TELEPHOTOGRAPHY CYRIL F, LAN-DAVIS. no CORNELL UNIVERSITY LIBRARY FROM 'isy. Jepji v:c D'^r-i'^tt Cornell University Library TR770 .L24 olin 3 1924 030 698 264 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924030698264 TELEPHOTOGRAPHY SOME USEFUL BOOKS LIGHT CAMPING KIT, AND HOW TO MAKE IT. By Jaimes H. Wood. With 5o*Diagrams and Illus- trations. Cloth, i/- net (postage lid.). " Should prove Useful to beginners who contemplate making their camp kit at home. It is thoroughly practicable and ably and clearly i 11 H st rate d , " — Ca nip inff. FIRST STEPS IN PHOTOGRAPHY. Full In- struction in the Simplest Terms for the Beginner. By J. C. H. Wali.sgrove. Fi/ik and Revised Edition. Crown 8vo, paper, 6d. net (postage id.). PHOTOGRAPHY : A Manual for Beginners in the Art and Practice of Photography. By Rev. A. H. Blake, M.A. With Illustrations. Third Edition. Crown 8vo, cloth, i/- (postage 3d.). HOW TO USE A CAMERA. A Practical and Up-to-date Manual for the Beginner. By Clive Hol- land. Fully Illustrated. Crown 8vo, cloth, i/- (postage 3d.). PHOTOGRAPHY IN COLOURS. A Text-book for Amateurs. By E. Lindsay Johnson. With 8 Coloured Plates and numerous text Illustrations. Crown 8vo, cloth, 3/6 net (postage 3d.). London : GEORGE ROUTLEDGE & SONS, LTD. TELEPHOTOGRAPHY CYRIL F. LAN-DAVIS, F.R.P.S. WITH SIXTEEN FULL-PAGE PLATES AND SEVEN DIAGRAMS LONDON GEORGE ROUTLEDGE & SONS LIMITED New York : E. P. DUTTON & CO. f\7o'^^^^^ CONTENTS PAGE List of Plates vii Tnteodtjction . ix CHAP. I. The Scale of a Picture — ^Positive and Negative Lenses . . 3 II. Telephoto Lenses in Relation to distant Objects , 1 1 III. Some Commercial Telephoto Lenses — ^Variable Types — ^Pixed Types . . 23 IV. The Telephoto Lens in Relation to near Subjects — Exposure — Depth of Focus — Perspective — Telephoto - Micrography — The Swing-back . . 39 V. Simple Measurements and Calculations — Rules and Tables . 75 VI. Working Data 111 Index . 120 LIST OF PLATES 1. MoiTNT Kenya, British East Apeioa. — Telephoto- graph, taken from Nairobi, 90 miles away, with a Dallmeyer telephoto lens . . . . 2 2. HnnALAYA Peak op over 22,000 Feet Altititde. — Telephotograph with the Adon . . .9 3. Telephoto snapshot with the No. 2 Series, X Adon f/6 . 21 4. Steamer Dunmail on the Books at Robin Hood's Bay, Yorks. — Taken with an Adon at 5^' camera extension from the cliffs above. Exposure, yj,^ of a second. Time, 5 o'clock on an April afternoon . 37 5. John Wyclippe's Church at Lutterworth . . 62 6. Taken with the Adon telephoto lens from the same standpoint as the picture in Plate 5, at 17" extension. Exposure, 3 seconds . 63 7. Taken with the Adon telephoto lens from the same standpoint as Plate 5, at 26" extension. Exposure with 10-times screen, 16 seconds . . 69 8. Camsbricht, Perthshire. — ^Taken with an 8" positive, 100 feet away . . . . . .80 9. Camsericht, Perthshire. — Taken with the Adon at 11" extension, a quarter of a mile away. . . .81 vii LIST OF PLATES PLATE PAGE 10. Window. — Taken with the Adon with the baseboard tilted at 13° to the horizontal, focusing screen per- pendicular to the baseboard. Here the inner frame appears wider at the bottom than at the top . 90 11. Window. — Taken under the same conditions as Plate 10, but with the camera back vertical. Here the inner frame is wider at the top than at the bottom . 91 12. SoKEW. — Taken with the Adon, 24" distant from the object, at 11" camera extension. Two magnifications. Exposure, 2 minutes . . 96 13. Empebor Moth. — Taken with the Adon telephoto lens. Exposure, 10 minutes . . 107 14. Mont Blanc Range. — Taken from Argentiere, 12 miles away, with 7^" Dallmeyer Stigmatic . .109 15. Taken from the same standpoint with a 7J" Dallmeyer Stigmatic and 3" Zeiss negative. Exposure, 10 seconds . . 115 16. Taken from the same standpoint with a 7 J" Dallmeyer Stigmatic and a IJ" Zeiss negative. Exposure, 30 seconds 121 INTRODUCTION The outstanding feature of the telephoto lens is its power of giving large direct pictures of distant obj ects. A mountain twenty miles away appears insignificant in an ordinary photograpk. A tele- photograpli of the mountain may be made from the same standpoint, showing it thirty times as large as before. The telephoto lens stands, in fact, in the same relation to an ordinary lens that a telescope does to the unaided eye. Details which are quite invisible to an observer can be plainly shown on a telephotograph. In the picture on page 109, there is a general view of the Mont Blanc range. In the next two pictures on pages 115 and 121, taken from the same standpoint with a telephoto lens, the very ridges and depressions in the snow can be seen. One of the earliest examples of telephoto- graphy was the famous picture of Mont Blanc X INTKUDUCTIUW taken from Geneva, forty miles away. An even more wonderful telephotograph of Mount Kenya in British East Africa is reproduced on page 2. This was taken with a high-power telephoto lens from Nairobi, ninety miles away. Apart from these striking uses, the telephoto lens is of great value for photographs of subjects near at hand, and particularly for artistic portraiture. Those unnaturally magnified hands and feet which too frequently disfigure photo- graphic portraits are conspicuously absent from pictures taken with telephoto lenses. It may be added that there are important advantages to be gained by the use of these lenses for the re- production of jewellery, natiiral history speci- mens and small objects of all kinds, and for astronomical and, particularly, solar photography. In the following pages I have endeavoured to outhne the theory of telephotography in the hope that this little book may be of service both to those who are at present using telephoto lenses and to those many others who think of so doing. In order that the essential simphcity of the subject may be shown, an attempt has been made to explain the theory in plain lan- guage without resort to complicated diagrams INTRODUCTION xi and matliematical abstractions. A number of numerical examples have been worked out, but for their comprehension a knowledge of simple arithmetic only is required. The literature of telephotography comprises some half-dozen volumes, all of which, with the exception of Mr. T. R. Dallmeyer's Telephoto- graphy, pubhshed in 1899, deal more particularly with the practical side of the subject. Since the pubhcation of Mr. Dallmeyer's book, there have been important advances in the construction and application of telephoto lenses, but there has been no general exposition of the theory of telephotography. CYRIL F. LAN-DAVIS. 5 ^ o a ,^ n a W" CHAPTER I THE SCALE OF A PICTURE THE SCALE OF A PICTURE Photographers speak commonly of a quarter- plate or a half-plate lens, meaning lenses that will fit quarter-plate and half-plate cameras respectively. Such lenses are, however, quite arbitrarily named, as a half-plate lens can be quite well used on a quarter-plate, and frequently also the quarter-plate lens will work well on a Ijalf-plate. The distinguishing feature between them is the difference in their " focal length," this being the name given to the most important measurement in a lens. It is usual for the manufacturer to engrave the focal length on each lens, and to state it in his catalogue. Methods of determining focal length will be given later. If we compare the pictures of a house given, say, by a quarter-plate lens of 5" focal length with that given by a half-plate of 7^" focal length, both usual sizes, we see that the house is half as large again with the 71" lens as with 6 TELEPHOTOGBAPHY the 5", and a proportionately less area of ground is included. That is, if the image of the house be 2" high with the 5" lens, it will be 3" high with the 1\" ; and we may say that for objects a considerable distance away, the scale of the picture depends entirely on the focal length. For a picture four times as large as the 5" lens we shall then require a lens of 20" focal length ; for eight times one of 40", and in proportion. Now with ordinary lenses the distance from the lens to the