SHUTTERS 



111 



mechanism of one of the shutters. Shutters of this t3rpe are never so fast as the 

 standard models with one set of blades. 



Another and different principle is found in the louver or Venetian-blind shutter. 

 One model was designed for use in an aerial camera and was located midway between 

 the lens and the plate. There have been few practical applications of this type. 



Efficiency and Accuracy of Shutter Testing. — Interlens Shutters. — ^Efficiency may 

 be defined as the ratio of light transmitted by a shutter during the exposure interval 

 to the amount of light that would have gone through if the full aperture had been open 

 during that interval. An efficient shutter is one whose blades open fast and close 

 fast; the wide-open position should be held for as long a part of the prescribed exposure 

 time as possible. Efficiency varies with the size of lens aperture that is employed. 



The efficiency ratio may be expressed graphically when the constants of the shutter 

 have been determined. A typical set of such diagrams are shown in Figs. 25, 26, 



f///////// //////// 



nr 



*g 



^-h 



^>/^//////. '^^^^/. 



^-mpm^ 



^^^a^^^^^^^■^^k^^^^^»,'.^«^p ^ 



Fig. 22. — Focal-plane shutter of Leica camera. The parts labeled are as follows: 

 c, roller for extension bands of lower curtain i; /, upper curtain; g, extension bands; h, 

 spring roller for bands g of upper curtain; i, lower curtain; j, spring roller for lower curtain; 

 fc, pin connecting rollers c and e; m, winding knob; n, coupling pin connecting winding knob 

 m with axle. 



and 27. In these the area DT (or % DT for Fig. 27) represents ideal performance or 

 full opening for the total interval at which the shutter is set to operate. The shaded 

 area in each diagram represents the time the shutter is actually open, and the ratio 

 of the two is the efficiency in percentage. 



The determination of the constants of any particular shutter requires special 

 equipment of a high order of precision. One really needs a slow-motion picture of the 

 shutter blades in the act of opening and closing, and the separate pictures must be 

 exactly timed. Such a set of pictures is presented in Fig. 28, and the time interval 

 between pictures in this case is Hooo s^c. The timing accuracy of the shutter is 

 shown also, being ^Hooo sec. for this particular exposure. Efficiency is here 

 measurable by simply comparing the total open area of the blades in all exposures on 

 the print and the ideal area of 11 pictures at the full circular aperture; in this case it is 

 about 60 per cent. 



