2.2.5.3.2. Photo Scale 
The photoscale S) must be as large as possible to ensure 
highest possible precision in image coordinate measurement. 
2,2.5.3.3.. Focus 
The lens equation: 
] 
1 
C f 
must be satisfied. (f is the focal length of the camera lens). 
+ 
x 
Z 
2.2.5.3.4, Depth of Field 
Obviously, the depth of field of the camera must cover 
the entire Z range of the object (or that part of the object 
under consideration). This requirement is sometimes found 
incompatible with a large photo scale (3) »« The depth of field 
could be increased by stopping down the lens (a feature 
available in some, but not all, stereometric cameras), but 
this obviously reduces the resolution and increases exposure time. 
2.2.5.3.5. Camera Calibration 
For any serious photogrammetric work with any camera, the 
geometrical qualities of the metrical chamber and of the lens 
must be thoroughly known. Most important of these qualities 
are the lens distortion, and the relative position of the fi- 
ducial marks with respect to each other and with respect to the 
center of projection. 
Since the metrical chambers of the stereometric cameras 
are, generally, not set for infinite focus, the calibration of 
such cameras cannot be accomplished using the standard calibra- 
tion equipment for aerial cameras. Special arrangements have