ground-glass, generally called the extension or back focus, is nearly the same as the focal length. A Eapid Rectilinear or an Anastigmat of 20" focal length requires nearly 20" extension, so that to enlarge our picture four times compared with the 5" lens we require a camera four times as long ; for eight magnifications we should require a camera eight times as long, and in proportion. Clearly considerations of bulk and weight will prevent any considerable enlargement in this manner. To obtain large pictures at short extensions we require, then, a lens of great focal length, needing only small extension ; and these desider- ata are united in the telephoto lens. POSITIVE AND NEGATIVE LENSES All lenses used for photography, such as Rapid Rectilinears, Anastigmats, and Portrait Lenses, give a real image on a piece of ground- glass placed at their focus. Such lenses are classified as " positive " lenses. With the lenses used in opera-glasses and diminishing-glasses a real image on a piece of ground-glass cannot be obtained, and these lenses are classified as " negative " lenses, and are said to give " virtual " images, such as those in a mirror. Long-sighted people wear spectacles with positive lenses, and short-sighted ones those with negative lenses. The two types may be easily distinguished by holding them a httle way from the eye and noticing that objects seen through positive lenses are blurred and indistinct, whilst objects seen through a negative lens are quite sharp, but less than natural size. If, now, we combine a positive lens with a suitable negative, we obtain another positive 8 TELEPHOTOGRAPHY lens, which may be used for photographic and other purposes. This is the construction of the telephoto lens, as rediscovered almost simultaneously in 1891 by Thomas R. Dallmeyer in England, A. Duboscq in France, and Dr. A. Miethe in Germany. Major- General Waterhouse has shown that the idea in connection with telescopes is as old as the time of the great astronomer Kepler, who flour- ished in the early seventeenth century. The first practical telephoto lenses were, however, not constructed until 1890 and 1891, since which time several different systems have been invented. ■ ^^1 H H jl^K^ ^^^^^^^^1 w fei^^'^^ 1 ■* * *-„ .-„» v^ B «^^^Hfli^^l^^^fll i>H{; ^m^iflR ^P^l M^M HRHM^BI^I Plate 2. — Himalaya Peak of over 22,000 eeet Altitude. — Tele- photograph with the Adon. (Kindly lent by Dr. W. Hunter Workman.) CHAPTER II TELEPHOTO LENSES IN RELATION TO DISTANT OBJECTS u TELEPHOTO LENSES IN RELATION TO DISTANT OBJECTS Positive lenses, as we have just seen, require extensions practically equal to their focal lengths, object and image being formed on opposite sides of the lens. Conversely, negative lenses require no extension at all, the object and the virtual image, which cannot be seen, being formed on the same side of the lens. When, now, positive and negative lenses are combined to form a telephoto lens, the result is that the focal length of the telephoto lens is much greater than the extension ; and this is the chief advantage of the telephoto lens over ordinary lenses. This disproportion between the focal length and the extension depends on the ratio between the focal lengths of the positive and negative lenses, or the "power" of the system. If, for instance, we take a positive of 5" focal length and. a negative of 2^" focal length, the ratio of 14 TELEPHOTOGEAPHY the focal lengths of the positive and negative is 1:2. Then with 7^' camera extension we obtain 20" focal length. If we change the negative to one of 1" focal length, making the ratio 1 : 5 instead of 1:2, the focal length at 7V' camera extension is 421" instead of 20". Now the scale of the picture depends on the focal length and not on the extension. If we focus a distant group of houses with an ordinary lens of 8" focal length, requiring 7V extension, and select one house the image of which is 1" high, then on substituting the telephoto lens, composed of the 5" positive and 2i" negative, for the ordinary lens, we should find that the image would be \", or 2|" high ; and with the telephoto- lens, composed of the 5" positive and V negative, the image would be 's-*, or 5-iV' high. It is usual to speak of the difference in scale as a number of " magnifications " or " diameters." Comparing the 8" ordinary lens wiibh the first telephoto lens, we may say that the number of magnifications or diameters is 2J, and that the second telephoto lens gives 5tV magnifications. In every case the magnification is the linear ratio of the size of picture giien by one lens to DISTANT OBJECTS 15 the size of picture given by another, this being the same as the ratio of their focal lengths. We can therefore say that the telephoto lens of 20" focal length gives four magnifications compared with its positive only of 5" focal length. Here the word " magnification " apphed to a telephoto lens means magnification compared with its own positive ; but when another lens is also mentioned, it means the ratio of the sizes of image or focal lengths of this lens and the telephoto lens. Clearly, then, by increasing the ratio of positive to negative we could get any magnification we pleased with a short-extension camera. But the positive and negative lenses must be separated a certain distance to give the required focal length, and this separation depends also on their focal lengths. The more we increase the ratio of positive to negative, keeping the extension fixed, the greater is the separation, a,nd consequently the total length of the lens. For instance, at 7J" extension with the 5" positive and 2^" negative, the separa- tion for 20" focal length is 2-125", and at the same extension with the 5" positive and V negative the separation for 42^" focal length is 16 TELEPHOTOGRAPHY 4:1". Moreover, as we decrease the focal length of the negative we necessarily also decrease its diameter ; so that with a given positive, increased ratio between the focal length of the positive and negative means decreased covering power and illumination, and conversely a lower ratio means increased covering power and illumination. Again, variation in the separation of the positive and negative lenses alters the focal length, extension, and covering power con- siderably. Taking again our 5" positive and 2|" negative, we can draw up a table to show the focal lengths and extensions at various separations. Separation of Elements. Extension. Focal Length. Magnifica- tion. Inches. Inches, Inches. .5 5 1 3| 2i 10 2 3* 31 12* ^ 3-1 7l 20 4 3 10 25 5 2-8 IH 40 8 2f 22| 50 10 2i Very large Very large Very large It will be noticed that very small variations in the separation make large differences in the DISTANT OBJECTS 17 extensions and focal lengths. If a greater sep- aration than 5", which is equal to the focal length of the positive, were tried, the ordinary picture given by the positive would be formed in its normal position, and the rays of hght would meet the negative lens beyond this position. We should then have a negative lens looking at a real object ; and this, as explained on p. 7, does not give any real image. As we decrease the separation the focal length and extension increase, until at iy separation they are both very large — that is, larger than any finite value we choose to put on them. If we wish to get a focus of 1000" we require a separation of 2-512", for 10,000". 2-501", and for infinite focal length and extension 2J" separation. We see, then, that with a 5" positive and 2J" negative we can obtain a sharp picture at all focal lengths above 5", and at all extensions exceeding zero. It is simply a question of the extension which is available, and of suitable mechanical means for varying the separation. Further examination of this table shows that the focal length is always equal to twice the extension in use added to the focal length of 18 TELEPHOTOGRAPHY the positive. The reason for doubling the extension is that the ratio of positive and negative focal lengths, or the power, is 1:2. If this ratio were 1 : 5, we should have to multiply the extension by five and add the focal length of the positive. We should also in that case obtain the same range of focal lengths from 5" to infinity, and extensions from zero to infinity ; but the extensions and focal lengths would correspond differently. If we make our 5" positive of l^" clear aperture, then the f/no. or intensity at each extension is found by dividing IJ" into the focal length. 