, produced on entirely independent lines, was the first to attain to any pronounced popularity. Ross, a little later, also brought out an actinic triplet, which again differed 66 PHOTOGRAPHIC LENSES. in construcflion from that of Dallmeyer. The construcftion of the triplet is clearly shown in the diagram (fig. 23). It is composed of two achromatic convergent meniscus lenses, between which is a third achromatic divergent meniscus, and close to the latter the stops are placed. It is fairly rapid, its maximum aperture being usually about //12 or//i3. With smaller stops the field is very flat and evenly lighted, the image, moreover, is entirely free from distortion. By removing the central dispersive lens, the focus of the combina- tion is shortened. This is sometimes recommended to be done on account of the gain in rapidity, which, however, is attained at the expense of all the other excellent qualities in the lens, for the field becomes very curved, astigmatism is pro- duced, and the extent of plate sharply covered is extremely small. For portrait or group work the lens should always be used intacfl. On the other hand if both the central and anterior, or front. Fig. 23. PHOTOGRAPHIC LENSES. 67 lenses be removed, the posterior or back lens alone being retained, a single view lens of rather long focus results, which for some purposes may- be useful. The triplet is a very fine copying lens, and is equally useful for enlarging, but in the latter case the position of the lens should be reversed, i,e,y the back combination should be next to the negative. SECTION III. THE CHOICE AND USE OF LENSES, ETC. CHAPTER XIII. UPON THE CHOICE OF A LENS. PRELIMINARY CONSIDERATIONS NO LENS EQUALLY SUITABLE FOR ALL KINDS OF WORK THE ONE- LENS " PHOTOGRAPHER A RAPID RECTILINEAR BEST A SECOND LENS ITS TYPE AND FOCUS INTER- CHANGING COMBINATIONS LENSES FOR STUDIO WORK AND PORTRAITURE ADVANTAGES OF THE PORTRAIT LENS FOR SMALL WORK ITS DISADVANT- AGES — WIRY DEFINITION DALLMEYER's DIFFUSION SYSTEM ADVANTAGE OF THE RECTILINEAR AS COM- PARED WITH THE PORTRAIT LENS LENSES FOR LARGE HEADS DIFFUSION FOCAL LENGTH AND APERTURE FOCUSSING EXAGGERATED PERSPEC- TIVE PRODUCED BY SHORT FOCUS LENSES — LENSES FOR LANDSCAPE WORK THE SINGLE LENS LENSES FOR ARCHITECTURAL WORK NON-DISTORTING LENSES ESSENTIAL VARIETY IN FOCAL LENGTHS ADVANTAGE OF RAPID LENSES FOR INTERIORS LENSES FOR COPYING POINTS TO BE CONSIDERED THE TRIPLET THE DOUBLE ANASTIGMAT LENSES FOR ENLARGING. The first point to be considered in chosing a lens is the kind of work intended to be done with 70 PHOTOGRAPHIC LENSES. it. No particular class of lens is equally well adapted to every kind of work ; recognising this fadl, opticians have endeavoured to supply photo- graphers with special forms of lenses construdled for particular and special purposes. For instance, in portraiture the acme of rapidity is, or rather was, required, to meet which the skill of the optician produced the portrait lens. Again, in architedlural work, the photographer usually wishes to include a great deal of subjedl from a near point of view, and in order to enable him to do so, wide angle lenses were introduced. Al- though each type of lens ma}^ be admirably adapted for its own special work, the finest results must not be expe61:ed when either is used for purposes other than those for which it was con- strudled. In order to assist the reader in making a proper seledlion, the different kinds of work with which he is likely to be confronted have been classified, and the objecflives which will be found most useful in each case specified. There is, however, one class of photographers^ and by no means a small one, to whom reference must be made, namely, those who seek to make one lens, as far as possible, do all their work. It is unnecessary to say that the finest results cannot, under such conditions, be produced. Elementary text-books in such cases usually advise the pur- chase of a lens of the rapid redlilinear type, and PHOTOGRAPHIC LENSES. 71 the advice is certainly sound, for probably no other form of lens can be put, with equally good effecft, to such a variety of purposes, and indeed, if a wide angle re(5lilinear can also be purchased, the range of utility covered by the two lenses when carefully used will be very great. The focus of the rapid lens should be about one and a half times the length of the longer side of the plate, and that of the wide angle about the length of the plate, or perhaps a little less ; a wider angle may occasionally be useful, but would require greater skill and judgment in using. The back combination of the wide angle would form an admirable single lens of slightly longer focus than the rapid lens used intadl, whilst the back com- bination of the last named would prove very useful as a long focus single lens for distant objecfls. In the event of the diameter of the cells and tubes of the two lenses being similar, addi- tional doublets might be formed, thus further increasing the number of focal lengths obtainable. (See Chapter XVIII.) To specify the lenses of any particular maker would be as unjust as it would be invidious, for of late years, probably as the result of increased competition and greater facilities for technical instrucflion, there has been a great levelling up, so to speak, in the optical industry, and although in the earlier days of photography the manufadlure of F 72 PHOTOGRAPHIC LENSES. really reliable lenses was confined to very few firms, yet the same can hardly be said of the present time, there being several firms of younger growth whose lenses are second to none in the perfecSlion of their optical qualities. Lenses for Studio Work and Portraiture. — For small work, up to cabinet size, the ordinary portrait lens cannot be surpassed, particularly for rapid exposures, as, for instance, when photo- graphing children, but for large diredl work the portrait lens is not so suitable owing to its lack of depth, and the wiry sharpness of the particular plane focussed. This defecft, however, is reduced in Dallmeyer's portrait lens, for by unscrewing the back combination, of which a certain amount of spherical aberration is introduced, producing the so-called effedl of diffusion, the acftual amount being determined by the extent to which the com- ponents of the back combination are separated. When the comparatively slow wet collodion pro- cess was in vogue, it was necessary to sacrifice a great deal in order to obtain the necessary rapidity, but the rapid plates of to-day permit of sufficiently short exposures being made with much smaller apertures, so that it is now quite possible in many cases to use a lens of the extra rapid rectilinear pattern in the studio. The advan- tages of so doing for large work are very great, the covering power and flatness of field PHOTOGRAPHIC LENSES. 73 of such lenses, even with full aperture, being, as compared with a portrait lens of similar focus, very marked. The adlual pradiical difference in rapidity between the two lenses is not so great, for although the portrait lens may possess a full aperture of //4, and that of the cemented or rapid doublet one of f/6 or f/y only, yet, even when the portrait lens is stopped down to the latter aperture its covering power will probably be inferior to its more modern rival. Further, the latter lens has the advantage of being the best that can be employed for groups, whether in the studio or out of doors. For the produdlion of large heads in the studio a single lens working at a large aperture cannot be surpassed. In portraits so produced, there is a softness of definition, and an absence of biting sharpness in any particular plane, that is difficult, if not impossible, to produce in any other way. The conditions under which these results may be obtained are fully dealt with in another chapter, and it is only necessary to observe that an experiment in this diredlion can be easily made at very little expense by those who are sceptical, by purchasing an ordinary cheap single lens of a focal length of about twice the length of the plate to be used, and if need be, opening out the diaphragm to about f/u, or even//8. Some care will be required in focussing, for with so large an 74 PHOTOGRAPHIC LENSES. aperture it is needless to say that critical sharp- ness in any portion of the field cannot be obtained, indeed, were it to be, the chief merit of the method would disappear. If evidence of the value of the single lens in portraiture were needed it is to be Fig. 24. found in the price lists of several noted opticians, who now catalogue such lenses specially manu- facSlured for the purpose. The subjedl of portrait lenses cannot be dis- PHOTOGRAPHIC LENSES. 75 missed without referring to a very important matter, namely, the focal length of the lens chosen. This is a point to which the majority of photo- graphers pay far too little attention. From motives of economy, and on account of the short- Fig. 25. ness of the studio, lenses of short focus are far too commonly employed, the result being, in many instances, the produdlion of an unpleasing effe(5l of exaggerated perspective, and sometimes abso- 76 PHOTOGRAPHIC LENSES. lute distortion. To this fadl: alone much of the alleged untruthfulness of photographic portraiture may be traced. Fig. 24 and fig. 25 are illustra- tions of the very different rendering to be obtained of the same sitter by using lenses of a different focal length. Fig. 24 was taken with a short focus lens, such as is commonly listed in opticians* catalogues to give a carte picfture. Note the exaggerated and unpleasant perspecftive of the hands and lower part of the body. Fig. 25, on the other hand, which was taken with a lens of longer focus, gives a pleasing and natural repre- sentation without any appearance of distortion. When large heads are taken under similar con- ditions, the results are still more displeasing, and acftual distortion of the features may result. Under no circumstances, where it can be avoided, should a lens of less than 7 inches focus be used for cartes, nor one less than 12 (and preferably 14) for cabinets. The additional expenditure involved in lengthening the studio and purchasing a more expensive objecftive, will be amply compensated for in the more pleasing and truthful results obtained. Lenses for Landscape Work. — For pure landscape the single lens is quite equal, and theoretically superior, to any doublet, on account of the lesser number of its refledling surfaces, although pradlically it might be difficult to prove PHOTOGRAPHIC LENSES. 