CHAPTER XX 



STEREOSCOPIC PHOTOGRAPHY 



By Vannevae Bush and John T. Rule 



General Stereoscopic Theory. Types of Vision. — Monocular vision consists in 

 the reception on a surface (the retina) of a single two-dimensional image, which has 

 been projected through the crystalline lens of one eye from a scene in space, and the 

 resultant interpretation b)' the brain of this image. 



Except for the lack of color and the presence of the texture of the film, such an 

 image is closely reproduced on the film of a single-lens camera making black-and-white 

 pictures. The resultant print when properly \dewed reproduces an image on the 

 retina similar to that which it would have received had the eye occupied the position 

 of the camera lens when the picture was taken. From this two-dimensional image 

 the brain infers the original three-dimensional space scene. 



Binocular vision consists in the reception on the two retinas of two images of a 

 single scene in space and the resultant fusion and interpretation by the brain of these 

 images. Since the position of the two eyes is not the same, different portions of objects 

 are visible and near objects obscure different areas of far objects. Thus the two 

 retinal images are not identical. The brain fuses these two different two-dimensional 

 images and interprets them as a single three-dimensional space scene in which depth 

 appears to exist as a definite reality. 



If two photographs are taken from the positions occupied bj^ the two eyes and the 

 resultant prints are properly viewed — the left eye viewing only the print taken from 

 the left and the right ej^e only that taken from the right — ^two retinal images are 

 produced which correspond to those which would have been formed had the eyes 

 occupied the positions of the camera lenses. With proper viewing aids the resultant 

 fusion and interpretation is substantially the same as that of binocular vision including 

 the sense of the reality of depth. This reproduction of the effect of binocular vision 

 by the use of two two-dimensional images is "stereoscopic vision." 



Factors Influencing Judgment of Depth. — For the purposes of this chapter the word 

 "depth" is always used to niean distance away from the eyes. 



The important factors which contribute to depth judgment in monocular vision are 

 perspective, light and shade, apparent size of known objects, obscuring of remoter 

 objects by nearer ones, decreasing sharpness of detail with distance, and motion of 

 foreground objects against background objects. 



All the factors entering into depth judgment in monocular vision are equally opera- 

 tive in binocular vision. In addition, without evaluating their importance here, the 

 following factors are present: 



1. Two Views. — As stated above the two views of binocular vision are different. 

 This difference is a major factor in the resultant depth effect. The physiology of the 

 resultant fusion and interpretation are not well understood. 



2. Convergence. — In looking at a point in space, an angle is formed by the two rays 

 from the point to the eyes. The eyeballs turn inward until the axis of each is coin- 

 cident with its entering ray. This "angle of convergence" grows larger as the point 

 approaches. It always has a fixed value for any fixed position of the point with respect 

 to a given position of a pair of eyes. 



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