20 BIRD STUDIES WITH A CAMERA 



less when used for slow exposures of two or three 

 seconds, a matter of much importance in making 

 time pictures of sitting birds, who are apt to turn 

 their head if they hear the click of the shutter. This 

 shutter, however, does not respond quickly in slow 

 exposures and is very heavy, a disadvantage in tele- 

 photography. 



The " Unicum " shutter is lighter, responds quick- 

 ly, has a lever to which a thread may be attached 

 for making exposures from a distance, can be easily 

 diaphragmed from the rear, but is not wholly noise- 

 less. There are also other shutters, each possessing 

 good points of its own, and the selection of any one of 

 them for use in medium rapid, slow, or time work 

 can be left to the photographer, who should, how- 

 ever, remember that the time scales on these shutters 

 represent degrees of difference and not exact meas- 

 urements of time, and that there is great variation 

 in the exposures of different shutters of the same 

 make when similarly adjusted. Thus the " one fifth 

 of a second" of one shutter may be equivalent to 

 the " one second " of another. The scale on most of 

 these shutters calls for a speed not exceeding a jfa 

 part of a second, but this is far too slow an exposure 

 to successfully photograph a flying bird at short 

 range where a speed of at least ? ^ 7 of a second is 

 required. 



For very rapid work the choice is limited to one 

 kind of shutter that is, the focal-plane, which in 

 effect is a curtain with an adjustable slit which is 

 placed directly in front of the plate. Great speed 

 with this shutter is in part secured by increasing 

 the tension of the spring, which acts as its motive 