77 its superiority by a comparison of results. It has, however, the advantage of being very much cheaper than a rectiUnear doublet of correspond- ing size, which is doubtless an important matter. For pure landscape purposes the reader may safely choose a lens of this type, or preferably a series of them, indeed, he may even use them, under conditions which are enumerated m the next paragraph, for archite(flural subjecSls without any obvious distortion resulting. Lenses for Architecfture. — For this class of work non-distorting doublets, either rapid or wide angle, must be used. If the best results are to be obtained, it is very necessary to have a variety of focal lengths, in order that the photo- grapher may be independent of his position, may be able to include upon his plate just the amount of subject required, without altering the position of his camera. For moderate or small angles the rapid redlilinear is very useful, par- ticularly for interior work, the comparatively greater brightness of the image rendering the operation of focussing easier than when a wide angle lens of smaller aperture is employed. Where a wide angle of from 70^ upwards is required a lens of the wide angle recflilinear type should be chosen, and its focal length should not be longer than two-thirds that of the plate used. A second lens with a focal length equal to the base of the 78 PHOTOGRAPHIC LENSES. plate, and a third (which might with advantage be of the rapid type) with a focal length equal to about one-and-a-half times the base of the plate, would form a battery that should enable the user to cope with almost any archite(5lural subjecft. In using lenses of the wide-angle redlilinear type, it should be noted that focussing with full aperture, and afterwards inserting a smaller stop, sometimes alters the plane of sharp focus, therefore, it is always advisable to have a final look at the focussing screen after stopping down, in order to make sure that the sharpness of the plane focussed for has not been disturbed. We have not, how- ever, noticed this property in lenses of the portable symmetrical type. Lenses for Copying. — If the subjecfl: to be copied contain straight lines, it is essential, in order to avoid distortion, that a non-distorting doublet be used. Almost any pattern or type may be chosen, but on account of convenience in focussing a rapid redlilinear has perhaps some advantages. In order to obtain critically fine definition it is advisable to seledl a lens constru(5led to cover a larger plate than that intended to be used, the field will then be flatter, a larger stop may be used, and the exposure consequently reduced. The triplet is an excellent lens for copying purposes, and if moderately stopped down gives a flat field and fine definition. It is at the PHOTOGRAPHIC LENSES. 79 present time largely used and greatly esteemed by those who have to do much copying and repro- duction work. Where definition of the highest possible order over an extended area is required, probably one of the new Goerz double-anastig- matic obje(5lives will give the best results. A description of these wonderful lenses is given in Chapter XL, to which the reader is referred. Lenses for Enlarging. — The rapid redli- linear is undoubtedly the best all-round lens for enlarging purposes, especially when daylight is used. For artificial light a portrait lens may be employed for the sake of its superior rapidity, but the marginal definition will generally be more or less defedl:ive. Single lenses are generally unsuit- able on account of their slowness, and the distor- tion which ensues when enlarging from a subjedl containing straight lines. CHAPTER XIV. THE CARE OF LENSES. WASHLEATHER BAGS EXTRA CAPS HINTS ON PACK- ING LENSES PARTITION CASES FITTING OF CAP place's EXPOSURE SHUTTER — ADAPTERS VerSUS SEPARATE FRONTS UNIVERSAL ADAPTERS THEIR DISADVANTAGES TAYLOR's ADAPTER NECESSITY FOR PRECISION IN TURNING THE THREADS OF ADAPTERS — ALUMINIUM LENS MOUNTS — ENHANCED LIGHTNESS DRAWBACKS PRECAUTIONS IN USE THE CARE OF LENSES CLEANING VARIOUS PRE- CAUTIONS INNER SURFACES MISPLACING LENSES. Having procured a good lens, care should be taken to keep it in good condition and to preserve it from injury. The best way to do so is to have a little bag made of soft washleather, padded with cotton wool, in which to keep each lens. It is a good plan, also, to have an extra cap to slip over the cell of the back combination when the latter protrudes beyond the flange. When several lenses are habitually used in the field, they should never be packed loose with the other apparatus, for the jolting of a vehicle or the constant vibration of a PHOTOGRAPHIC LENSES. 8l railway carriage may create sufficient fricflion to cause serious injury to the lens. A velvet lined leather case, with a separate partition for each lens, should be procured, which may either be carried in the camera case with the rest of the apparatus or slung over the shoulder in opera glass fashion. Speaking of lens caps, every lens should be provided with two in case of accidental loss, which may easily happen, especially if the cap fits, as it should do, loosely. With large lenses and light cameras it is essential that the cap should fit so loosely as to permit of it being taken off without communicating the least shake or tremor to the plate during exposure. The writer seldom uses a lens cap to make an exposure, preferring a blind shutter of the window type acSluated by pulling a cord, such as that sold by Place, of Birmingham. With it any exposure from the /oth part of a second downwards may be given without the slightest fear of shaking the camera. Where lenses of different sizes are used on the same camera, it will be found better to use ^'adapters" than to screw each lens on to its own flange in a separate front. What are called universal adapters" can now be obtained. They are constru(5led on the Iris diaphragm principle* but although the idea is ingenious and convenient, the ordinary form of adapter would appear to ensure greater safety, especially in the case of 82 PHOTOGRAPHIC LENSES. heavy lenses. The form of adapter made by Messrs. Taylor, Taylor & Hobson is shown in fig. 26. It possesses certain special and advantageous features, which entitle it to recognition. The screws on the lenses and on the adapter are so formed that by placing the lens at the arrow mark the threads engage at once, three turns only bringing the lens home. It is very necessary that the work of fitting several lenses to one flange Fig. 26. by means of adapters should be carried out with precision, for if the thread be in the least drunken" or untrue, the axis of the lens will be disturbed when it is screwed home, and really accurate work rendered impossible. There is a great rage at the present time in certain quarters for extreme lightness, and opticians have endeavoured to respond to this demand by substituting aluminium for brass in the construdlion PHOTOGRAPHIC LENSES. 83 of lens mounts. There is nothing to be said against aluminium as it is now used, though at first, owing to the difficulty of finding a suitable alloy, the excessive softness of the metal was an objecflion. With reasonable care, an aluminium-mounted lens should have as long an existence as one mounted in brass — the risk of fracfture, if dropped, would be about equal in either case. The thread of the screw in the case of the aluminium lens will doubtless become worn sooner than would be the case with the brass-mounted lens, and extra care should therefore be taken in screwing not to cross the threads or screw the lens up unnecessarily tight. The writer has worked for some time with lenses mounted in aluminium, and, so far, has found no signs of wear, whilst the gain in lightness is considerable. Reverting, finally, to the care of lenses, it is necessary to warn the thoughtless against the too common pradlice of polishing or cleaning the surface of a lens with, for instance, such articles as a cotton handkerchief, or the focussing cloth. Every time this is done, the fine polish produced by the optician is, imperceptibly at first, but nevertheless surely, to a certain extent impaired, and this fridlion, constantly repeated, ends finally in a perceptible scratching or degradation of the finely-polished surface. Neither should any foreign substance, such as whiting, rouge, or any description 84 PHOTOGRAPHIC LENSES. of glass-polishing powder be used, a very soft old linen handkerchief or a piece of the finest chamois leather (with the aid of so much moisture as can be obtained by breathing on the lens) applied with a slight wiping — not rubbing — acflion, being quite sufficient to keep the surfaces of the lens clean and free from dirt. If the lens be fitted with an Iris diaphragm, it will seldom or never be necessary to unscrew the cells in order to clean the inner surfaces of the combinations, for such diaphragms, if properly made, should pracfhically exclude all dust particles. With Waterhouse or rotating diaphragms it is, however, sometimes necessary to do so, in which case care should be taken in putting them back to see that in the case of a portrait lens the lenses are not misplaced. CHAPTER XV. UPON FOCUSSING. CAREFUL FOCUSSING ESSENTIAL TO THE PRODUCTION OF SHARP PHOTOGRAPHS FOCUSSING SCREEN OUT OF REGISTER TESTING FOR THE SAME ACCIDENTAL DISPLACEMENT OF FOCAL PLANE THE FOCUSSING SCREEN VARIOUS DEVICES FOR RENDERING THE GRAIN FINER CELLULOID SCREENS THEIR DRAW- BACKS — THE FOCUSSING GLASS USE OF RULES TO BE OBSERVED IN FOCUSSING THE SIZE OF THE " STOP " EXCEPTIONS AND PRECAUTIONS HOW TO SECURE GOOD GENERAL DEFINITION FURTHER HINTS A POPULAR FALLACY FOCUSSING PORTRAITS FOCUSSING GROUPS A USEFUL PRECAUTION. As the sharpness of the photograph will depend upon the degree of care bestowed upon the focussing of the image on the screen, it follows that it is a matter deserving of consideration. Indeed, if one may judge from the number of fuzzy, indistindl photographs always to be met with (leaving out of consideration the so-called naturalistic " photo- graphs in which the absence of definition is intentional) it may be presumed that the operation 86 PHOTOGRAPHIC LENSES. of focussing is not quite so easy as it would at first sight appear to be. A very common cause of badly defined photo- graphs may be traced to the dark slide being out of register with the focussing screen. It is, of course, absolutely essential that, when the focus- sing screen is removed, and the dark- slide inserted, the sensitive plate should be at exactly the same dis- tance from the lens as was the screen before its removal. Further, the rack and pinion should work with sufficient stiffness to prevent any Fig. 27. accidental shifting of the camera back either in or out when placing the dark slide in position. Good lenses have frequently been condemned on account of the above mentioned defedl:, and, therefore, it is always advisable, before bringing a new camera into general use, to carry out the following simple test. Cut from a piece of card a wedge two or three inches long as in the diagram (fig. 27), put a couple of exposed plates in the dark-shde, close it, and draw one of the shutters. Now place a flat PHOTOGRAPHIC LENSES. 87 rule or straight-edge across the sUde, so that the ends rest on the front of the sUde, and sHp the wedge underneath, its flat side resting on the plate. Mark accurately with a pencil the point where the ruler and wedge touch. Repeat the operation with the focussing glass, and if the two points coincide, the screen and slide are in register. Any variation between the two must be compen- sated by adjusting the focussing glass, either by deepening the rebate, or filling it up, until, on testing, the pencil marks coincide. We may now turn our attention to the focussing screen itself, which, even in good cameras, is sometimes of so coarse a grain as to render accurate focussing extremely difficult. Ground glass varies very much in this respedl, but it is possible to obtain samples with very fine grain indeed, and such should be procured. Some photographers advocate rubbing the ground sur- face of the glnss with oil, and for delicate focussing, a more transparent efFe(fl is obtained, but the greasy nature of the oil quickly attra(5ls dust and dirt, and the objecSl in using it is frustrated. A better plan is to attach, by means of transparent cement, a disc of thin glass, such as is used in microscopy, to the centre of the screen, upon which, with the aid of a focussing glass, the most delicate details may be brought to a maximum of sharpness Celluloid is now sometimes substituted for ground G 88 PHOTOGRAPmC LENSES. glass, and if thin, affords a good medium for focussing, it is then, however, very Hable to buckle, and lack of sharpness may easily arise from that cause ; while, if the thick variety is used, its yellow colour is an obje(5lion, and the loss of translucency on this account makes the operation of focussing more difficult. Many sub- stitutes for ground glass have been suggested, some of which are useful in cases of emergency, but for pradfical purposes a piece of really finely ground glass affords the best screen. For delicate focussing it is always advisable to use a focussing glass. One of good quality, corredled for achromatism and flatness of field, should be chosen. Its construction is simple, it being usually formed of two tubes telescoping one within the other, in the upper end of which a pair of magnifying lenses are fixed. It should first be carefully adjusted to the sight of the user, and a mark made on the tube so that the adjustment may be afterwards made quickly and with exa(51:itude. The more brilHant the image appears on the screen, the easier will the operation of focussing be. It follows, therefore, that it is easier to focus with a large stop than a small one, and, subje(5l to one or two exceptions, this plan may usually be adopted. The principal exception is in the case of copying with a lens of the portrait type, the PHOTOGRAPHIC LENSES. 