90. 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 zt> 
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 34, 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 
