EXPOSURE AND EXPOSURE DEVICES 221 



Ordinarily this equation is of little practical importance, although occasionally it 

 may be found useful where a neutral-density filter is advantageous in cutting down 

 the light without affecting the other quantities involved, as may sometimes be neces- 

 sary when photographing bright sources of illumination. 



For example, let the reference conditions be So = 16, io = Ko sec, and /o = 16. 

 It is desired to determine the filter factor for a neutral (gray) filter when the Weston 

 film speed is /S = 40, the aperture is/ = 4, and the shutter speed is i = Ho sec. The 

 magnification, change in light, and transmission through the lens may be neglected. 

 The required filter factor is then 



Ho ^40 

 Ho 16 



iW- 



F=^X^^XI^] =40 (26) 



If a neutral-density filter is used, it should have a transmission of J^o or a density of 



D = logio ^ = logio 40 = 1.6 (27) 



This equation cannot be applied to filters having selective absorption unless the spec- 

 tral characteristics of the filter are correlated with those of the light source and fiilm 

 in arriving at the desired result. 



Shutter Speed. — The shutter speed may have two important functions in photog- 

 raphy. Usually its most important function is to determine the quantity of light 

 reaching the film by establishing the time during which the light passes through the 

 lens to the photographic material. Its other function is to limit the time of exposure 

 so that the relative motion between a moving object and its background does not 

 produce a blur on the negative. These two functions of the shutter are entirely 

 separate and distinct, although it may happen that the shutter speed for obtaining one 

 of the desired conditions is also that for obtaining the other. 



Since these two functions are distinct, it is necessary to determine them separately 

 for the exposure and sharpness of image desired. It is then necessary to select the 

 most suitable shutter speed and to adjust the aperture, if necessary, for the film speed 

 and prevailing light conditions. If the shutter speed required to stop motion is longer 

 than that required to give adequate exposure, the shutter speed determined for proper 

 exposure should be given. On the other hand, if the shutter speed required to stop 

 motion is less than that required to give proper exposure, the shutter speed should be 

 used which will prevent blurring, and the /-number should be decreased to that value 

 which will produce the desired exposure. 



In order that the apparent motion of a moving object may be eliminated in a 

 photograph, it is necessary that the shutter speed be sufficiently rapid that no appreci- 

 able motion of the object is apparent while the shutter is opened. The actual exposure 

 time required to stop motion will depend upon the focal length of the lens, the distance 

 between the camera and the moving object, and upon the speed of the moving object. 

 It will also depend upon the direction of motion with respect to the camera, for a 

 slightly longer exposure can be given, for a specified velocity of travel, if the object is 

 coming toward or is receding from the camera rather than moving directly across the 

 field of view. 



Table III indicates the longest exposure times which may be regarded as being 

 satisfactory to stop motion for the conditions stipulated. Faster shutter speeds may 

 be given if unusually sharp negatives are desired, but light conditions, fUm, speed, 

 aperture stop, and other matters affecting exposure must be taken into account to 

 ascertain that the film is not underexposed. Slower shutter speeds should not be 

 given unless some blurring of the image is tolerable. 