89 efFecfl: of inserting a small stop after focussing with open aperture being to lengthen the focus. The same thing also occurs with some forms of doublets, particularly those of the wide-angle type, but lenses of the rapid recSlilinear type may generally be focussed with their full aperture which may afterwards be diminished without fear of the plane of sharp definition being altered. The camera having been drawn out and clamped at about the approximate focal length of the lens, the image when examined on the ground glass will appear more or less indistin(ft and con- fused. Upon racking the ground glass in or out, various portions of the subjedl become in turn sharp, but it will be found impossible to bring objedls situate at different distances from the camera into sharp focus at the same time. This is owing to the lens not possessing what is termed depth of focus " q.v. The problem to be solved is how to secure the best general definition. Photographers at the present time are somewhat divided in opinion as to how much sharpness a photograph should possess. The writer does not intend to discuss any such debatable point here, but assumes that what is commonly referred to as a sharply defined image " is required. It is often sufficient for the purposes of picflorial definition to concentrate the focus on the principal obje(5l in the pi(5lure, whatever that may be, 90 PHOTOGRAPHIC LENSES. leaving all the rest more or less out of focus. In such cases all that we have to do is to carefully focus the objecft of interest, and if necessary insert a slightly smaller stop in order to make it sufficiently sharp. But when it is necessary to distribute the definition as evenly as possible over the whole of the plate, the procedure will be somewhat different. The mistake is frequently made of focussing sharply the most central portion of the image ; this plan, unless the diaphragm subsequently inserted be a very small one, seldom gives the best general definition all over the plate. It is far better to selecft a point about one-third from the margin of the screen^ and sharply focus that, for by so doing the definition will be more evenly distributed, and probably a larger diaphragm may be used. As a general rule in landscape work the fore- ground, or some prominent objedl in it, should be focussed, a smaller stop being inserted to bring the distance into fair, but not biting sharpness. Atmosphere and roundness notwithstanding all statements to the contrary, are perfedlly com- patible with sharp definition, and the photo- grapher should learn to obtain them by having careful regard to the selection of the point of view, and, more particularly, to the manner in which the subjecft is lighted. In focussing a portrait the eyes of the sitter PHOTOGRAPHIC LENSES. 91 afford the best guide to obtaining good general definition, and upon them the focus should be concentrated. In regard to groups the rule above given, namely, to focus upon a point about one-third from the margin of the screen should generally be followed. When a great number of persons are grouped together to be photographed better marginal and general definition will be obtained if, instead of placing the sitters in a straight line, they are arranged in a semi-circle. This, by accommodating the focus to the natural curvature of field possessed by the lens, will enable a larger stop to be used, and consequently shorten the exposure (see curvature of field). CHAPTER XVI. UPON ANGLE OF VIEW. FACTORS WHICH DETERMINE ANGLE CONFUSED NOTIONS PRACTICAL ILLUSTRATIONS EFFECT OF FOCAL LENGTH ON ANGLE OF VIEW THE SIZE OF PLATE BEING CONSTANT THE ANGLE INCLUDED IS INCREASED AS THE FOCAL LENGTH IS DIMINISHED THE EFFECT OF DECREASING THE DISTANCE BETWEEN THE COMBINATIONS OF A DOUBLET INFLUENCE OF FOCAL LENGTH UPON PERSPECTIVE ILLUSTRATIONS ASCERTAINING ANGLE OF VIEW THE USE OF THE PROTRACTOR SMITH's TABLE OF VIEW ANGLES FIELD OF VIEW NOT ALTERED BY STOPPING DOWN A PRACTICAL DEMONSTRATION. That a great deal of misconception exists among photographers on this very important subjedl, the correspondence columns of any photographic journal will abundantly testify. Supposing the reader to be working with a plate of a given size, the angle of view included by any particular lens is entirely dependent upon its focal length, ^'.^.,the shorter the focus of the lens, the greater the angle of view, and, conversely, the longer the focus the narrower the angle. Confusion arises in the » PHOTOGRAPHIC LENSES. 93 minds of beginners partly on account of the terms which opticians have applied to different forms of lenses, referring to them as wide angle/' narrow angle,'* medium angle," etc. A wide- angle lens is a lens construcfled to cover a plate which is considerably longer than its focus, and it thus includes a wide angle. But if of the same focal length, and used on the same sized plate, a lens that is called a narrow angle lens will include just the same angle of view, no more and no less, as the wide angle. It must, however, be borne in mind that taking the two lenses, each being of the same focal length, the wide angle will cover a much larger plate than would the narrow angle. By way of illustration, if two negatives are taken upon the same sized plate from the same position without moving the camera, with a rapid recftilinear of 5|- in. focus and a wide angle recflilinear of the same focus, it will be seen that as regards the amount of subjedl: included, the two pi(ftures are identical ; therefore the angle of view on that particular size of plate given by the two lenses must be the same. If we wish to include a wider angle, or more subjecSl, upon the plate, we must use a lens of shorter focus, but it will then appear upon a smaller scale. These experiments show clearly that, keeping the size of the plate the same, the angle of view will increase as the focal length of the lens is diminished. 94 PHOTOGRAPHIC LENSES. The influence of focal length upon angle of view is exemplified in figs. 28 and 29, which are photographs of a sitting-room in the house of the writer. Fig. 28 was taken upon a half-plate with a rapid recflilinear of 8| inches focus. It will be seen that so little of the subjedl is included, i,e,, the angle is so small, that picflorially the photo- Fig. 28 graph has little merit, being more a representation of certain articles of furniture than a domestic interior. Fig. 29 was taken with a wide angle recftilinear of 5^ inch focus from the same point of view, and it will be seen gives a far more truthful and pleasing rendering of the subjecft. The question is not unfrequently asked (it has PHOTOGRAPHIC LENSES. 95 several times been put to the writer) Can a rapid doublet be converted into a wide angle by shorten- ing the lens mount, and so bringing the combina- tions closer together ? " If the lens is to be used on a plate of the same size the answer is No," because the shght alteration in the distance between the combinations will not appreciably Fig. 29. shorten the focus of the lens, and consequently the angle of view will, for that sized plate, remain pradlically the same. It may, however, be found that the shortening of the tube will enable the lens to cover a larger sized plate than it would other- wise, if used intacSl, do, but that is another matter. When the optician uses the expression wide- 96 PHOTOGRAPHIC LENSES. angle lens " he intends to signify one that is capable of covering a large size plate in relation to its focal length, which a " narrow angle lens," of course, could not do. Pradlically the angle of view given by a lens on a particular size of plate is a very important matter to the photographer, because, as we have Fig. 30. seen, it involves the question of focal length, and therefore in order to obtain different angles of view upon the same plate he must employ lenses of different focal lengths. This also involves the question of perspecftive, a most important matter in pracftical and picflorial photography. It is of paramount importance to the photographer who PHOTOGRAPHIC LENSES. 97 wishes to produce a truthful representation of his 3ubje6l that, having seledled his view point, he should be able to include any given amount of subjedt, but in order to be in a position to do so he must possess a battery of lenses of different focal lengths, from which he can seledl: the one which gives him approximately the angle of view Fig sr. required. Figs. 30 and 31 are pradlical illustrations of this, fig. 30 being taken with a lens of inch focus on a half-plate, and fig. 31 with a lens of 12 inches focus, the position of the camera being the same in each case. In the first photograph the pi(5lure angle, after trimming, is about 46^, and 98 PHOTOGRAPHIC LENSES. in the latter 28°. Here the long focus lens gives a far more truthful rendering than the short focus» The simplest way to ascertain the adlual angle of view included by any lens upon a given sized plate is to use an ordinary protracflor obtainable from any mathematical instrument maker for a small sum. To use it, draw on a sheet of paper a line equal to the length of the plate, then from the centre of the first line and at right angles to it, draw a second line equal in length to the equivalent focus of the lens, the angle thus described can then be read off by placing the protradlor upon the diagram. The following table of view angles is due to Mr. George Smith, and for the purposes of ready reference will be found very useful. Focus of Base Line of Plate. Lens. 3i in- 4 in. 4t in. 5 in- 6 in. 3 inches. 57° 67'=' 53° 71° 4 46° 39° 64° 53° 74° 5 36" 30^ 62° 6 „ 37° 45° 53° 7 26^ 32^ 34° 39° 46° 41° 8 „ 23^ 28° 30° 35° 31° 9 20^ 27° 37° 10 18° 23° 24° 28° 33° II „ 17° 21° 22" 26" 31° 12 „ 15° 19° 20° 24° 28° For lenses of double the focal length the PHOTOGRAPHIC LENSES. 99 same angle is embraced on a plate of double the size.