112 



HANDBOOK OF PHOTOGRAPHY 



The apparatus required to make such an analysis, as shown in Fig. 29, was designed 

 by P. G. Nutting for use in the Kodak Laboratories. It consists of an arc lamp as 

 light source, a constant-speed motor to drive a cylinder on the rim of which are 

 20 small flat mirrors, a holder for the shutter, a small lens to form an image of the 

 blades, and a rotating drum to hold the film that is to record the separate images. 

 The mirror cylinder must turn at exactly 50 r.p.s. (or 3000 r.p.m.) if its 20 mirrors 



Fig. 23. — Mechanism of metal focal-plane shutter used in the Contax camera. 



Fig. 24.- 



-Linked metal bars (lower left) and assembly of bars in focal-plane shutter for 



Contax. 



are to give exactly 1000 flashes per second on the shutter blades. It is impossible to 

 make an accurate determination of efficiency without special apparatus like this. 

 Photometric methods are accurate if properly carried out, but they involve consider- 

 able labor for a complete analysis. 



Focal-plane Shutters. — In the case of focal-plane shutters, efficiency is more easily 

 determined. All one needs to know is the essential lens and shutter dimensions 

 given in Fig. 30. Examination of this drawing shows that the distance E between 

 the shutter curtain and the film is the determining factor in efficiency. Only when 



