106 PHOTOGRAPHY FOR BIRD-LOVERS. 



whereas a bird can zig-zag horizontally or up and 

 down at will, and so the difficulty is immeasurably 

 increased. 



The Exposure. 



Nor is this all. Were birds large creatures like 

 a horse, the photography of them in flight would 

 be vastly easier than it is, since then we could 

 secure a satisfactory picture from a distance many 

 times as great as is now possible ; and this, though 

 a purely photographic matter, has such direct 

 bearing on the subject, that some notice must be 

 taken of it. There is a rule which says that the 

 exposures for moving objects must vary directly 

 with their distances from the camera, other factors 

 remaining constant, i.e., the duration of the 

 exposure decreases as the distance is reduced. 

 To apply this to our own case. Suppose that the 

 right exposure for a certain bird flying steadily 

 along at a distance of forty yards from the camera 

 is 1 /200th part of a second, then to secure a picture 

 of the same bird from twenty yards the shutter 

 would require to be set at l/400th part of a second, 

 while at a distance of five yards the exposure 

 would require to be l/1600th part of a second : a 

 speed rarely reached even with the focal-plane 

 shutter. Thus, while it may be easy to secure 

 a picture of a distant flying bird, a near study of it 