*' A popular fallacy, not altogether confined to beginners, is the belief that the angle of view is altered according to the size of the stop used. In other words, that by stopping down a lens the angle of view is increased in the same ratio as the size of the stop is diminished. That this is not so may easily be demonstrated. The entire field produced by a lens is circular, as may be seen by mounting a short focus lens on a large camera, or, if the latter be not available, attaching the lens to the shutter of a darkened room and placing a sheet of ground glass behind to receive the image. For example, a six inch recflilinear is chosen, if the circle of light be carefully measured, it may be found to be nine inches in diameter, the lens being used at its full aperture, of /"/8. Let the lens now be stopped down to//i6, and the field again care- fully measured, it will be pra(5lically of the same diameter, and stopping down to //32, or less, will not increase the angle of view subtended by that particular lens. But it may be asked, what does an optician mean by stating in his catalogue that by using smaller stops the angle of view will be increased. What is meant can be easily explained. Let us again examine the circular image cast upon the ground glass. We shall notice that only the central portion is at all sharp, the defini- lOO PHOTOGRAPHIC LENSES. tion rapidly falling off, and becoming worse as the margin of the circle is approached. With the full opening of the lens, the extent of useful sharpness will probably barely equal a quarter-plate in dimensions, though by using a smaller stop, the extent of sharp image may be sufficiently extended to give sharpness over the whole of a 5x4 or possibly a half plate. This, then, is what is meant when the statement is made that by using a smaller stop a larger angle is included, but the result can only be arrived at by using a larger plate; keeping to the same sized plate the mere stopping down of a lens cannot affedl the angle subtended. CHAPTER XVII. DISTORTION: AND ITS AVOIDANCE BY THE USE OF THE SWING BACK. CURVILINEAR DISTORTION ANOTHER CAUSE AN INVIOLABLE RULE CONVERGENT DISTORTION DIVERGENT DISTORTION HOW PRODUCED — HOW TO USE THE SWING BACK DIFFICULTY OF OBTAINING SHARP FOCUS PRACTICAL RULES — SMALL STOPS IMPERATIVE ANOTHER USE FOR THE SWING BACK EQUALIZING THE FOCUS WHEN USING LARGE STOPS IN LANDSCAPE OR FIGURE SUBJECTS THE RISING FRONT AN EFFICIENT SUBSTITUTE FOR THE SWING BACK. Curvilinear distortion produced by the single lens has already been referred to, but distortion of another kind may be produced with any symmetrical or non-distorting doublet if certain precautions are disregarded. When photographing buildings, diagrams, plans, or any obie6t containing straight lines, the back of the camera must be kept parallel with the subjecft or distortion will ensue. If the back portion of the camera containing the ground glass leans inwards towards the building, as, for instance, when the camera is tilted downwards, I02 PHOTOGRAPHIC LENSES. the vertical lines in the photograph will diverge and present the appearance shown in fig. 32. If, on the contrary, the lens is pointed upwards^ causing the back to lean away from the building, then the distortion will be of a contrary nature, and the top of the building will Fig. 32. appear narrower than the base (fig. 33). Therefore it follows that vertical lines occurring in any subjed will only be reproduced as vertical lines and free from distortion when the back of the camera is itself vertical, or phimb, with the subjedl. PHOTOGRAPHIC LENSES. These conditions are fulfilled when the camera is kept square with the subjecft, and neither tilted up nor down. But in pradlical photography, especially when working in a confined situation with a short-focus lens (for example, when photo- graphing a very high building), it is often found that. Fig. 33- keeping the camera level, the roof of the building does not appear on the plate, and there is, moreover, an inordinate amount of foreground. So far, however, the lines are straight, and there is no distortion. How, then, are the roof and upper part H PHOTOGRAPHIC LENSES. of the building to be brought into their proper position on the plate ? Clearly, the only way in which this can be done is by tilting the camera upwards, but immediately this is done, although the \yhole of the building is now included, the lines are no longer straight, but converge towards the top. Fortunately we have a remedy provided for this serious difficulty in the swing back, which should be regarded as an indispensable adjuntfl: to any camera. To use it, the milled heads at the sides must be loosened and the back swung or tilted until it is exacSlly vertical. To facilitate this operation, a plumb or level should be attached to the camera. If the image be now examined on the ground glass, distortion will no longer be apparent, and the lines of the building will appear straight. But a new defecfl: will be observed, for whereas previously the whole of the subjecSl was in fairly good focus, the greater part of it will probably now appear more or less fuzzy and indistincft. Moreover, on attempting to re-adjust the focussing screen, it will be found impossible to bring the whole of the image into sharp focus, and good definition will not be obtained until a small stop has been inserted. When the camera is tilted, and the back swung, the plate is no longer at right angles to the axis of the lens, and the latter is, consequently, more or less strained, owing to the {a.6i that the image of PHOTOGRAPHIC LENSES. the roof of the building, being furthest from the lens, is brought to a focus at a point nearer to the lens than the image of the foreground, which, being nearest to the lens, comes to a focus at a point further from it. It will be found in pracflice. that the more the camera is tilted, the smaller must the stop be in order to secure good general definition. The use of the swing back, therefore, in preserving the redlilinearity of lines, necessitates the employ- ment of small stops, and is generally incompatible Fig. 34- with large apertures and quick exposures. The swing back is also sometimes used in landscape and portrait work for an entirely different purpose, namely, to obtain a better general focus of near and distant obje(5ls without resorting to the use of small stops. It is, however, a power which requires to be used with the greatest caution, in order to avoid the distortion or untrue drawing which may result becoming unpleasantly conspic- uous. Its chief advantage will be found in certain I06 PHOTOGRAPHIC LENSES. classes of instantaneous landscape work. For example, a pi(5lure containing near and distant planes, such as is afforded by a river view (fig. 34), with a bed of rushes in the foreground, A, a boat sailing in the middle distance, B, and a background of hills in the rear, C. Now the image of each of these points comes to a sharp focus at varying distances from the lens, the rays forming the foreground being farthest from, and those forming the distant hills nearest to, the lens. Therefore it follows that if we focus sharply for a point intermediate between the two — in this instance the boat — we can, without unduly disturbing their sharpness, accommodate and bring to a better focus the longer rays forming the foreground by swinging the back outwards as shown in the diagram, the image being, of course, inverted. The same principle is sometimes applied to portrait work in the studio, but great judgment and care is required in order to avoid exaggerated perspedlive. In the case of a sitting figure, the whole of the body may be brought into a better general focus, when a large stop is used, by slightly swinging the back ; but this, if carried too far, may result in serious distortion of the image. In short, the swing back, although an indispensable auxiliary in architedlural and reprodudlive work, requires to be used with care and judgment, as its employment necessarily strains the optical powers PHOTOGRAPHIC LENSES. 107 of the lens. It should never be unnecessarily used, and in this conne(5lion it is well to point out that a liberal use of the rising front will, in many cases, obviate the necessity for using the swing back. In photographing architecftural subjecfts, therefore, the rising front should be first raised to its full extent, in order to see whether, by that means, the whole of the subjedl: can be included upon the plate without tilting the camera. It is true that in so working, the centre of the field will no longer be in the centre of the plate, and if the covering powers of the lens be limited, the corners of the plate may be **cut," through the lens being raised too high. But in an extreme case a compromise can generally be efFe(5led, for if the front be first partially raised, the camera will not require to be tilted to such a degree as would otherwise be necessary. CHAPTER XVIII. COMBINATION LENSES, CASKET LENSES, AND THE USE OF BACK COMBINATIONS, ETC. DARLOT'S combination casket lenses THEIR UTILITY UPON COMBINING ELEMENTS OF DIS- SIMILAR LENSES SUPPLEMENTAL LENSES FOR PORTRAITURE UNSYMMETRICAL DOUBLETS, PRACTI- CAL ADVANTAGES OF PRECAUTIONS TO BE OBSERVED IN COMBINING LENSES UNIVERSAL LENS MOUNT INCREASE OF SEPARATION FLATTENS THE FIELD, BUT DECREASES THE ANGLE DECREASE OF SEPARATION INCREASES COVERING POWER USEFULNESS OF BACK COMBINATIONS FOR SECURING LARGE IMAGES OF DISTANT OBJECTS EFFECT OF POSITION OF THE STOP ON DEFINITION AN ADJUSTABLE DIAPHRAGM ALTERATION OF FOCAL LENGTH BY SUPPLEMENTAL LENSES PRACTICAL DISADVANTAGES OF THE SYSTEM PORTABLE SYMMETRICALS INTERCHANGING COM- BINATIONS OF CASKET LENSES OF ENGLISH MAKE. Several French opticians, notably Darl6t, issue caskets of single lenses of different focal lengths, set in cells all of which fit one mount. Some of these sets are intended only to be used as landscape lenses, others are intended either to be PHOTOGRAPHIC LENSES. IO9 used in combination as doublets, or as single landscape lenses. Convenience is, perhaps, the chief merit of such systems, more or less variation in focal length being obtainable by simply inter- changing the combinations ; but, as a rule, the optical perfecftion of the image is not quite equal to that produced by doublets construdled for special purposes. Now and again somebody makes the important "discovery?" that by combining the back com- bination of a recftilinear, of for instance, twelve inch focus, with the front combination of one of six inch focus, or vice versa ^ a lens with an intermediate focus of about nine inches will result. There is really no novelty in such a suggestion, for Darlot's system, which is identical in principle, has now been in vogue for many years. Indeed, supplementary negative lenses had been applied to ordinary portrait lenses in order to flatten the field and lengthen the focus long previously to the issue of the Darlot caskets. Although the designation symmetrical " is applied to certain classes of doublets, the term is often a misnomer, for in many instances the front combination of a lens so designated will be found to be longer in focus than the back. This, in one respedl, is an advantage to the user, he having pra(fl:ically three focal lengths in one instrument. When combining lenses in the no PHOTOGRAPHIC LENSES. manner referred to, the best results will only be obtained when the elements of instruments of similar type are combined. That is to say, one element of a rapid recflilinear should be combined with the element of another rapid redlilinear, and not with the element of a wide-angle redlilinear, and vice versa. So used, the resulting combinations may probably be used with the full aperture of the lens with good efFe(5l. Not so, however, when one element of a rapid recflilinear is combined with an element belonging to a wide-angle redlilinear, for, although a useful combination may result, it will probably be necessary to stop down considerably in order to get a sharply-defined image. If the reader is in possession of several obje(5lives which he wishes to use in combination in the manner suggested, he should get an optician to construdl for him what may be described as a kind of universal mount, or lens tube, which will not only receive the various elements, but, by a telescopic motion, permit of the distance between them being varied at will. When a certain combination is decided upon, the exact distance which the lenses should be apart may be ascertained by experiment. Attention to this matter is very important if the best results are to be obtained. By increasing the amount of separation the field is rendered flatter, but the angle of view becomes more limited ; conversely, PHOTOGRAPHIC LENSES. Ill by bringing the elements