SHUTTERS 



113 



E = 0, i.e., when the shutter is at the emulsion surface, is a focal-plane shutter 

 100 per cent efficient. The aperture ratio of the lens also enters into the calculation, 

 so that a focal-plane shutter 80 per cent efficient for an //4.5 lens may be totally 





D,= 2/5D-~|_ 



Fig. 26. 



Fig. 25. — Efficiency diagram of interlens shutter. The total time of operation of the 

 shutter is T; the time during which the leaves are entirely open is T2. During the interval 

 Ti the shutter is opening, whereas during T^ it is closing. D represents the diameter of 

 the aperture. The efficiency is the ratio of the shaded area to the area of the entire 

 rectangle, and may be expressed as 



V 



DiT2 + HTi + ]4Ti) 

 DT 



In this case, the efficiency is 86.5 per cent. 



Fig. 26. — Efficiency diagram for interlens shutter. The efficiency of the shutter 

 represented in this case is less than that of the shutter of Fig. 25 because the ratio of To/T 

 is much less than in the former case. Efficiency is 61.1 per cent. 



Fig. 27. — Efficiency diagram of interlens shutter, illustrating that for a given shutter 

 speed, the shutter efficiency, rj, increases as the diameter of the aperture, D, is increased. 

 Efficiency is 84.5 per cent. 



Fig. 28. — Shutter test strip. The single exposures are made with a time interval of 

 Mo 00 sec. each. Shutter efficiency is about 60 per cent for this case. 



Fig. 29.- 



-Shutter-testing outfit of P. G. Nutting, a, arc lamp; b, motor drive; c, mirror- 

 wheel assembly; d, shutter holder; e, auxiliary lens; /, drum for film strip. 



unsuitable when an f/2.0 lens is put on the same camera. In Fig. 31, slit width and 

 efficiency are plotted for three diiferent values of e, and typical efficiency curves 

 are drawn to show how efficiency varies with lens aperture. 