5 At 5" focal length the intensity is f yt, or f/4 ; „12^" „ „ „ f^^orf/10; „20" „ „ „ ffr, orf/16; so that as we increase the extension and focal length we necessarily get a less rapid lens, requiring increased exposure. Another way of putting this, is to say that our positive of 5" focal length and IJ" clear aperture works at f/4. At 2| magnifications the focal length has been increased 2h times, but the clear apertm-e is mialtered. The f/no. then, which. DISTANT OBJECTS 19 as before, is found by dividing the aperture into the focal length, has been increased 2| times, or the new f /no. is found by multiplying the stop of the positive by the magnification, i.e., ij^- by 2|, giving f/10 as the result. And f/10 requires not 2\, but six times as much exposure as f/4. Exposure, then, increases theoretically as the square of the magnification ; but for distant subjects taken without a colour-screen, a rule of practice, founded solely on this observation, will generally lead to over-exposure. A safe working rule is that exposure should be increased as one-half the square of the magnification. The apparent disagreement between theory and practice is due to the greater actinic value of distant subjects. When a properly adjusted colour-screen is used, the full exposure may be given, as described more fully on pages 119 to 124. Again, as we increase the extension and focal length of our telephoto lens we increase the covering power. At a small extension only a small plate is covered, and at long extension a large plate is covered, the covering power increasing in proportion to the extension. The 20 TELEPHOTOGRAPHY field of view included scarcely varies, but the size of each individual object increases. If our film or plate increased equally, the field of view would be httle altered ; but as the telephoto lens is generally used on a camera with adjust- able extension but fixed size of plate, the field of view steadily decreases as the magnification increases. In connection with this subject of covering power, it must be remembered that increase in the separation of the elements decreases the covering power, so that the field of view at 2^' extension will be small. But using the lens on a quarter -plate, it may be said that with extensions of 5" and upwards, the field of view decreases in proportion as the magnification increases. B^ ^m^m "J HIp^^^B ^^^^^H .; .-.:?, ^^^^^^^^H^ 1 ^ ''^y^^H ^^^^^^^^^^^^^^B -<»•■, ,■■. ^^^HjHIt''- ^^H I^K^'; .^***^^ tj fllH^B •. v.- V ^^ft! '^ ■l^% ^'' i ^^^M^:-'"-''f''-''- ■g^:^'^ * P ■w ♦^-H^^-^r;^ ^^^JMH hi^'^ilB^Jt^.- ^i^S MldH iKilhiiiSliiilli *^^ v^^^^ a mnii^ Plate 3. — Telephoto snapshot; with.the No. 2 Series X Adon f/6. CHAPTER III SOME COMMERCIAL TELEPHOTO LENSES- VARIABLE TYPES— FIXED TYPES 23 SOME COMMERCIAL TELEPHOTO LENSES- VARIABLE TYPES— FIXED TYPES The earliest telephoto lenses experimented with consisted of simple positive and negative lenses, the separations between which could be varied. They gave high magnification and produced some remarkable results, but they were difficult to use. The Simple Telephoto Lens patented by T. R. Dallmeyer in 1892 was of this type, but it was soon discarded for a more perfect system in which the positive was a well-corrected photographic lens, and the negative was composed of a pair of doublets. This Telephoto Attachment can be fitted to any good photographic lens working at a reason- able aperture, the whole then forming a telephoto lens of variable focal length and covering power. In the illustration, a quarter-plate Dallmeyer Stigmatic of 5-3" focal length working at f/6 is shown combined with one of these attachments of 2 J" focal length. This telephoto system 26 TELEPHOTOGRAPHY covers a quarter-plate at extensions over 5h", and at this extension gives a picture on three times the scale given by the Dallmeyer Stig- matic alone, and at one-third of the intensity. At 10" extension the magnification is five diameters, the intensity correspondingly reduced to one-twenty-fifth, and the area illuminated sufficient for a half-plate. Any magnification Fig. 1. over three diameters may be obtained by choosing a suitable extension. If a 3" negative be substituted for the 2^", the new telephoto system will cover a quarter- plate at extensions over 4f ", and at this extension will increase the scale of the picture 2| times, thus showing that the lower -power system has greater covering power for less magnification. This combination is to be preferred for flower studies and portraiture when only a Uttle greater focal length than usual is wanted. For COMMEECIAL TELEPHOTO LENSES 27 a good-sized head-and-shoulders portrait, one can with this lens keep ten to twelve feet from the sitter, and thus avoid the unnatural disproportion in the size of the features so apparent in photo- graphs made with lenses only a few feet from the subject. If a li" negative be fitted in place of either the 2|* or 3", very much higher magni- fication is obtained, and the least extension at which the plate is covered is proportionately lengthened. A negative of this size may either be manufactured in the ordinary way, or it may be built up of other negatives placed in contact. Thus the 2" and 3" negatives used together give this focal length. If the camera be used at a fixed extension of say 10", then with the 3" lens we get four magnifications, with the 2h" five magnifications, and with the two together ten magnifications. Captain Owen Wheeler has gone even farther in this direction by combining three or four negatives, though the loss of light by absorption and reflection with such a com- bination may be considerable. In order to simplify the calculations for exposure, many makers engrave a scale of magnifications on the telephoto tube. This 28 TELEPHOTOGRAPHY method, if the figures are not too small to read, is sufficient where the lens is used for distant subjects ; but the figures must not be relied on for near subjects, as the conditions are entirely altered by distance. An alternative method is the use of the Dallmeyer Calculator, a small spring tape-measure which is marked with the number of magnifications corresponding to each extension. A third method is sometimes adopted in which the " optical interval " is engraved on the lens mount. The magnification is then ob- tained by dividing the scale reading of the optical interval into the focal length of the negative. Telephoto Attachments are made by the majority of opticians, including Messrs. Ross, Dallmeyer, Voigtlander, Zeiss, and Goerz, to suit their particular types of lenses. It is not, however, every positive lens that can be satisfactorily used in this way, as the negative lens magnifies the aberrations of the positive. Chromatic aberration, for instance, the presence of which means that the visual and photographic images do not coincide, increases as the magni- fication. Thus some lenses which are reasonably well corrected for ordinary work fail altogether in telephotography. COMMEECIAL TELEPHOTO LENSES 29 For this reason the best results are to be obtained with complete telephoto lenses, the aberrations of which have been balanced through- out the system. The best known of this type is the Adon Telephoto Lens, invented in 1899 and since greatly improved, optically and mechani- cally. Fio. 2.