closer together the covering power will be increased, but at the expense of flatness of field. In addressing beginners it is, perhaps, necessary to point out that lessening the distance between the com- binations will not alter the angle of view given on a particular sized plate, but bringing the combina- tions closer together will permit of a larger plate being covered than would be possible when they were separated to a greater extent, the mount in the latter case cutting off the marginal rays, and limiting the covering power of the combination. In addition to combining the elements of dissimilar combinations in order to obtain dilferent focal lengths, either the front or back combinations of most doublets may be used alone, as single lenses of long focus, and so used, are extremely useful for distant landscapes, or for getting architectural details on a large scale. It will generally be necessary, however, to use a rather small stop in order to obtain crisp definition to the corners of the plate. This is because the stop is placed too near to the lens, and better marginal definition, with a larger aperture, can be obtained by placing the stop a little further from the lens. Any working optician will fit a short piece of tube, of a diameter equal to that of the inside of the lens tube, with one or two circular stops, so that it may be adjusted within the lens tube by sliding to 112 PHOTOGRAPHIC LENSES. and fro until the maximum of flatness is obtained. It must, however, be borne in mind that by improving the flatness ot field of a single lens in this way, the covering power is diminished, and at the same time its distortive qualities increased. When, however, as is generally the case, the lens is used upon a small plate as compared with its focal length, only the centre of its available field is utilised, therefore both the drawbacks alluded to above may be disregarded. The focal length of a doublet may be altered by placing a supplementary lens between the combinations. The lenses used may be of the most inexpensive description, ordinary spectacle lenses answering the purpose. If it is desired to lengthen the focus, a negative of dispersing lens is used, a magnifying, or convergent lens producing a contrary effect. The writer has experimented a great deal with different types of lenses in this way, but has come to the conclusion that where critically fine definition is desired, the system can hardly be recommended, although, under certain circum- stances it may prove useful. The optical quality of the image is more satisfactory when a negative lens is used to lengthen the focus, for when it is attempted to shorten the focus by the interposition of a magnifying lens, the field becomes exceedingly round and the definition of a low order, unless a very small stop is employed. Mr. Blackwood took PHOTOGRAPHIC LENSES. out a patent for an analagous system, but Mr. Traill Taylor and others had referred to the matter long anterior to the date of the patent. The most convenient method of ringing the changes " by combining lenses of different focal length has been referred to already. Two or three lenses of the portable symmetrical type, the screws and cells of which are identical, and therefore permit of their being easily inter- changed, should be procured. The writer for many years past has used this system with the most gratifying results, and can therefore recom- mend it with confidence. Recognising a demand for greater variety in focal lengths, several well-known opticians now issue what are called ^' casket " lenses. These, however, are not usually ^'combination'* lenses, in the sense in which the term has been used in this chapter, the casket usually containing three or more doublet lenses, each being either in its own mount, or arranged to fit into a mount common to the set, the system of interchanging the combinations of different doublets being one not usually recognised by opticians. The casket system in fadl provides the novice with several lenses of various focal lengths, carefully selecfted by the optician, so that different angles of view may be included upon the same plate. CHAPTER XIX. "HOW TO TEST A LENS." SCIENTIFIC LENS TESTING PRACTICAL TESTING NECESSITY FOR ACCURATE REGISTER HOW TO TEST FOR ACHROMATISM — APPEARANCE OF IMAGE NO CRITERION THE FOCIMETER A HOME-MADE SUB- STITUTE TESTING FOR COVERING POWER THE FIELD OF VIEW HOW TO TEST FOR CURVATURE OF FIELD A STANDARD OF COMPARISON NECESSARY THE TEST DESCRIBED ADVANTAGES OF A CURVED FIELD DISTORTION SINGLE LENSES RARITY IN DOUBLETS ASTIGMATISM SPHERICAL ABERRATION ALWAYS PRESENT IN SINGLE LENSES WITH APER- TURES LARGER THAN f/iI A SIMPLE TEST FOR FLARE APPEARANCE OF FLARE-SPOT IN SINGLE LENSES ITS CAUSE AND CURE FLARE IN RECTI- LINEARS ITS REMEDY INCORRECT CENTERING A DIFFICULT TEST FOR THE NOVICE A MAKESHIFT METHOD REMEDIES OTHER DEFECTS AIR BUBBLES THEIR INFLUENCE ON THE QUALITY OF THE LENS STRI^ TESTING FOR EFFECT UPON IMAGE SCRATCHES, ETC., NOT NECESSARILY FATAL TO THE EFFICIENCY OF THE LENS. To form an accurate opinion upon the relative merits of two lenses, each being the produdlion of PHOTOGRAPHIC LENSES. an optician of repute, demands far more skill than the average photographer is likely to possess, and the tests involved are necessarily of a highly scientific charadler. But in these days, when lenses can almost be obtained for as many shillings as they formerly cost pounds, it may be useful to many readers, who perchance cannot afford to purchase high-class and expensive instruments, to learn how to apply certain simple tests by which a rough idea may be formed of the capabilities of a particular lens for performing the work required of it. To such testing, therefore, will the following direcftions be confined. Before any accurate judgment can be formed of the excellence, or otherwise, of any lens, attention must be paid to the focussing screen, in the first place to see that its surface is finely ground, and in the next to ascertain that it occupies the same plane as the plate when the dark slide is inserted. Both these matters are fully dealt with in the chapter on focussing," to which the reader is referred. Testing for Achromatism. — It sometimes happens in lenses of cheap construdlion that the visual and actinic foci are not coincident, and although composed of two different kinds of glass, the lens has been either under, or over corredled. In such a case, although the image may appear quite sharp upon the screen, the resulting negative Il6 PHOTOGRAPHIC LENSES. will be more or less out of focus. It is obvious, however, that the mere examination of the sharp- ness of the image affords no real test of the value of such a lens, although many photographers in purchasing are quite content with such an examination. The optician when testing for achromatism employs what is termed a focimeter. A rough model serving every practical purpose may be easily made. Seven fan-shaped pieces of cardboard, numbered from one to seven, are fixed to notches cut in a bar of wood (a piece of broom handle answers admirably) in such a way that all the numbers may be visible, there being an interval of about a couple of inches between each. Carefully focus the central card, which will be that marked 4, and, using the full aperture of the lens, proceed to expose a plate. If upon develop- ment the number focussed comes out sharpest, it may be assumed that the lens is properly corrected, but if on the contrary, a lower or a higher number appears sharpest, then the lens is either over or under correcfted. In either of the latter alter- natives occurring, the lens should be rejedled. Testing for Covering Power. — Lenses are advertised by the optician to cover certain sized plates, and if we attempt to use them upon plates of larger dimensions, though we may often do so successfully, we do so at our own risk. Now, as we have already seen, the field, ix,^ the area of PHOTOGRAPHIC LENSES. 117 image covered by any lens, is circular, but the plates in general use are rectangular, so that if the lens properly covers the plate we cannot see the full extent of its field. The field should be sufficiently large to allow of the lens being raised or lowered, or moved from side to side, in order to B B B B A Fig. 35. permit of the adjustment of the subjecft, without bringing the margin of the plate outside it, and so producing black corners. In fig. 35 the circle A represents the field of the lens, B B B B being the diagonals of the plate which it is intended to cover, the dotted lines showing the degree to 1 18 PHOTOGRAPHIC LENSES. which the lens may be adjusted without cutting off the margin of the field, and producing black corners. Testing for Curvature of Field. — It will be somewhat difficult for the novice to form an opinion upon the excellence, or otherwise, of a lens in this respecft, unless he can obtain another lens of a similar size and type by an esteemed maker with which to make the comparison. Otherwise the test can be very simply carried out by pinning up an uncreased advertisement sheet of a daily newspaper in a good light, and sharply focussing its image, using a large stop and con- centrating the focus on the centre of the screen. Make a small mark on the sliding portion of the baseboard, and then proceed to focus the extreme edge of the print, the difference between the two positions will represent the amount of curvature present. This, however, as we have already observed, will not be of much assistance to the beginner, but by comparing the performance of the lens to be tested with another of known quality, he will be able to form a fair idea of the quality of the instrument he desires to purchase. The larger the surface of the print reproduced clearly, the flatter will be the field and the better the lens. It should, however, be pointed out that a certain amount of curvature of field is in some cases a positive advantage, as for instance when PHOTOGRAPHIC LENSES. 119 photographing a subjedl which is not situated in a plane, but in which the marginal objecfts approach nearer to the camera. A street scene in which the buildings at the sides of thepidlure are nearer than those forming the background, is one example, an avenue of trees another, in either case a lens with slight curvature of field would give better general and marginal definition than one in which the correcflion for flatness was more perfedl. The new lenses of Zeiss and others, possess remarkable flatness of field where the reproducftion of an object situated in one plane is concerned ; and some photographers, lacking an elementary know- ledge of optical laws, have expressed disappoint- ment with these lenses because they would not give an equally flat image of a subjecfl occupying a position equivalent to the segment of a sphere. No lens has yet been made which performs equally well under these widely diverse conditions. Distortion. — Distortion is pradlically con- fined to the use of single lenses, and the conditions under which they may safely be used for the reprodudlion of objecfls containing straight lines, have already been fully discussed. Few modern doublets, however common in quality, are so badly construdled as to show this defecft. Astigmatism. — This defe(5f is far more likely to cause inconvenience than the last named, but it is scarcely one that will come within the J I20 PHOTOGRAPHIC LENSES. province of the novice to test for. The optician in making his calculations endeavours to effecft a compromise between curvature of field, and astigmatism, for by making the field too flat, astigmatism is introduced. Spherical Aberration. — The cause of this defecft has been already explained. It manifests itself in a general indistincflness of image, or an inability to secure a sharp focus in any portion of the field. It is always present in single lenses when used with an aperture larger than //ii, and is more pronounced in those of long than those of short focus. It is also present, though in a less degree, in rapid lenses of the Euryscope type, when used with the full aperture. Focus a candle or gas flame sharply by the aid of a focussing glass, using the full aperture of the lens, if a halo exists around the flame the lens is not free from spherical aberration. The use of smaller stops correcfls the defecSl, though, of course, at the expense of rapidity. Flare. — There are several kinds of flare. In its most pronounced form it generally appears as a halo or disc of light in the centre of the focussing screen, varying in size and brightness with the aperture of the stop of which it is in fadl a refle(5led image. This kind of flare is generally termed flare spot," and is very often met with in cheap single lenses. It may be remedied in most cases PHOTOGRAPHIC LENSES. 