62 



THE AMERICAN MUSEUM JOURNAL 



note that, to produce a 4" x 5" still picture 

 on the same basis as I do the 1" x %" mo- 

 tion picture with this 17" lens, it would be 

 necessary to use a lens of about 13" diame- 

 ter. This comparison will give an idea of 

 how the question of speed is met in produc- 

 ing telephoto lenses for cinematography. 

 Owing to the difficulties described, cine- 

 matographers have so far but little used 

 telephoto lenses of a longer focus than eight 

 inches. 



Several years ago I realized the possiltili- 

 ties for usefulness of telephoto lenses of 

 long foci when applied to bird work. I 

 therefore secured the two most powerful 

 cinematograph telephotos I could find and 

 I will now confess it was without a full com- 

 prehension of the difficulties I had in store 

 for me. But, as "necessity is the mother of 

 invention," the problems were gradually 

 met. 



Having worked for many years at bird 

 still photography, I observed that individual 

 song birds would permit me openly to ap- 

 proach them to within a more or less defi- 

 nite distance, which was rather constant. 

 This distance I term the critical distance 

 and it could be determined for each indi- 

 vidual bird. 



It then occurred to me that if I couLl 

 work on birds outside their critical distance 

 and still secure photographic images of a 

 satisfactory size, a new, and, as far as I 

 was aware, an unused field would be opened 

 up to me. This reasoning combined with 

 some simple mathematical calculations led 

 to the procuring of the cinematograph tele- 

 photos already mentioned. 



As a concrete example of this type of bird 

 photography I will say that it is manifestly 

 an impossibility, without the use of a blind, 

 to approach and photograph the average 

 adult robin at a distance of three feet. And 

 yet by employing my seventeen-inch lens 

 and working at a distance of twenty-five 

 feet, which is thoroughly practical, and 

 which is outside the generally accepted 

 robin critical distance, I can secure an image 

 of the same identical size on the film as I 

 would be able to do with the standard two- 

 inch lens at a distance of three feet. 



I often have recourse to blinds for bird 

 work and they are many times essential, but 

 I must confess it is a source of still greater 

 pleasure to me to meet song birds in the 

 open and to be able with their full know- 



ledge and consent to secure motion pic- 

 tures i which occasionally depict interesting 

 and characteristic habits. 



It will be noted that in what I have writ- 

 ten so far the application of the telephoto 

 lens has been confined to comparatively close 

 work. The photographing through the cine- 

 matographic telephoto lens of larger animal 

 life at a distance from the camera, often 

 introduces the additional problem of the 

 light rays passing through air strata of dif- 

 ferent and varying densities, with the con- 

 sequent fluctuating distortions of the objects. 

 This defect can only be met by selecting a 

 suitable time for making the pictures. The 

 best time for such distant work is in the 

 early morning and the late afternoon, when 

 the heat radiation from the earth is less 

 than it is during the middle of the day. 



Telephoto work through great aerial dis- 

 tances, such as distant mountain views, pre- 

 sents a problem that cannot usually be 

 successfully met, except with special tools, 

 which must be intelligently employed. 

 Those who have ever tried to make distant 

 still telephotos know the unsatisfactory flat 

 and foggy negatives that usually result, ir- 

 respective of the exposure and development. 

 This oljjectionable quality of negative is due 

 to the action of violet and ultra-violet rays 

 which are present to an excessive degree be- 

 tween the lens and distant objects. This 

 defect, to a small extent, can be and should 

 be reduced by employing a light shield in 

 front of the telephoto lens for cutting off all 

 extraneous light rays. 



Investigations in this subject by such ex- 

 pert scientists as the Belgian worker, M. 

 Andre Collier, and Dr. C. E. K. Mees, of the 

 Eastman Company, seem to indicate that 

 the suspended particles of water vapor, 

 which are present in the atmosphere, espe- 

 cially near the earth's surface, are trans- 

 parent for the longer waves of light, which 

 compose the red end of the spectrum. 

 Therefore, as the human vision is more 

 affected by the red end of the spectrum 

 than is the standard photographic emul- 

 sion, our vision is not materially affected by 

 tlie intervening presence of water vapor in 

 a landscape view, unless present to an ex- 

 cessive degree. On the other hand, it is 

 supposed that the passage of even moderate 



^ Of the bird pictures which I have shown at the 

 American Museum of Natural History, one entire 

 reel, as well as parts of other reels, was made in 

 the manner described. 