— (Full size.) This lens is of far simpler construction than the system just described. The front portion is a cemented doublet of 4|" positive focal length, and the back a cemented doublet of 2" negative focal length. An iris diaphragm is fitted to the aluminium mount, and the separation between the elements can be varied by rack and pinion movement. For greater portabihty the front 30 TELEPHOTOGRAPHY is carried in a light sliding tube, which should be pulled out before the lens is used. The entire Adon weighs only 5 oz., and is thus suit- able for light hand-cameras, as well as for instru- ments of long extension and substantial build. The ratio of the focal lengths of the components is a little over 1:2, with the result that for medium extensions the Adon gives a picture on three times the scale of an ordinary lens used at equal extension. It covers plates of from 3o X 2| in. to 15 x 12 in. in size, provided there be camera extension available. The focal lengths, extensions, intensities, and magnifica- tions compared with 5" and 8" ordinary lenses are shown in the table on p. 31. A model of the Dallmeyer Calculator marked with the f /no. and focal length at each extension is made, by the use of which all calculations and tables are dispensed with. The Adon can also be obtained with a higher ratio of positive and negative, permitting of higher magnification in proportion to extension. The Adon telephoto lens is really a reversion to the earlier forms, with the important difference that the ratio of the focal lengths of the com- ponents is low. It was the striving after COMMERCIAL TELEPHOTO LENSES 31 abnormal magnifications and striking results which for so long prevented telephotography from being successful. Apart from the light weight and small size of the Adon, it has the important advantage of possessing no internal, reflecting surfaces. If an anastigmat and tele- photo attachment are used together, there are inevitably two or more such surfaces, and reflections from these surfaces have a consider- able effect on the brilliancy of the pictures. Camera Extension (measured] from "Adon" Flange). Focal Length. F/No. Linear Magnification compared with 6" Lens. Linear magnification compared with 8" Lens. Inches. Inches. Diameter. Diameter. 4 12 10 2 5 16 13 3 ■2 6 18 15 7 20 17 "i 8 22-5 19 9 24-5 21 5 10 27 22-5 11 29 24 "6 12 31-5 26 4 13 34 28 i 14 36 30 15 38 32 16 40-5 34 8 17 43 36 18 45 38 9 19 -17 39 6 20 49-0 41 10 32 TELEPHOTOGKAJr'liY On the other hand, there is more distortion with simple systems than with the more com- phcated types. But this distortion appears only at the edge of the field, and its undesirable effects can be entirely avoided by using the lens at a shghtly longer extension. The Adon will, for instance, cover a quarter-plate at 5^" exten- sion. If, however, it be used at extensions of 8" and over, there is no distortion on the quarter- plate. The table on p. 31 shows that the intensities available range from about f/10 downwards. In favourable circumstances, snapshots may be made with the Adon, particularly on the sea- coast, where the light is strongly actinic ; but for general purposes the aperture is rather small. The Grandac Rapid Telephoto lenses partly overcame this difficulty. They work at apertures of f/10, and have yielded remarkable results in the hands of capable photographers, one of whom, Mr. A. E. Dugmore, obtained strikingly beautiful results with the Grandac in photo- graphing wild animals in Uganda. Another form of rapid telephoto lens was introduced by Messrs. Busch in 1905, which differed from previous types in that it worked COMMEECIAL TELEPHOTO LENSES 33 at a fixed extension and was of fixed focal length and rapidity. Busch Bis-Telars are made in a variety of sizes, and work at apertures of f/7 and f/9. Messrs. Zeiss make also the Magnar, working at f /lO ; and Messrs. Dalhneyer manufacture New Large Adons, working at f /lO, BRITISH MADE Fig. 3. f /6 and f/4-5. Each of these lenses works well only at one particular extension, and for distant subjects gives one degree of magnification only. By placing the subject close to the lens, pictures on a larger scale can, of course, be obtained. This fixed-focus type is just as simple to use as an ordinary lens of the same aperture. There 3 34 TBLEPHOTOQRAPHY are no calculations whatever to be made. The f /6 New Large Adon, for instance, is attached to the camera, focused on the ground-glass, and exposures are made just as for any other lens at f/6. The focal lengths in the several series are from 2 to 4 times as large as the extensions, and the pictures accordingly are on a scale of from 2 to 4 times the usual size. But to obtain the same rapidity as ordinary lenses at the same extension, the glass diameter must be increased in a like proportion. Rapid telephoto lenses are therefore of necessity larger and more expensive than ordinary lenses, and they have also less apparent depth of focus. They are therefore particularly suitable for reflex cameras, where the image may be focused up to the moment of exposure. The ratio of the components varies in the lenses of different makers, but a high ratio always means a large picture in proportion to the extension, and, either medium rapidity and small size, or high rapidity and large size. The No. 4 Series VII. i-plate New Large Adon, for example, of 20" focal length, rapidity f/10 and extension 8", is of the same size as the No. 2 Series X. J-plate New Large Adon of 12" focal COMMEECIAL TELEPHOTO LENSES 35 length, rapidity f/6, and 5|" extension. The front of each of these lenses is mounted in a light shding tube which is pulled out in use and closed up for packing. The lenses closed, measure only 3|" and 3f " respectively over all, and they screw into a flange of 2" inside diameter. In many cameras, particularly those designed for films, the removal of the entire lens is dif- ficult. The front combination, however, may be easily removed and replaced by the Junior Adon Telephoto lens. When the back lens and " Junior Adon " are used together, a magni- fication of nearly two diameters is obtained with short camera extension, whilst the intensity is almost as great as that of the original lens. There is a shght loss, but not such as causes any serious prolongation of exposure, although it is desirable to ensure even illumination by doubhng the exposure. With greater extension, higher magnification is given. The Junior Adon is of the fixed-focus type, and must be set for a definite camera extension. It is best used where the extension available is 6" and over, as in the 3A and larger Folding Pocket Kodaks. At shorter extensions the entire film is not covered. 36 TELEPHOTOGKAPHY A simple telephoto lens which will show many of the properties of this construction may be made of a pair of spectacle glasses, one positive and the other negative. If these are momited in cardboard tubes, and one tube arranged to shde over the other, the increase in focal length with decrease in separation and intensity may be immediately observed. Let us sum up the advantages of the telephoto lens for distant subjects. Large-scale pictures are obtained direct on small cameras, thus sav- ing the provision, of heavy and bulky apparatus. The size of the picture and the angle included can be varied at will without change of stand- point. Details previously invisible are shown in the photographs, and the trouble, loss of quahty and granular effect due to ordinary enlargement are eliminated, though telephotographs may be subsequently enlarged to get still higher mag- nification. ^ te M ■< dS m "13 ^-1 CO ■S U o o I 1^ 1 o o3 a o ,ii 13 P^ -2 § ^ s "u i oT M p^ O H j^ T3 f] O -)j |z; g O c o > ■^ «>-< "9 to i o a ^ PM TELEPHOTO-MICROGRAPHY Small objects, such as insects, articles of jewellery, screws, and the like, may be re- produced in natural size or on an enlarged scale with much less extension than is needed with a positive lens alone. The subject may also be placed farther away and natural perspective preserved. Using the combination of 5" positive and 2V negative for an object 20" away, an extension of 5" only gives the image in natural size, and at an extension of 12|" the image is twice as large as the original. At 10" extension, with the object 10" away, the image would be on five times the scale ; at 20" it would be on nine times the scale, and any enlarge- ment