121 by slightly varying the position of the diaphragm, i.e., moving it nearer to, or further from the lens until the flare spot disappears ; some lenses, however, are very difficult to cure. It is also to be met with in other lenses, some recflilinears of the rapid type being very much so affecfled, particularly when worked towards the sun. It may be reduced b}^ shielding the lens from direcft rays during exposure. When a doublet gives flare under normal conditions of working, it may be cured by increasing the distance between the combinations, i,e,, having the lenses mounted in a longer tube, but as this treatment will slightly curtail the field which the lens would otherwise cover, the amount of separation should not be greater than is sufficient to efFedl the objedl: aimed at, sometimes the mere unscrewing of the cells a couple of turns being sufficient. Flare is also occasioned by the careless mounting of the lens, a portion of the lens tube, or the cell, being left bright ; this can be remedied by painting over the bright parts, with a dead black varnish, such as that sold by Park or Tyler. Incorrect Centering. — Cheap foreign lenses frequently suffer from this defecSl, which is a very serious one. The test, however, is rather difficult, and requires to be applied with care and skill. The most satisfadlory and usual method is to put the lens in a lathe, and whilst it is rotating care- 122 PHOTOGRAPHIC LENSES. fully watch for any movement in the various refledlions of a candle flame placed in front. If they remain steady, the lens is properly centered ; but if, on the contrary, there is the slightest move- ment or wobbling, it has been badly centered. A writer in the International Annual for 1888 gives a method, based upon the same principle, which although permitting the test to be carried out without the aid of a lathe, is not sufficiently delicate to allow of the detedlion of a small amount of error. The camera is firmly fixed to a bench or table, and the lens screwed in ; a lighted candle placed in front, and the various reflecfted images carefully examined as before, the rotation being effe(5led by carefully unscrewing the lens. The defecft may be due to either of two causes, the first and most common being attributable to careless- ness on the part of the optician when grinding or edging the lens prior to mounting. This of course cannot be remedied by the photographer. It may also be caused by carelessly mounting the elements of the combination in too large a cell ; this defecft, presuming the lens itself to have been accuratel}/- centered, may be more easily remedied. Mechanical and Other Defects. — The most common of these are striae, and air bubbles. The latter are frequently to be met with even in lenses coming from the workshops of noted opticians ; with large lenses it is often impossible PHOTOGRAPHIC LENSES. 123 to avoid their presence, as to do so would involve the rejecflion of so large an amount of glass as to render the price of such objedlives prohibitive. Fortunately small bubbles do not prejudicially affecfl: the defining power of a lens, their presence may therefore be disregarded, and the instrument should not be reje(5ted on this account. Striae on the contrary is a very serious defedl. To ascertain whether it exists hold the lens to be examined in front of a strong gas or lamp flame in such a way that the surfaces of the glasses are strongly illuminated. By slowly turning the lens round, any wavy or uneven appearance of the glass will denote the presence of striae. The defedl is by no means uncommon in foreign lenses of cheap make, which are rarely subje(5led to very careful testing. It cannot be remedied, being due to a want of homogeniety in the glass, and any lens so affedled should be rejedled. Its presence would cause unequal refracftion, and a more or less blurred and distorted image. Scratches, etc. — Although the freer the surfaces of a lens are from blemish of any kind the more perfedl will its optical properties be, yet the presence of a scratch, or even a more serious injury, may not materially impair its efficiency. The writer is acquainted with several instances where lenses having received serious injury by scratching or abrasion, yet, nevertheless, continue 124 PHOTOGRAPHIC LENSES. to work with undiminished excellence. Such instruments may sometimes be purchased for less than the value of the brasswork. The writer has in his possession a cabinet lens by Ross, the back combination of which has, by the carelessness of some former owner, become badly scratched in the centre, to such an extent indeed, as to present a central patch almost like ground glass of about half an inch in diameter. This lens works so perfecflly that not even an expert, judging from photographs taken with it, could detedl the existence of any- thing abnormal in the lens. Instances indeed are not unfrequent of lenses which have sustained injury continuing to work satisfadlorily, but enough has been said to direcft attention to the fadl that accidental injury may not have such disastrous consequences as might be imagined. It must not, of course, be inferred from the above, that the optical perfedlion of an injured lens will in all cases remain unimpaired, for such may obviously not be the case, and the matter is only referred to for the guidance of those who may, perchance, be in possession of expensive lenses, which at some time or other have received accidental injury, and which on that account might be permanently laid aside as worthless. CHAPTER XX. LENSES OF FOREIGN CONSTRUCTION. HIGH-CLASS FOREIGN OBJECTIVES CHEAP CON- TINENTAL LENSES GENERAL CONSIDERATIONS PREJUDICE THE WRITER'S VIEWS QUALITY AN INDIVIDUAL CHARACTERISTIC — COMMERCIAL TESTING PROS AND CONS RISK OF OBTAINING AN INFERIOR INSTRUMENT MINIMISED BY APPROVAL SYSTEM LOW PRICED LENSES — PRECAUTIONS TO BE OBSERVED IN TESTING CHEAP LENSES — OPTICAL PERFECTION MORE DIFFICULT TO SECURE IN LARGE THAN IN SMALL LENSES A COROLLARY EVASION OF THE MER- CHANDISE MARKS ACT DEPRECATED THE KEW TEST. In speaking of lenses of foreign constriKftion reference is not, of course, made to the obje(5lives of eminent opticians such as Voightlander, Stein- heil, Suter and others, their produdlions as a rule being at least equal in quality to those of the most esteemed English houses, but to the cheap unnamed instruments of continental wholesale houses, which are almost universally supplied to the trade, generally without any guarantee as to quality. It would be impossible to entirely ignore in a 126 PHOTOGRAPHIC LENSES. handbook the avowed objecfl of which is to impart information of a pradlical nature, the subjecfl: of cheap foreign lenses, so many thousands being now in every-day use. It is, however, for tolerably obvious reasons, more difficult to give useful advise upon the purchase of a cheap French lens for example, than upon the selecftion and purchase of one from the shelves of an English manufadlurer of repute. The first consideration which arises is by no means unimportant, and is the one which usually prompts the question, Shall I be able to get as good results with a cheap foreign lens, as I could with an expensive one by an esteemed maker ?" It is the, to many, all important one of economy. For a very long time a great deal of prejudice existed in the minds of many photographers against cheap foreign lenses, and this was fostered, quite naturally perhaps, from a commercial point of view, by those interested in the sale of more expensive instruments. This feeling still exists in some quarters, but to a much more limited extent than formerly, as the fadl that many photo- graphers of the highest skill and standing now employ cheap foreign lenses will abundantly testify. The opinion of the writer, formed after using and testing many hundreds of such lenses, is that where econom}^ must be studied, the purchase (by a photographer) of a cheap foreign PHOTOGRAPHIC LENSES. 127 lens in preference to one of more costly construc- tion, should not necessarily lead to any apparent deterioration in the technique of his photographs. It has already been pointed out that quality in a lens is to a large extent an individual or personal equation, if grammarians will pardon the expression — it belongs to a particular instrument rather than to a class — and this of necessity applies with tenfold force to cheap, unnamed foreign lenses. An optician of repute subjecfls all lenses manufadlured by himself to certain tests, more or less critical, and those that do not pass a certain standard are rejedled. With the majority of foreign lenses, however, no such system of testing is adopted — or when adopted, it is not of a very critical characfler. Herein, therefore, lies the cnix of the whole question — by paying the higher price the purchaser gets a guarantee of a certain standard of quality, whilst by paying the smaller sum he runs the possible risk of becoming the possessor of an instrument of inferior quality. The risk, however, is not so great as might be imagined, and may be guarded against by stipulating for approval. Although the expression foreign lenses " is commonly used as a kind of generic term, the instruments to which it is applied vary exceedingly, both in quality and price. The very cheap and common qualities should always be looked upon with suspicion, they being 128 PHOTOGRAPHIC LENSES. sometimes made from glass bad in colour, or suffering from striae or other defecfls. Recently periscopic redlilinears have been offered for sale at very low prices, these also should be eschewed. It follows from what has been said that a foreign lens should always be tested before pur- chase. Full direcftions for so doing will be found in another chapter, and there is therefore no necessity for repetition. Emphasis must, however, be laid upon one point, namely, that the mere inspedlion of the image upon the focussing screen affords, at any rate to the novice, no useful criterion of the excellence or otherwise of the lens. In every case it is imperatively necessary that a negative should be taken with the largest working stop with which the lens is provided. The most useful all-round test for practical purposes in such a case, especially where time is limited, is to pro- cure an auctioneer's advertising poster, or the advertisement sheet of the Times, and having pasted it on a flat sheet of cardboard, proceed to make a negative, focussing for the centre of the printed matter. In order to form a basis of com- parison a second exposure should be made, using a lens of similar construdlion and focal length, but made by a maker of repute. This will give a good idea of the performance of the cheaper lens in regard to covering power, flatness of field, and definition. PHOTOGRAPHIC LENSES. 