468 SIZE OF SCREENS [Cn. XII 



In the above example 

 f is unknown. 



the size of the object is 7.5 cm. 3 in. 

 d the distance of the screen image is n meters 36ft. 



1 the size of the screen image is 5 meters 16.5 ft. 

 Substituting the values in the formula we have, for metric values, 



f ll 7-S XI1 r r .LI. t.- .,_ 



= or f = - - = 16.5 cm., focus or the objective. 

 7-5 5 5 



For English values 



- = or f = - = 6.5 in., focus of the objective. 

 3 16.5 16.5 



638. Size of screen image for moving pictures. As the 

 scenes depicted by the moving picture are so largely of human 

 action, and thus resemble a theater play, one would think that the 

 standard should be the representation of people in their natural 

 size. The fact is, however, that in most picture theaters the people 

 represented are of heroic or semi-heroic size, being from 1^2 to 

 two times the natural size of ordinary people. 



The large size of the moving picture on the screen has come about 

 naturally, as the details of movement and the facial expression of 



636a-637a. In the formula here given it is assumed that the objective will 

 always be at its principal focal distance from the object regardless of the screen 

 distance. This is not strictly true, but as the screen distance is so great rela- 

 tively to the distance of the objective from the object, the slight error involved 

 in the above assumption is negligible. If the screen distance and the principal 

 focal distance were more nearly the same, the error would be altogether too 

 great to be neglected (see fig. 210). 



It follows, naturally also, from this formula that, if any three of the elements 

 are given, the fourth can be found. Ordinarily, it is the proper focus of the 

 objective to use that is unknown, but any one of the elements might be desired, 

 and it can be found if one knows three of them. 



As it is the focal length of the objective that is most often required, the 

 following may be of assistance; it simply states in words what the formula 

 shows: 



To find the focal length of the objective needed, the screen distance and the 

 size of the screen image being known: Multiply the screen distance in meters 

 by 7.5, and divide the product by the size of the screen image in meters and the 

 result will give the focus of the objective in centimeters. For English measure : 

 Multiply the screen distance in feet by 3 and divide the product by the size of 

 the screen image in feet, and the result will give the focus of the objective in 

 inches. 



