The Telephoto Lens in Cinematography 



WITH ESPECIAL REFERENCE TO ITS APPLICATION 

 TX THE PROP,LE:\rS OF BIRD PHOTOriRAPHY 



By NOR M A X M c C' L I X T () C K 



Illustrations from tho Author's motion iiicture films taken with regular lens and telephoto 



THE application of the telephoto lens 

 to still photography is well known. 

 Those who have ever tried telephoto 

 work are, of course, thoroughly familiar 

 with the well-known law, that the in- 

 crease in the exposure required is directly 

 proportional to the square of the magnifica- 

 tion used. This law is based upon the as- 

 sumption that the diameter of the positive 

 element of the telephoto system is constant. 

 When the telephoto lens is applied to cine- 

 matography all of the ditficulties encoun- 

 tered in still photography are present, with 

 the addition of new problems. For instance, 

 it is generally impossible materially to in- 

 crease the duration of the exposure in cine- 

 matographic work, in order to conform with 

 the above-mentioned law, for the reason that 

 sixteen pictures per second must be made 

 and the longest permissible exposure for 

 each picture is about one thirty-second of 

 one second. In view of this exposure limita- 

 tion, it will be obvious that the only alterna- 

 tive in motion picture work is to employ a 

 telephoto combination having larger glass 

 cells than commonly used in still photog- 

 raphy, so that sufficient light can be had to 

 make exposures of not less than one thirty- 

 second of a second duration. This will ex- 

 plain why it is advisable in cinematographic 

 telephoto work to make each telephoto lens 

 complete in itself and not consisting of a 

 negative element which is to be added to the 

 regular positive lens. 



The cinematographic telephotos have their 

 diaphragm stops graduated on the lens 

 mounts, as on all ordinary lenses, and ac- 

 cordingly no special calculations for the 

 exposures are required. 



Another, and all important, problem in 

 cinematograph telephotography is accurate 

 focusing. Owing to the great magnification 

 on the screen of the motion picture positive 

 film, which is sometimes as much as, or even 

 more than, three hundred diameters, the 

 greatest care in focusing is necessary. And, 



on the other hand, the longer the focal 

 length of the lens used the greater is the 

 difficulty, by reason of the rapid decrease in 

 the so-called depth of field with the increase 

 in the magnification. In the telephoto bird 

 work which I have done, I have used an 

 auxiliary magnifying lens for the sharp 

 focusing of objects on the ground glass. 



The most serious difficulty, however, to be 

 met with in making telephoto motion pic- 

 tures is the question of vibration. Vibration 

 is an ever-present enemy of the field cine- 

 matographer, which has to be constantly 

 fought, even with the use of regular lenses, 

 and when telephotos are used the problem is 

 far harder, for not only is the desired image 

 enlarged by the use of a telephoto lens but 

 any vibrations present are correspondingly 

 magnified. 



It is the act of grinding the camera com- 

 bined with the necessity of having a com- 

 bination of interior moving parts that is apt 

 to set up vibrations, which may be most 

 disastrous as far as satisfactory screen re- 

 sults are concerned. I use the term "screen 

 results" advisedly, for one may secure a 

 perfect series of individual still pictures on 

 a motion picture film and yet show a most 

 disagreeable and prohibitive relative vibra- 

 tion between the positions on the screen of 

 the separate pictures when projected. This 

 question of vibration is, of course, a me- 

 chanical and not an optical problem. 



In my bird work I use a heavy camera, a 

 heavy tripod, and brace the telephoto lenses. 

 Even then, I am often troubled with vibra- 

 tion, especially when using my largest tele- 

 photo lens, which has an equivalent focus 

 of seventeen inches and a maximum speed 

 of F 6. This lens gives a magnification of 

 eight and one half diameters, in comparison 

 with the standard two-inch lens. 



In order to make a comparison with still 

 photography, to show the relative size of 

 this particular telephoto lens to the size of 

 the picture secured, it will be interesting to 