129 The diiBculties of lens constru(5lion increase with the size of the lens, ix,, it is much easier to make a lens of small size work satisfadlorily than one of large dimensions. Therefore it follows that the difference in quality between cheap French lenses, and those construdled by opticians of repute, is much more marked in large sized instru- ments, for example, from i2xio upwards, and greater care and circumspedlion is necessary in purchasing such. Again, a difference in quality is more likely to occur in a lens of the wide-angle than in the ordinary rapid recflilinear type, probably owing to the facft that the deeper curves of the former require if possible greater skill and precision in grinding than the latter. So far as the sale of foreign photographic lenses is concerned the Merchandise Marks Acfl would seem to be pracfti- cally a dead-letter, for it is the pracftice of very many dealers in photographic apparatus to import continental objedlives and engrave upon the mounts their own names and trade designation. Although the morality of such a course may be open to question, the majority of photographers would probably not be misled into believing that in purchasing such lenses they were buying instru- ments acflually manufadlured by the firm whose name they happened to bear. Such lenses, how- ever, come within the scope of our remarks, and 130 PHOTOGRAPHIC LENSES, although the name of the firm appearing upon the mount may be to some extent a guarantee of quality, yet the lens should only be purchased subjedt to test and approval. If the photographer is diffident of his own power of discrimination, all doubt may be solved and trouble avoided by sending the lens to the testing department at Kew, where for a fee of 7s. 6d. it will be scientifically tested, and returned with a certified report as to its quality. CHAPTER XXI. ON PURCHASING SECOND-HAND LENSES. VARIOUS CONSIDERATIONS SPURIOUS LENSES FORGED NAMES AND DESCRIPTIONS HOW TO DETECT APPROVAL SECOND - HAND DEALERS THE writer's EXPERIENCE ALTERED LENSES FRAUD CONCLUDING ADVICE. A pra(5lical hand-book upon lenses would be incomplete without some reference to the purchase of second-hand objedlives, therefore the present chapter has been added. At the same time it must be confessed that the question, Is it advis- able to purchase a second-hand lens ?" is by no means an easy one to answer. There are, of course, several considerations to be weighed, of which, perhaps, economy is paramount, for if the opportunity occur of purchasing a high-class- instrument at possibly half its original cost, a great saving of expenditure may be effedled. A good deal depends upon the source from which the bargain comes. Worthless lenses are sometimes made up to imitate the produdlions of noted opticians, whose forged names and descriptions 132 PHOTOGRAPHIC LENSES. appear on the mounts, and unscrupulous individuals not unfrequently attempt to dispose of such instruments through the medium of an adver- tisement. Such forgeries are generally not difficult for an expert to detecfh, but in cases of doubt the lenses should be submitted to the inspecflion of the opticians whose name they bear, who in their own interests are invariably willing to report upon the genuineness or otherwise of the instruments. In buying a second-hand lens, especially when the transacftion takes place with a stranger, approval should always be stipulated for, and the would-be purchaser should not part with his money until he is assured of the genuineness of the article he is purchasing. In the above remarks the writer is, of course, referring to transacftions taking place between private individuals. There are several photographic dealers of the highest respecflability who for years have carried on an extensive business in second-hand lenses. That the reader will be quite safe in putting himself in their hands, the writer from his own experience, extending over a period of more than 20 years, can abundantly testify. As a matter of facfl: in pro- curing a lens from such a source, the purchaser usually runs little risk of obtaining a forged instru- ment, for the critical eye of the dealer is seldom deceived, and any doubt is solved by submitting the lens to the reputed maker. There is, however, PHOTOGRAPHIC LENSES. another consideration which has a very important bearing, and which incidentally has already been adverted to ; excellence in a lens must be regarded to a large extent as an individual quality, and although the fadl of a lens being issued by an optician of repute is in itself a guarantee that a certain standard of quality may be expecfted, yet out of a dozen such lenses one or two may generally be selecfled which are superior in quality to the rest. Further, it is impossible to trace the history of an obje(5live purchased second- hand, nor is it possible to tell the vicissitudes which it may have undergone. The author on one occasion purchased a second-hand rapid recftilinear, bearing the name of one of our most honoured optical houses, which was apparently genuine and intacfl. It was a half-plate lens, but upon affixing it to the camera, its focal length, which should have been approximately eight inches, turned out to be nearly ten. Subsequent examination proved that the back combination had been at some previous time removed, and another of much longer focus substituted. Indeed, it is not an unknown thing for unscrupulous persons to remove the original combinations from an obje(5live, replacing them with lenses of a common quality, and then to dispose of the instrument as a genuine article. Upon the whole, unless the reader is able to 134 PHOTOGRAPHIC LENSES. place himself in the hands of a respectable dealer who will treat him fairly, or can obtain the advice and judgment of some competent person, he will generally find it to his advantage to purchase diredl from an optician with whom the manufadlure of photographic lenses is a speciality. CHAPTER XXII. DALLMEYEr's tele-photo lens ADVANTAGES OF UTILITY OF FOR BOTH NEAR AND DISTANT OBJECTS SOME NEW LENSES — DENNIS TAYLOR's NEW TRIPLE LENS FLATNESS OF FIELD FREEDOM FROM ASTIGMATISM AND FLARE THE COLLINEAR. The use of the back combination of a doublet in order to secure a larger image of a distant objecfl has already been referred to, but the same result can be obtained in a more scientific manner by employing the tele-photographic lens, of Dallmeyer. This consists essentially of a negative combination or attachment, which can be adapted to the photographer's own lens. By adjusting the distance between the supplementary lens, and the lens to which it is adapted, enlarged images of near or distant objedls can be obtained, the degree of magnification being dependent upon the amount of separation. The advantage of the system over the use of a special long focus lens, or a back combination, is to be found not only in the range obtainable with one instrument, but also in the facft that these results can be secured without any undue extension of the camera. For example, K 136 PHOTOGRAPHIC LENSES. No. 2 tele-photo lens, when used on an ordinary whole plate double-extension camera with a focal range of about 22 inches, gives a range of foci between 29 and 62 inches, the aperture ranging from//i5 to//32, becoming less as the magnifica- tion is increased. It may be stated that the most suitable lenses to employ with the tele- photographic attachment are portrait lenses, and rapid recftilinears, it being necessary to use objecSlives with comparatively speaking large apertures, so that the intensity of the combina- tion may not be too much reduced. In acftual pracftical work, the loss of intensity is not very serious, for in photographing distant obje(5ls the exposure has necessarily to be very much cur- tailed, and, therefore, very quick, or even shutter exposures, may be given even when the degree of magnification is large. It must not be supposed, however, that the utility of the instrument is confined to what may be called long range" or extremely distant work, for in architectural photo- graphy especially, the system would appear to have a wide range of application, enabling the photographer to secure, at comparatively close quarters, details of architecSlure and construdlion, which would be difficult if not impossible otherwise to obtain. Since the foregoing chapters were written, indeed, after going to press, two new lenses have PHOTOGRAPHIC LENSES. been introduced to the attention of photographers, namely, the new triple combination lenses of H. Dennis Taylor, and Voightlander's collinear lens. The writer was fortunate in hearing Mr. Taylor describe the constru(51:ion of his invention before the Royal Photographic Society, and later, through the kindness of Messrs. Taylor, Taylor & Hobson, Fig. 36. has had an opportunity of pracflically testing the lens. Mr. Dennis Taylor's lens, though a triple combination, possesses unique properties which differentiate it from any previously existing form of the triplet. The diagram (fig. 36) shows the constru(5lion of the lens, which will be seen to 1^8 PHOTOGRAPHIC LENSES. consist of two simple positive lenses of unequal curvature, with an achromatic negative lens placed between them. It is claimed for this lens that it will give an absolutely flat field on a plate, theHonger side of which is about equal to its equivalent focus, astigmatism being at the same time pracflically eliminated. The lens is also corrected for both spherical and chromatic aberra- tion. A by no means unimportant feature of the new lens is the facSl: that it can be construdled at a very moderate cost. It is, moreover, a rapid lens, working with a maximum intensity of f/5.6. The particular lens submitted to the writer for inspedlion was one of a dete(ftive series about to be introduced, working with the above aperture, having a focal length of 6.6 inches, and intended for use upon 5x4 plates. When copying a number of advertisements, pasted upon a large flat sheet of cardboard, the flatness of the field, when compared with an ordinary rapid redlilinear of the Steinheil type, was remarkable, both lenses being used with their maximum apertures— //5. 6. With the second stop //8, the whole of the plate right up to the corners was critically sharp, small print at the extreme margins being clear and distindl. The absence of astigmatism was very marked, vertical and hori- zontal lines running across the diagram being rendered equally sharp. During the discussion which followed Mr. Dennis Taylor's description of PHOTOGRAPHIC LENSES. his invention, the possible danger of flare in a triple combination was adverted to ; the writer, therefore, subjecfted the lens to several somewhat severe pradlical tests, in order to ascertain whether such an apprehension was well founded. The camera was placed in such a position that the sun, which was shining, was in front, but except when the adlual direcft rays of the sun entered the lens, no flare spot, or ghost, was discernible. Further experiments were made on strongly lighted in- teriors exhibiting extreme contrasts of light and shade, with equally satisfactory results. There is, it should be noted, no lens in existence which will not show a flare spot when the dire(51: rays of powerful sunlight are allowed to fall upon it. We also tested this lens in another way, and one in which its special feature, namely, flatness of field, was remarkably demonstrated — we refer to the producftion of enlargements. Placed on the en- larging lantern, and tested against the rapid redlilinear before adverted to, its superiority in flatness, even with the full aperture of //5.6, was remarkable. The negative, an architedlural subjecfl, was microscopically sharp to the corners, and was enlarged to four diameters without any perceptible loss of definition even at the margins. Mr. Dennis Taylor is to be congratulated upon having produced a lens (possessing properties 140 PHOTOGRAPHIC LENSES. hitherto only to be found in certain very costly lenses of German origin), which being simple in design and construcfted mainly, if not wholly, of English optical glass, can be offered to photo- graphers at a price not greatly exceeding that charged for an ordinary rapid redlilinear of good quality. Of the second new lens, Voightlander's col- linear, to which reference has been made, we cannot speak from personal knowledge, not having had an opportunity of pradlically testing it. In the printed pamphlet which has been issued, the makers state that they have succeeded in pro- ducing such a symmetrical double lens, consisting of two equal tripartite cemented halves, which will satisfy the most exacfting requirements as to the anastigmatic flatness of image, large range of vision, and intensity of light The collinear produces sharply-defined images of 86*^ angle of image even at //7.7 to //g. They combine with a great intensity of light the specific proper- ties of wide angle lenses, and consequently are of the greatest advantage for wide angle instan- taneous exposures with large stops, //6.3, //y.y, or//9." It is needless to add that a lens fulfilling these conditions will prove a very valuable addition to the pradliical photographer's lens cabinet. INDEX. PAGE. Abb6-Schott Glass 57 Achromatism 12 Testing for . . 