could be obtained, provided sufficient extension and separation between the lenses were available. With a positive only, we should have to use a lens of 2J" focal length to give the image in 5 66 TELEPHOTOGEAPHY natural size at 5" extension, and the object would only be placed 5" away. For an image on five times the scale, the extension must be 15", and the object only 3" in front of the lens. The illumination with the lens so close to the subject would be a serious problem, whereas with the telephoto lens the increased working distance makes it easy to arrange satisfactory lighting. The same advantages of decreased extension and improved perspective, as were found when photographing on a reduced scale, are thus obtained by the use of the telephoto lens for enlarging. The depth for equal glass diameters is much greater and the focusing is considerably easier, but for equal exposures there is no ad- vantage in depth, as explained on p. 54. With "fixed camera extension one may obtain any degree of enlargement by bringing the object close enough and adjusting the separation of the elements. To get the greatest magni- fication on a particular camera, the bellows should be fully extended, the elements separated as widely as possible, and the object moved until the image is sharp. Any further magni- fication can then only be obtained by an TELEPHOTO-MICROGRAPHY 67 increase either of the separation or of the extension. Exposures are best found by trial and error, as conditions vary so much. A guess as to exposure should be made and the negative developed. If under-exposed, twice the previous exposure should be given, or if over-exposed, half should be tried. Successive exposures ought generally to be in the ratio of 2 : 1, as the effect of less proportionate increase or decrease can seldom be seen. Three or four trials will thus enable the photographer to get a good idea of the correct exposure. When the correct exposure has been found for one size of picture, it may be easily calculated for any other size according to the following rule : The exposure increases directly as the square of the magnification, and in- versely as the square of the distance of the object from the lens. For example, with our 5" positive and 2J" negative used at 10" extension, an object 17|^" from the front lens is reproduced at twice its natural size. With the same extension the object must be placed 10" away in order that its image may be five times as large, and the 68 TELEPHOTOGRAPHY separation of the elements must be increased sufficiently to focus the picture. If at 2 magnifications the object required 1 minute exposure, the new exposure = £), 1-^x^ = 2 mmutes. Had we increased the size of the picture by altering the extension and distance of the object without altering the lens, we should have had to place the object 16" from the front lens and in- crease the extension, the new exposure then 5^ 16^ , . = 2-2 yju = 5 mmutes. There is thus a decided advantage in in- creasing the size of the image by adjustment of the lens rather than by alteration of the extension. The illustration on p. 97 was taken with an Adon at 24" distance. The extension was 11", the separation of the lenses 4", the magni- fication nearly two diameters, and the exposure in poor hght 2 minutes. Focusing was done roughly by moving the object, and finalty by the rack and pinion on the lens. T3 ^1 TO +J OJ I ; >, C-; «> -^ a ^ 1=1 S ^ Ph THE SWING-BACK One of the earliest applications of the telephoto lens was to architecture. Fine work on the ceiKng, sculptural figures high up on the walls, gargoyles and other interesting details, must necessarily be photographed at a considerable distance. The telephoto lens gives the subject on a satisfactory scale, and enables the photo- grapher to make a picture of an isolated piece. Good general views of a cathedral are fre- quently difficult to obtain, owing to the proximity of other buildings. A more 'satisfactory picture can, however, often be obtained a mile or more away, when the smaller buildings and trees no longer obscure the view. The most suitable position can then be chosen, and the lens modified to include only that portion of the subject which is wanted. For professional work, records of the state of the walls, of the appearance and spreading of cracks in buildings, of the permanence of the 72 TELEPHOTOGRAPHY bricks, can be admirably made with the help of the telephoto lens. The resulting photographs are hkely to be of great service in any disputes that may arise, and by the use of suitable colour-screens much additional knowledge as to the age of different parts may be obtained. The camera must usually be tilted to include the upper portions of high buildings. Messrs. E. A. & G. R. Reeve ^ have pointed out that when this expedient is employed there must be a departure from the ordinary practice of keeping the back of the camera vertical. If the back be kept vertical, divergent distortion is intro- duced ; that is to say, the image of a window is wider at the top than at the bottom. If the back is kept square to the baseboard, there is convergent distortion, but of small amount only. The correct position Kes between these two, as shown by Plates 10 and 11. In addition to this geometrical distortion, sHght pin-cushion distortion is also generally present. The best position in which to set the swing-back should therefore be found by ex- periment, but an absolutely true reproduction cannot be obtained with the camera tilted. A ' Brit. Journ. of Photography, Deo. 1910. THE SWING-BACK 73 square is transformed into a rectangle. This effect is not of great importance for ordinary work, but it should be remembered when exact measurements are in question. The best defi- nition is obtained when the back is square to the baseboard and the geometrical distortion is then small. To sum up, the advantages of the telephoto lens for near subjects are : the increased distance between object and lens giving improved per- spective, the reduced exposure consistent with equal depth of focus for objects at moderate distances away, and the high magnification obtainable with short extension. CHAPTEE V SIMPLE MEASUREMENTS AND CALCULATIONS —RULES AND TABLES THE MEASUREMENT OF FOCAL LENGTH. POSITIVES As the focal length of a positive lens is nearly the same as the extension, a good idea of its value can be obtained by measuring from the position of the diaphragm to the ground-glass, when distant objects are in sharp focus. For an accurate determination there are a large number of methods, one of which is as follows : Cut two pieces of paper exactly alike, say two rectangular pieces each measuring 3" x 1". Set one up in front of the lens at a distance of about double the extension for infinity, and focus on the ground-glass. The image will appear of nearly the same size as the other piece of paper, and it may be made to appear of exactly the same size by placing the first piece a httle farther away or a little nearer and refocusing. When image and object are of exactly equal size, a mark should be made on the 77 78 TELEPHOTOGRAPHY baseboard at the edge of the screen, and a distant object, a quarter of a mile or more away, should be focused by moving the screen, keeping the lens fixed. The distance between the two positions is the measurement of the focal length. When a number of lenses have to be measured it is convenient to arrange the test described above for a collimator. A piece of fine wire should be stretched in the focal plane of the collimating lens, and another piece of fine wire of a known length should be placed two or three inches in front of the collimating lens, the whole being illuminated by an electric lamp at the back. The image of the front wire should then be focused and the positions of the lens and screen manipulated until the image and object appear to be of exactly equal size. The wire at the focus of the colhmator is then focused by pushing in the ground-glass, and the distance between the two positions gives the focal length. c . <3^ PL, NEGATIVES As the image of an object formed by a negative lens cannot be received on the ground-glass, the focal length cannot be determined in the ways suggested above. An. easy method which gives a near result is as follows : Measure with a pair of compasses the clear aperture of the negative and describe ,on a card a circle with this measurement as radius. Place the negative in a beam of sunhght or in the path of the rays from a colHmator, and move the card until the disc of hght is of exactly this size. The focal length of the negative is then equal to the distance from the card to the middle of the glass. To obtain an accurate measurement, the negative should be fitted for use with a positive in adjustable mount. The size of image of a distant object thrown by the positive only is first measured. The negative is then attached and the system ad- 84 TELEPHOTOGKAFHY justed until the same object is four times as large. The position of the focusing screen should be noted and the separation between the lenses again adjusted until the new image is eight times as large as the original image, the negative not being moved. A quarter of the shift of the ground-glass divided by the size of image thrown by the positive is then an accurate measurement of the focal length. If there is not sufficient extension available, the magnified images may be made two and four times as large as the original respectively, in which case one-half the shift divided by the size of image thrown by the positive is equal to the focal length of the negative. SIMPLE CALCULATIONS DISTANT SUBJECTS FARTHER AWAY THAN ONE HUNDRED TIMES THE FOCAL LENGTH. I HAVE already mentioned the use of a Dall- meyer Calculator for the purpose of avoiding calculations when distant subjects are con- cerned. To those who desire to work out the results for themselves, two ways are open. All data must, of course, be taken in one standard of length, either the inch or the centimetre. First, we may treat the telephoto as an ordinary positive lens of known focal length. This method is excellent with systems like the Adon. (1) To find the focal length. Multiply the extension by the power of the system and add the focal length of the positive. (2) To find the intensity. Divide the focal length by the clear aperture. 86 TELEPHOTOGRAPHY For instance, with a 5" positive of IJ" clear aperture and a 21" negative and 10" extension, 5 Power of system = ^ = 2 Focal length = 2.10 + 5 = 25 25 Intensity = -, i = f/20 and this f/no. is available for use with any exposure meter. The covering power here is equal to the extension, so that a whole plate would just be covered. Secondly, we may consider that the image is formed first by the positive and is subsequently magnified by the negative. This method is better for a positive with telephoto attachment. (1) To find the magnification — (a) Divide the extension by the focal length of the negative and add unity ; or (6) Divide the focal length of the negative by the optical interval. (2) To find the intensity — , Multiply the f/no. on the positive by the magnification as found by either of the above methods. SIMPLE CALCULATIONS 87 (3) To find the focal length- Multiply the focal length of the positive by the magnification. Taking the same .example as before — 5" positive working at f/4, 2V' negative, 10" extension, and optical interval Y (a) Magnification = p; + 1 = 5 (&) -^ -5 Intensity Focal length = f/4x5 = f/20 = 5 x5 = 25 To find the circle of illumination, the .expres- sion on p. 95 should be used. When the power of the system is two, the diagonal of the largest plate illuminated is about equal to the extension. When the power is higher, a smaller plate is covered ; and, correspondingly, a lower-power system has more covering power. DISTORTION DUE TO TILTING OF THE CAMERA. i^B I^^H Plate 10. — Window. — Taken witli the Adon with the baseboard tilted at 13° to the horizontal, focusing screen perpendicular to the baseboard. Here the inner frame appears wider at the bottom than at the top. DISTORTION DUE TO TILTING OF THE CAMERA. Plaie 11. — Window. — Taken under the same conditions as Plate 10, but with the camera back vertical. Here the inner frame is wider at the top than at the bottom. NEAR SUBJECTS HOW TO FIND THE EXTENSION FOR A PARTICU- LAR SIZE OF IMAGE OF AN OBJECT AT A GIVEN DISTANCE It is possible to determine the magnification by treating the telephoto system as a positive lens, the positions of the principal points of which are known, but it is generally easier to spht the effect up into the work done by the positive and the negative separately. We therefore determine the magnification by the positive alone, and find how much this image is magnified by the negative. The positive magnification for a given lens depends only on the distance of the object from it. The negative magnification depends only on the extension used, and remains constant whatever the positive magnification and the distance of the object be. The principal points of the positive and negative respectively may be taken to coincide 94 TELEPHOTOGRAPHY with the lenses themselves, as only small errors are introduced by this assumption. Measure- ments from the object should be made to the centre of the positive, and measurements of extension to the centre of the negative. The circle of illumination depends on the extension, and on the position and diameter of the stop. In built-up systems such as that composed of a positive with telephoto attach- ment, the stop is usually placed incorrectly, with the result that the field illuminated is diminished by stopping down. In the Adon and the fixed-focus type, reduction of aperture has scarcely any effect on the circle illuminated. It is usual for the intensities of the positive and negative lenses to be equal, so that the ratio of their diameters is equal to the ratio of their focal lengths or to the power of the system, and this permits of the formulae generally given being considerably simplified. Let M be the magnification produced by the whole system, m^ and mg the positive and negative magnifications respectively, m the power of the system, fj and f^ the focal lengths of the positive and negative, d^^ and d^ their diameters, and v the extension. M and m. NEAE SUBJECTS 95 are greater than unity when the image is smaller than the object, and less than unity when the image is larger than the object. Distance of object = (mj + 1) /i Negative Positive magnification magnification ~ Total magnification mi_ V ^ Total magnification Positive magnification = x negative magnification Extension v = (mj - 1) f^ {m -l)v + f,{l+^) Separation of elements = m. Focal length ^mv±h Diameter of circle _ 2mv d^ covered at full m — \ /a aperture ■^ a o , '- 50 :: o o IW CHAPTER VI WORKING DATA m WORKING DATA Most cameras can be satisfactorily used for telephotography, provided that the lenses attached to them can be removed either entirely or in part. If the entire lens can be taken away, as is the more usual, any of the lenses described in Chapter III. may be fitted. Lenses of the fixed- focus type are very suitable for Reflex cameras ; whilst lenses such as the Adon, or a telephoto attachment fitted to the existing lens on the camera, can best be used on cameras with a considerable range of extension. It is an ad- vantage to have a rigid camera, as vibration destroys definition, particularly with high magni- fication. At the same time, it is not necessary to carry about the very heavy apparatus that was at one time thought necessary, provided that reasonable care is taken in manipulation, and ample time is given for the apparatus to come to rest after focusing and the hke operations have been carried out. For this 114 TELEPHOTOGRAPHY reason a simple shutter may advantageously be fitted. Medium magnification fixed-focus telephotos are used exactly as ordinary lenses, and no special precautions apart from care in focusing and avoidance of vibration need be taken. Fine instantaneous work may be done with them, as with ordinary lenses working at f/6, f/4"5, and