576 HANDBOOK OF PHOTOGRAPHY 



lengths are maintained, is generally fixed at infinity. Consequently both absolute 

 convergence and accommodation are of little importance. 



Eliminating these factors, each of the other depth factors has its own tendency as 

 follows : 



All the normal factors of monocular vision, perspective, known size of objects, etc., 

 tend to make the resultant image of the proper size and at its proper location in space. 



Relative convergence tends to place the parts of the image in their proper relation 

 to one another according to the final location of the image as a whole. 



The tendency to find a fixation point in the plane of the plate is the greater the 

 more obvious the texture or grain of the plate. 



The enclosing frame around the fused image tends to make any point near the edge 

 appear to lie in the plane of this plate. A stereoscope scene may appear quite natural 

 as a whole. Yet if successive points near the edges are fixed, they will all appear to 

 lie in or near the plane of the frame, regardless of the fact that they are obviously in 

 different planes in the whole view. 



The enclosing frame is supplied either by the edge of the plate around the stereo- 

 graph or by the restricting parts of the stereoscope. Since the eyes normally see over 

 an angle of about 120° each, a camera capable of covering a field of 120° would be 

 necessary to eliminate this frame. A stereoscope would then be necessary which 

 would permit both eyes to cover this range such as a mirror stereoscope where the 

 mirrors superimpose the views. In a lens stereoscope the right eye cannot be per- 

 mitted to see the left view and vice versa. The usual camera and the usual lens 

 stereoscope both permit angles of less than 60°. Consequently the enclosing frame 

 is always present in one form or another. 



The total resultant image is a compromise on the part of the eyes and brain between 

 these various factors. It is more perfect when the factors which place it correctly 

 are more obvious and the factors which draw it forward and decrease its size are more 

 nearly eliminated. 



In normal photographic work the depth of any object in the scene is not exactly 

 known to the observer. Consequently he has no means of judging the accuracy of the 

 location and size of the stereoscopic image of it. So long as the illusion of reality is 

 maintained the tendency is to accept the image as correct. 



From the above discussion the photographer should remember: 



1. Extremely simple objects which primarily must depend on convergence to give 

 depth judgment will tend to appear smaller, nearer, and somewhat distorted. If 

 the accurate judgment of the size and shape of rather uniform objects, such as cubes 

 or geometrical figures, is desired, other elements should be added to the picture which 

 will aid in this judgment. 



2. Scenic views appear quite natural though they have a tendency to be cramped. 

 As more elements are added which normally aid depth perception in monocular vision, 

 the scene will more closely approach its true size and shape in space. 



3. Every effort should be made to eliminate print or plate textures and the obvious- 

 ness of the enclosing frame. 



4. No absolute criteria for depth judgment exists; consequently the experience 

 and capabilities of the photographer will inevitably play a large part in determining 

 the degree to which the illusion of reality is approached. 



It must be realized that the entire discussion of depth judgment in stereographs is 

 put forth only tentatively in the absence of a more exact knowledge of the subject. 

 The sense of depth in a stereograph is so striking that no great sensibility to its vari- 

 ability has yet been developed. As stereographs become more common the interpreta- 

 tion of their realistic and artistic effects will become more acute. The field offers an 

 enormous opportunity for research on the part of the photographer. 