115 Actinic Doublets, Ross' 47 Adapters 81 Adjusting Position of Stops. Ill Advantages of Curved Field 119 Advantages of Single Lenses 42 Air Bubbles in Optical Glass 122 Alteration of Focus 78, 89, lOJ, 112 Aluminium Lens Mounts . . 83 Angle of View 92 Architecture, Lenses for 77 Arrangement of Large Groups 91 Astigmatism 119 Back Combination, Use of. . 71 Back Fo(ms 14 B«g for Lenses 80 Blind Shutter 81 Brilliance of Image 42, 88 Care of Lenses 80 Casket Lenses 109, 113 Celluloid Focussing Screens 87 Cheap Lenses 128 Choosing a Lens 69 Chromatic Aberration 10 Cleaning Lenses 83 Coincidence of Foci 117 CoUinear Lens 140 Combination Lenses 108 Combining Dissimilar Lenses 109 Composition of Light 8 Concentric Lens, Ross' .... 58 Conjugate Foci 14 Copying, Lenses for . 78 Covering Power 99 Covering Power, Testing for 117 Curvature of Field 20, 118 Curvature, Testing for 118 Curvilinear Distortion . . 28, 41 Dallmeyer's Portrait Lens 53, 72 Dailmeyer's Rapid Recti- linear 45 PAGE. Dallmeyer's Rectilinear Landscape Lens 39 Dallmeyer's Tele-photo Lens 135 Dallmeyer's Triplet 65 Dallmeyer's Wide Angle Rectilinear 48 Dallmeyer's Wide Angle Single Lens 38 Dark Corners 46, 107 Darlot's Lenses 109 Definition 17 Dennis Taylor's Triple Lens. 137 Depth of Focus 28, 89 Diaphragm, influence upon Distortion 24 Diaphragm, Waterhouse 27 Diffusion of Focus 53, 72 Direct Portraits, Large 41 Distortion 22, 41, 119 „ Avoidance of . . 101 „ in Portraiture . . 75 Double Anastigmat 60 Effect of Focal Length on Angle 94 Effect of Stopping Down . . 27 Enlarging, Lenses for 79 Equalising Focus with Swing Back 105 Equivalent Focus 14 Eury scope Lens 120 Exa(:i,'gerated Perspective. . 25,75 Extra Caps 80 Field of Lens 99,117 "Fixed" Focus 29 Flare 27, 120 Flatness of Field .... 61, 78, 110 Focal Length 13,75, 93 Focimeter 116 Focussing, Effect of Stops upon 78 Focussing Screens 87 ,, Glass 88 Foreign Lenses 125 Utility of .. 126 Forged Lenses 132 „ „ Detection of 133 Functions of Diaphragms . . 28 142 PHOTOGRAPHIC LENSES. PAGE. • Goerz Double Anastigmat. . 59 Ground Glass 87 Groups 54, 55, 72 Grubb's Single Lens 37 Harrison's Globe Lens . . 47, 64 How to Focus 85, 90 Improving Focussing Screens 87 Incorrect Centeriner 121 Intensity Ratio of Stops 32 Interchanging Combina- tions 50, 113 Introduction 8 Iris Diaphragm 27 Jena Glass 45, 57 Kew Test 130 Large Heads 73 Lenses for Copying 78 ,, Er)larging 79 ,, Landscape Work 76 Lengthening Focus 89 Light, Theory of 8 Marking Stops 31 Mechanical Defects j 122 Non-registration 86 Non-symmetrical Doublets . 109 Obsolete Lenses 65 " One Lens " Photographers 70 Orthoscopic Lens 63 Packing Lenses 80 Periscopic Lens 47, 65 Perspective 96, 97 Pictorial Definition 89 Pin-cushion Distortion 24 Place's Shutter 81 Portable Symmetrical 49 Portrait Lens 51, 72 Portraiture, Lenses for 72 Position of Diaphragm 111 Preface 3 Properties of Light ... - 9 Protractor. Use of 98 Purchasing Second-hand Lenses 183 Rapid Rectilinear 44 Rapidity 30 Rectilinear Landscape Lens 39 Refraction 8 Remedy for Flare 121 PAGE. Rising Front, Utility of 107 Ross' Actinic Doublets 47 ,, Portable Symmetrical. 49 „ Concentric 58 Rules for Focussing 90, 91 Scratches 123 Second-Hand Lenses 132 Single Lenses 35,41 Single Lens, for Portraiture. 73 ,, Landscape . 77 Smith's Table of View Angles 98 Solar Spectrum 8 Spherical Aberration 19 „ „ Testing for 120 Steinheil's Rapid Aplanat . . 44 „ Wide-Angle Ap- lanat 48 StriBB 122 Substitute for Cap 81 Supplementary Lenses 112 Sutton's Panoramic Lens . . 47 Swing Back, Use of 101, 104 „ Use of in Por- traiture 105 Table of Rapidity 56 Taylor's New Triple Lens . . 137 Taylor's Patent Adapter 82 Tele- photographic Lens 135 Testing Lenses 115 „ for Coincidence of Foci 116 „ Covering Power. . . , 117 „ Curvature of Field.. 118 „ Distortion 119 „ Flare 120 „ Incorrect Centering 122 Striffi 123 Tilting the Camera, Effect of 103 Triplet Lens 65, 78 Unequal Illumination 46 Uniform System of Stops . . 33 Universal Adapters 81 Lens 54 „ Mounts Ill Use of Back Combinations 41, 111 Verticality of Lines 102 Voigtlander's Collinear Lens 140 Wedge for Testing Register . 86 Wide- Angle Rectilinear 48 Wiry Definition 72 Zeiss Anastigmatic Lenses.. 59 Zeiss, Flatness of Field of . . 119 PHOTOGRAPHIC LENSES. A third of a century's reputation for perfect optical construction and unrivalled workmanship. DALLMEYER UJ I H I- O H UJ CO 5? z o > I H m ;^ CO ^1 m > - CO CO LENSES New Illustrated Catalogue of Lenses and Apparatus sent gratis and post = free on application. J. H. DALLMEYER, Ltd., „.Xt.„ 25, Newman Street, London, W. PHOTOGRAPHIC LENSES. The 'Lund' Library of Photography. These books cover collectively a wide field, and include not only text books on the numerous branches of both art and science, but also volumes presenting photography in a popular aspect. Those dealing with the technical side aim at plain wording and explicit teaching. Uniform style. Crown 8vo. size. In boards, 1/6; in cloth, 2/0. The following have already been issued : — The Stereoscope and Stereoscopic Photography. Translated from the French of F. Deouin by Matthew Surface. 180 pages. More tlian 100 illustrations. " The matter is dealt with very th.oro\vj,hly " ~ Photographic Beviev) of Reviews. "Interesting and entertaining tliroiigliout — PIiotogru]))ii/. The Half-Tone Process. A Practical Manual of Photo-Engraving in Half-tone on Zinc and Copper. By Julius Vejifasser. 126 pages and 59 illustrations, with three full-page blocks in half-tope by the Author. " The whole proce-s is h*^re described in consi ierable detail." — Literary World. " This clear and concise demonstration of haU-tone process, as evolved by Mr. Verfasser, is sufficient, in our opinion, to give any ordinary intelligent person a very good notion ' f the general principle involved." — invention. The work should (enable the veriest ty o t'< perform the necessary operations in making a half-tone cut. Well illustrsLtad.'" — Photographic Times. Photographic Lenses: How to Choose and How to Use. An Elementary and Practical Guide to the selection and use of Photographic Objectives. By John A. Hodges. 140 pages and 34 original illustrations, including eight half-tone engravings. The following is in active preparation : — Sensitized Papers: How Made and Used By Dr. H. C. Stiefel. All about making, printing, toning, fixing, developing, and everything else of interest in connection with sensitized papers. Many others to follow. Full List of Photographic Publications revised to date on application. PHOTOGRAPHIC LENSES. "The Junior Photographer" P.,cB6D .^cH. Series of Handbooks. These volumes are the first of a series dealing with the Elementary and Popular Side of Photography. The books are got up in unique style, freely illustrated, and supply a need long felt for cheap literature on photography, suited to the purses and requirements of beginners Now Ready. No. 1. WOO sold on day of publication. Snap-Shot Photography; or, the Pleasures and Advantages of Hand-Camera Work. By Martin J. Harding. With 20 Illustrations in Half-tone from the Author's Photographs. " This booklet has the charm of making the reader long to devote himself to so fascinating a pursuit. ' — Printing Times. " More than fulfils the promise of its title." — Wilson*s Photographic Magazine. Now Ready. No. 2. The Dark-Room and its Equipment. By H. J. L. J. Masse. Principal Contents : — Fitting up a Dark-Eoom — Lighting: Day, Oil, Gas, Electric — Ventilation — Chemicals and Bottles required — Weights and Measures — Home-made Apparatus and Appliances — Notes on some Developing Formulas — Photographic Poisons and their Antidotes — Storing of Plates and Films. " In it the inexperienced photographer will find much to make easy his path in the acquirement of the special knowledge which is requisite to make an adept, while the would-be photographer of limited means will find useful hints which will enable him to make inexpensive substitutes for many things required in the exercise of the art." — Puhlishe. s' Circular. Now Ready. No. 3. Lantern Slides: Theip Production and Use. By J. Pike. Contents : — Introductory — A Consideration of Negatives — Printing by Contact — Printing by Keduction — Printing by the Carbon Process — Clouds — Exposure and Development : Formuhie — Toning and Intensification — Mounting and Finishing — Conclusion. Many others to follow at intervals of about six weeks. OF ALL DEALERS. Percy Lund & Co., Memorial Hall, London, E.G. Full List of Photographic Publications, revised to date, on appli; all: n. PHOTOGRAPHIC LENSES. PERKEN, SON, & PAYMENT, "^iSd^' LONDON, MANUFACTURE "OPTIMUS" RAPID EURYSGOPE LENS. The Aperture is F/6. The Lenses are of Special Optical Glass, constructed with the nicest precision of curvatures, so maintaining good marginal definition, coupled with the most extreme rapidity. 5x4 6x5 7x5 8x5 9x7 10 x8 63/- 78/- 94/6 110/- 126/- 220/- 380/- OPTIMUS" WIDE-ANGLE EURYSGOPE LENS. F/9*5o This Aperture is exceedingly open for wide-angle work. The definition, however, is in no way sacrificed, as the curvatures are most perfe<5tly accurate, and the minute detail in archite<5tural and interior subjedts is rendered with the maximum of crispness, and a total absence of distortion, To cover Price 5x4 7x5 9x7 10 X 8 63/- 94/6 126/- 220/- 380/- OPTIMUS " RAPID RECTILINEAR LENS. Aperture F/8. Second only to the above for rapidity, therefore well suited for instantaneous effedls, outdoor groups and views, as well as interiors ; copying and enlarging are also within the capabilities of this lens - in fa(5t, its work may be styled Univeksal 5x4 6x5 7x5 8x5 9x7 10x8 12x10 15x12 18x16 36/- 50/- 56/- 70/- 90/- 142/- ISO/- 225/- 275/- OPTin/iUS" WIDE-ANGLE SYMMETRICAL LENS. Aperture F/16. Specially adapted for Architedlure. Can be used to advantage when very close to the subjedt. 5x4 7x5 12x10 15x12 18x16 42/- 58/- 90/- 142/- 180/- 225/- 275/- " OPTIMUS " QUICK-ACTING PORTRAIT LENS. Aperture F/4. Specially constru<5led for short ex- posures in Portraiture. They are second to none, definition being maintained by their perfeft optical qualities. 4^ in. Focus, Carte-de-Visite, i B, diameter 2 in. 90/- 2i in. 120/- 3i in. 180/- 5I in. ,, Cabinet 2 E, 9 in. ,, Grand Cabinet, 3 B, Catalogue of Apparatus POST FREE 99, Hatton Garden, PERKEN, SON, & RAYMENT, YiSasS LONDON. PHOTOGRAPHIC LENSES. Percy Lund & Co.'s Publications ....