114 



HANDBOOK OF PHOTOGRAPHY 



Shutter Tests. — To test the accuracy of a shutter one does not need the special 

 equipment required to make a measurement of efficiency. Neon tubes operated on 

 60-cycle a.-c. line give 120 flashes per second, ^Yhich is suitable for testing the slow 

 range of shutter speeds. Such a test requires only a good tripod or a steady hand. 

 The pictures must be taken at night. Pick out a neon sign in which one vertical 

 line is well separated from the rest, take the picture from about 6 ft. distance and 

 from a position such that other lights do not fog the film, and swing the camera 

 steadily during the exposure so that the image of the neon tube moves steadily across 

 the center of the field during the time the shutter is open. This will give one flash 

 of light on the film for every K20 sec. that the shutter was open, (for 60-cjrcle source), 



and by simply counting the number of lines the 

 length of the exposure can be calculated. 



For faster exposures a rotating automobile 

 tire is a suitable test object. The idea is to 

 measure with the shutter the length of travel 

 of a piece of white tape stuck on the wheel in a 

 radial position to extend all the way from the 

 outer rim of the tire, the point where it touches 

 the road, to the center of the hub cap. In an 

 open space where there is plenty of light, jack 

 up one back wheel of the car. Use a long piece 

 of white tape ^o. in. wide, mark it with two lines 

 as shown in Fig. 32, and stick it firmly on the 



tire and hub. If the speedometer is correct 

 Fig. 30.— The essential lens and ^^^ ^^^^^ ^^ y^ ^^^^^^ ^^^ ^^^^^.^ -^ ^^^^^^ 



shutter dimensions required lor deter- ,• ,.,1 ,• -n ,1 1 ^ ^' 



mining the efficiency of focal -plane ^^e outer portion of the tire will then be rotat- 

 shutters. C, curtain of focal-plane ing at 30 m.p.h., when one wheel only is rotat- 

 shutter; D, diameter of the lens ing. This speed is 158,400 ft. per hr., 44 ft. per 



sec, or 528 in. per sec. (0.528 in. per Mooo sec.) 

 The photograph should be taken with the 

 camera in line with the axle and at a distance 

 such that the wheel and tire fill about half the 

 film area. By measuring on the print and 

 scaling from the 5.28-in. distance marked on 

 the tape, the travel of the H-in. width of the 

 tape during the shutter opening can be meas- 

 ured. One must remember that the tape is 

 actually }4 in. wide. It should appear 3-i plus 0.528 in., or 1.028 in. wide for a shut- 

 ter exposure of exactly Hooo sec. Furthermore, the travel of any point of the tape 

 not on the outermost portion of the tire is proportional to its distance from the 

 center of the hub, and its rate of travel can be easily determined. On a 16 by 5.50 

 tire, for example, the outside diameter is 27 in., radius 13.5 in. For a point 10 in. 

 from the hub the speed ratio is 10 divided by 13.5 times 0.528 in.^which equals 0.391 

 in. in Kooo sec. 



The results of this method of testing will be found difficult to interpret in the case 

 of focal-plane shutters, because of the distortion factor previously mentioned. It 

 does give a true measure of the ability of a shutter to stop motion and to that extent 

 is a reliable test of accuracy when carefullj^ carried out. 



Caxe of Shutters. — It is in order to list a few precautions for the user of photo- 

 graphic shutters of any type. A good shutter is built like a good watch and should be 

 treated with the same consideration. It should not be oiled, greased, cleaned, or 

 taken apart by anyone except an expert repairman. If the shutter fails to work 

 properly, send it to the manufacturer for repair. Keep it as free from dust and dirt 



aperture; E, distance between shutter 

 and photographic material; F, focal 

 length of lens; L, camera lens; P, plane 

 of photographic material; S, width of 

 slit in shutter curtain. For focal- 

 plane shutters, the efficiency is given 

 by 



1 SF 



V = 



1 + 



ED SF + ED 



w 