similar apertures. The rest of this chapter refers, therefore, to lenses with which high magnification is under- taken. Vibration can in such cases be avoided by the use of a light strut, with one end on the front portion of the camera, the other end being attached to one of the legs of the tripod. This small additional support steadies the whole apparatus in a marked way. Focusing. — A focusing glass of fairly high power should be used in order to reduce the " accommodation " of the eye. The depth of focus is generally considerable, and for this reason focusing is best done by the rack and pinion motion of the lens. A very slight turn of the pinion makes a considerable difference to the image, whilst the camera baseboard may WORKING DATA 117 sometimes be moved backwards and forwards two or three inches without making any appreci- able difference. I prefer, therefore, to set the camera to a definite extension, giving the magnification wanted, and to focus by means of the lens. Mechanical arrangements, such as the Hook's Joint Handle, may be employed, or an assistant can turn the pinion. It is, however, only in unusual circumstances that very long extensions are wanted, as it is often simpler to use a short-focus negative and thus reduce camera extension. If, for instance, one is limited to an extension of 12" on which a tele- photo lens consisting of a 6" positive and a 3" negative is used, the maximum obtainable is 5 magnifications. Then by simply replacing the negative by one of 2" focal length, one increases the magnification to 7 diameters without alter- ing the extension ; and by using a negative of f of an inch focal length, one can increase the magnification to 17 diameters, without the need for at all long extension. Magnification up to 40 and 50 diameters is sometimes obtained in this way, and the short camera extension required makes this method very convenient. At the same time, the limits imposed by the 118 TELEPHOTOGRAPHY necessarily small diameter or high complexity of the high-power negatives should be remembered. Very finely-ground glass should be chosen for the focusing screen. Entirely clear glass is troublesome, as on clear glass the image looks sharp in all positions. Some years ago Mr. Douglas Carnegie ^ sug- gested a method of focusing which avoids straining the eyes. Eor this purpose, a clear disc should be left in the ground glass, over part of which a strip of thin brass or tinfoil should be stuck. The magnifier must then be set for the strip of brass and the picture focused approximately. A portion of the image which comes over the edge of the tinfoil should be selected, and the eye moved sideways across the field of the magnifier. When the object is in perfect focus, the part of the picture selected and the edge of the tinfoil will move together, whereas in all other positions one will move relatively to the other. Stopping down must frequently be resorted to in order to get sharp definition, and apertures as small as f /220 and f /360 are quite common. It has been said that no smaller aperture than '■ Brit. Joiirn. of Photography, Oct. 1907. WORKING DATA 119 f /71 should be used ; but Lord Rayleigh, who is usually quoted as authority for this statement, in a letter to the author disclaimed responsibility for the limit suggested. Difiraction, with consequent impairment of definition, may occur with small stops, but the point at which this occurs depends on the brightness of the object. A much smaller stop could be used for such a subject as a distant mountain, than for a luminous body Hke the sun. Exposure. — As with an ordinary lens, the important thing to ascertain is the f/no. at which the combination is working. In the fixed-focus type this f/no. is stated on the lens. In the case of the variable types this f /number must be ascertained in one of the ways already described. The f/no. so found may be used with any exposure meter, and the time of exposure for distant subjects obtained. The f /ratio for a telephoto lens is exactly the same as for an ordinary lens, and is used to determine the exposure in the same way. It will be remembered that the f/nos. run f/2, f/2-8, f/4, f/5-6, f/8, f/11, f/16, f/22, f/32, f/45, 120 TELEPHOTOGEAPHY f/64 f/90, f/128, f/180, f/256, f/360, etc., and that each f/no. requires double the exposure of the previous. If, for instance, 1 second be correct at f/4, 2 seconds are required at f/5'6, 4 seconds at f /8, 8 seconds at f /ll, 16 seconds at f/16, 32 seconds at f/22, 64 seconds at f/32, 128 seconds at f /45, and in proportion. For instance, the Adon at full aperture and at Q" extension, works at f/15. If the Wynne speed number of the plate used be 90, and if the time taken for the sensitive paper to darken be 20 seconds, then the correct exposure, as shown by a Wynne actinometer, is half a second. Having determined the exposure for the Adon at full aperture, the exposxire at smaller stops is obtained by multiplying the exposure so found by the number on the iris handle. If, for instance, one stops the Adon down to No. 4 stop, the exposure in the above case would be 2 seconds, at No. 16 stop it would be 8 seconds, and in proportion. Again, if we are using an f/8 positive lens at 4 magnifications the f/no. of the system is f /32- If the positive only required | of a second exposure, the telephoto at f/32 will require 4 seconds, just sixteen times as long. Ml at CD 02 -^ c8 13 O 4J ■43 <^ rf c d bD 5 tn © CO £-1 :3 i iH p^ X ^ H WORKING DATA 123 Although the exposure for the Adon deter- mined by the Wynne meter is half a second, the character of the subject must be taken into consideration. An exposure which is correct for an open view of the sea with ships would not be right for one of the ships alone, which probably would be all that was included in the telephotograph. The instructions given with exposure meters state that additional exposure must be given for dark objects in the foreground and the Uke ; and these differences should be borne in mind. Besides, the atmosphere is itself luminous and affects the plate, giving the effect of flatness. When a screen is not used, only one-half to one-third the calculated exposure should there- fore be given. H a suitable screen is used — and its use is to be recommended — the atmospheric haze is eliminated, and exposures according to calculation are correct. Dr. C. E. K. Mees, however, in a communication to the author, advises a sHght diminution from the calculated times. He observes that there is a lower range of contrasts in distant subjects by reason of the removal below the hmit of resolving power of most objects forming the extremes of contrast 124 TELEPHOTOGRAPHY in the subject, and that as exposures must be in proportion to the scale of contrast, the calculated times may be decreased. For subjects at moderate distances, the calcu- lated exposures are correct ; but for those quite close, trial and correction of error is the best way. The optical correction of the colour screens should be good, as defects are magnified by the negative lens. The screens should therefore be made of optically-worked glass, or of gelatine cemented between flats. Kj, Ko, Kg, and Gr screens are all satisfactory. Reasonable results can also be obtained by fitting less perfect screens to the back of the negative and stopping down. Focusing should always be done with the screen in position, as the interposition of the screen usually alters the focus. A long hood is of great advantage, because it cuts off much of the stray hght, which has a tendency to fog the plate. The hood thus increases the brilliancy of the pictures and im- proves the contrast. The simplest and most convenient form consists of a tube, or a set of tubes, sliding on the lens mount. These are entirely out of the way when not wanted, and WORKING DATA 125 they may be extended to a considerable distance in front of the lens. There are other forms also which consist of miniature cameras, and allow of very exact adjustment. Plates should be fully developed to make sure that all detail