586 HANDBOOK OF PHOTOGRAPHY 



length as the camera, either a stereoscope must be ol)tained of approximately equal 

 focal length or, if the stereoscope has a greater focal length than the camera (generally 

 the case), the resultant prints should be enlarged. 



Reference to formula (1) shows that, if the stereoscope has a focal length m times 

 that of the camera, the print must be enlarged m times for orthostereoscopic viewing. 

 As an example, the most common American hand stereoscope has a focal length of 

 20 cm. With this instrument and a camera of 6 cm. focal length, the prints should be 

 enlarged 3^3 times. 



As the centers of the prints should remain a distance apart equal to or less than the 

 distance between the optical centers of the stereoscope, any considerable enlargement 

 will cause the prints to overlap necessitating trimming. If a portion is trimmed off 

 one print, the corresponding portion must be trimmed off the other. Since the left 

 and right sides must be trimmed off the right and left views, respectively, both sides 

 must be trimmed off both views. If this is to be done, the photographer must remem- 

 ber when taking the picture that objects near the edge of the finder maj^ be out of the 

 final stereograph. 



To avoid overlapping much smaller enlargements than are theoretically called 

 for can be used for general work. Though the resultant depth will be exaggerated 

 this may add to the effectiveness of the stereograph rather than detract from it. 



A compromise is usually the best answer. It is recommended that the prints be 

 enlarged to the point where overlapping begins, taking into consideration the possible 

 maximum separation of the prints. The result can then be viewed in the stereoscope 

 to determine if the scene is objectionably distorted. 



Alignment of Prints. Horizontal Separation between Prints.— On convergence 

 principles the distance between the two views of an infinitely distant point on the 

 plates should be equal to the distance between the lenses of the camera and of the 

 stereoscope. This can easily be done by picking some sharp point in the background 

 of one view and measuring the required distance in the proper direction for the same 

 point in the other view. 



Mounting should be done by measurement whenever possible. However, the 

 distance between prints can be considerably lessened without any effect on the result- 

 ant image. With split-lens stereoscopes it is customary to mount the prints with less 

 separation. 



Thus it is possible for the photographer to mount the prints arbitrarily at a chosen 

 distance less than the theoretical distance or by adjustment while viewing them in the 

 stereoscope. If the latter method is used and the photographer has a greater than 

 average interocular distance, he should be careful to place the prints somewhat closer 

 together than his maximum fusion distance. 



Vertical Alignment. — A line drawn between the two views of any point must be 

 parallel to the line between optical centers, i.e., parallel to the line through the eyes. 



The eyes actually allow a small amount of vertical variation between the two 

 views. The physiological reasons for this are not clearly understood. 



The best way of achieving proper alignment is to place a straightedge over the 

 center of the two prints parallel to the lower edge of the mounting card. Then adjust 

 the prints so that the straightedge intersects the same points on each. This assures 

 both vertical and rotational alignment. The center of the card and the center of the 

 prints should be used, as this insures the prints being in the center of the viewing area 

 with a consequent minimum of aberration. 



Proper alignment may be obtained by observation through the stereoscope. The 

 breakdown of fusion as the prints get out of alignment is quite sharp. One print can 

 be placed in the center of its viewing area and the other moved until it is in the center 

 of the fusion area. 



