222 HANDBOOK OF PHOTOGRAPHY 



Table III. — Shutter Speeds Required to Stop Motion on the Photograph 



The maximum shutter speed which is attainable with between-the-lens shutters 

 is about Hoo sec. Faster shutter speeds may be obtained with focal-plane shutters. 

 However, when a focal-plane shutter is used to photograph objects in rapid motion, 

 distortion of the object is very likely to result. This is because the slit in the focal- 

 plane shutter travels across the negative at a comparatively slow speed, while the 

 image of the object travels across the plate. Depending upon the direction of the 

 shutter motion relative to that of the moving object, focal plane distortion of this 

 sort may result in foreshortened, lengthened, or tilted images. There is no remedy 

 for this type of distortion, although it becomes less serious as the speed of the moving 

 object is reduced and as the distance between the moving object and the camera is 

 increased. 



Changes in shutter speed which may be required when changes are made in aper- 

 ture stop, film speed, intensity of light, or other factors may be found from the relation : 



='Kt)a)<--"<T)(^) 



(28) 



For example, suppose that the exposure table is calculated for reference Weston film 

 speed of So = 16, reference aperture /o = 16, and basic exposure time to = H5 sec. 

 and that it is desired to determine the exposure time for a film speed S = 8 at an 

 aperture / = 32 with a filter factor F = 1.5 when the subject is so small that M -^- 0. 

 Using the above equation we find the new exposure time to be 



t = >25 X 1.5 X 1% X (^he) ' X P = i%5 ^ }{ 



(29) 



Film Sensitivity or Speed. — The sensitivity or speed of negative materials is an 

 important factor in determining the exposure required to produce a given deposit of 

 silver. Various methods of determining the speed of photographic materials are in 

 use; these methods are discussed in the chapter on Photographic Sensitometry and 

 will not be treated here. It is sufficient to say that in all the common methods of 