has appeared. Covering Power. — This term is sometimes used in two senses. It may refer either to the extent of field illuminated, or to the extent of field sharply defined. The maximum circle of illumination depends on the extension, and is sometimes diminished by stopping down. On the other hand, the field sharply defined is generally less than the circle of illumination, and may be increased up to this limit by reducing the aperture. To ensure .that a particular size of plate be illuminated, the diameter of the circle of illimiination must be equal to or greater than the diagonal of the plate. As stopping down in variable-type lenses may decrease the area illuminated, a somewhat longer extension than the minimum should be chosen. With lenses in which the focal length of the positive is double that of the negative, the diameter of the circle 126 TELEPHOTOGRAPHY of illumination at the largest stop is about equal to the extension, so that the area covered is very easily measured. This area depends on the extension, and is unchanged, whether the lens is focused for near or distant subjects. To make sure that the entire plate is covered, a lighted candle or other bright object should be passed across the field, and the apparatus arranged in such a manner that the image, even if out of focus, can be seen to the extreme corners of the plate. View-finders. — The angle included by a telephoto lens is small compared with that of an ordinary lens. The regular view-finder there- fore includes far more subject than actually appears on the plate. With the fixed-focus type of telephoto lens, it is quit^ easy to mask aown the ordinary finder ; but for the variable- focus the task is rather difficult, as the field included varies with the magnification. A very simple finder can be made out of a piece of brass rod fitted at one end with a small brass plate, pierced with a sight hole. Along the rod a rectangular wire frame can be fitted, and this frame can be moved backwards and forwards WORKING DATA 127 along the rod to show the field included at various extensions. Another method is the use of one of the view-meters, which consist of the lenses of an opera-glass reversed in position. In the Dalhneyer form the distance apart of these lenses can be varied in accordance with the magnification of the telephoto lens. Such a finder may be made to serve for a fair range of extension, but it can scarcely be employed for all focal lengths from zero to infinity. INDEX Aeeeeation, chromatic, 28, 59. spherical, 55, 59. Adon. 9, 29, 30, 31, 32, 33, 37, 63, 68, 69, 81, 85, 90, 94, 97, 104, 105, 106, 107, 113, 120, 123. ., Series VII., 33, 34. „ X. and XL, 21, 33, 34, 59. ., Junior, 35. Back focus, 6, 44. Bis-Telar, 33, 59. Busch, 32, 33. Cardinal points, 42. Carnegie, D., 118. Circle of illumination, 87, 94, 95, 99, 101, 105, 125. Collimator, 78. Colour screens, 19, 72, 123, 124. Covering power, 16, 19, 20, 26, 86, 87, 125. Dallmeyer. 28. T. R., 8, 25. Dallmeyer- Bergheim, 59, 60. Calculator, 28, 30, 85 Stigmatio, 25, 109, 115, 121. View-finder, 127. Depth of focus, 34, 51, 52, 53, 54, 55, 56, 57, 73, 114. Diameters, 14. Diffraction, 119. Distance of image, 47, 48. of object, 40, 47, 48, 93, 95, 99, 100, 101, 102, 103, 104. Distortion, 72, 73. Duboscq, A., 8. Dugmore, A. E., 32. Exposure, 19, 27, 49, 54, 57, 67. 68, 73, 119, 120, 123. meters, 119, 123. Extension, 6, 13, 16, 17, 44, 45, 47, 48, 50, 67, 68, 73, 86, 93, 94, 95, 99, 100, 102, 104, 117, 127. Field of view, 20. F/no., 18, 19, 86, 119, 120. Focal length, 5, 6, 14, 15, 16, 17, 42, 43, 44, 45, 46, 55, 85, 86, 87, 85, 102, 103, 125, 127. „ measurement of, 77, 78, 83, 84. „ points, 42, 44, 45. Goerz, 28. Grandao lens, 32. Hood, 124. Intensity, 18, 85, 86, 87. Kepler, 8. 130 INDEX Magnar, 33. Reeve, E. A. & G. R., 72. Magnification, 14, 15, 20, 27 45, Ross, 28. 4H, 47, 66, 73 ,86, Rules for calculations, 102, 103 87, 93, 94, 101, 104. 117. negative, 93, 94, Scale of image, 6, 14, 45, 102 95, 99, 100, 101. 103, 104. „ positive, 93, 94, Separation. 15, 16, 17, 48, 66 95, 100. 95, 99, 100, 104. Mees, Dr. C. E. K., 123. Simple telephoto, 25. Miethe, Dr., 8. Swing-back, 71, 72, 73. Negative lenses, 7, 13, 14, 15 17, Table for the Adon, 31. 25, 44, 83, 84, 86, 93, 94. ,, .. fornear sub- Nodal points, 42. jects, 106, „ of focal lengths, etc., 16. Optical centre, 43. Telephoto attachment, 25, 28, interval, 28, 86, 87. 113. micrography, 65-68. Perspective, 58, 59, 60, 65, 66, ' 101. Positive lenses, 7, 13, 14, 15, 25, 41, 44, 86, 93, 94. Power of system, 13, 18, 45, 86, 94. Principal points, 42, 43, 44, 46, 47, 48, 50, 55, 93. PuUigny and Puyo, 60. Rayleigh, Lord, 119. View-finders, 12(1 Virtual images, 7. Voigtlander, 28. 13. Waterhouse, Major- General, 8. Wheeler. Captain, 27. Wynne, 120, 123, Zeiss, 28, 3.3, 115, 121, J^rititcd I MoHRisoN & GiBB LiMrrED, Edinburgh. }Fith 167 Illustrations and 5 Full^iage Plates. Demy Svo, Cloth, 396 pages. Price I2S. fid. net {postage $d.). THE ADVANCE OF PHOTOGRAPHY ITS HISTORY AXD MODERJT APPLICATIONS By A. E. Gabrett, B.Sc, Fellow of the Physical Society of London, Fellow of the Royal Astronomical _ Society, Fellow of the Royal Geographical Society, etc. SOME PRESS OPINIONS. " An elaborate description of all kinds of photography, dealing with the subject from the historical, scientific, and practical point of view." " A (.ompreliensive work. The various aspects of photographic science are set forth and explained in a lucid and practical manner ; and the chapters devoted to the most recent developments in the applications of photography will be specially valued by skilled practitioners and students alike. The book constitutes an admirable text-book and work of re- ference of a high standard." — Scotsman. " This is a handbook for the serious scientific student, not for the one who merely desires the most superficial knowledge, but at the same time the points are clearly put, and the volume should be found on the shelves of most students. It is a sm'prisiag summary of what photography has done and may do, and should be a useful text-book for some time to come." — Morning Post. ' ' Mr. Garrett's practical and comprehensive volume is the latest word on all subjects relating to photography, and as fresh discoveries are suc- cessively made in photography, will be the standard work to be revised year by year." — English Mechanic and World of Science. " Will be welcomed by those who wish to master the principles that underlie one of the most fascinating pursuits of modern times." Chemical News. " The book is well and clearly written, elaborately illustrated, and thoroughly practical. " — Westminster Gazette. " An invaluable text-book of the subject, well-written, and easy to understand." — Liverpool Daily Post. " A useful text-book for those who desire to know the theory as well as the practice of modern photography. It provides a very complete sum- mary of the advances which have been made in recent years in photographic chemistry, optics, etc. , in surgery, and in book and newspaper illustration." Glasgoio Herald. KEGAN" PAUL, TRENCH, TRUBNER & CO., Limited, Bro.\dway HorsE, 68-74 Carter Lane, London, E.G. ROUTLEDGE'S COUNTRY BOOKS Cloth gilt, 7s. 6d. net; half morocco gilt, lOs. fid. net. Each with numerous Coloured Plates. "This unrivalled series." — Daily Telegraph. British Birds in Their Haunts. By the late Rev. C. A. Johns, F.L.S. Edited throughout by J. A. Owen. With 64 full-pago Coloured Plates (256 Figs.) by William Foster. ■Flowers of the Field. By Bev. C. A. Johns, F.L.S. New and Enlarged Edition with a special chapter on British Grasses. Edited throughout by Clarence Elliott. With 96 Coloured Plates (268 Figs.) by Miss E. N. Gwatkin. British Ferns and Their Varieties. By C. T. Dbubey, F.L.S. With 40 magnificent Coloured Plates, numerous Cuts in the text, and 96 Plates of Nature-Prints. British Fungi. By George Massee, of the Kew Gardens. With 40 Coloured Plates by Miss Massee. British Trees and Shrubs. Edited by E. T. 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