- 
GV-98 PHOTOGRAMMETRIC ENGINEERING 
MAGAZINE 
SCALE INDICATOR 
| LENSES 
~~" PRISMS 
^ ANNULAR- 
ILLUMINATOR 
/ OBJECT 
GRID PLATE 
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|| TRANS- 
~'y ILLUMINATOR 
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| - - ^ —— 
| F16. 1.—Donaldson Stereo-Camera set up for macrophotography. 
| to the plane of the grid. The entire camera is racked along the vertical column 
until the grid is in focus. 
The Donaldson Stereo-Camera does not include facilities for a time exposure 
inasmuch as it is designed solely and specifically for electronic flash in all opera- 
tions. Remove the camera housing and actuate the mechanism to retain the 
reflex mirror (which also serves as a shutter) in the up position. 
A minimum of two elements of camera calibration must be determined: 
1) FOCAL LENGTH OF THE OPTICAL SYSTEM 
The nominal focal length of the matched stereo-lenses is 80 milli- 
meters. However, two large rhomboidal prisms are located in front of 
the lenses. The prisms can be rotated and are indexed relative to scale. 
The camera base is variable but is constant for a given scale. The focal 
length of the 80 millimeter lens, in combination with the prism, must be 
determined. 
| 2) PHOTO BASE OF THE STEREO-CAMERA FOR A GIVEN SCALE 
The scale selected for the evaluation is 2 (the image size being twice that 
of the object size). The photo base is equal to the camera base at the 
scale of the photograph. 
The camera calibration is performed without determining lens distortion 
or the location of the principal point, owing to a lack of facilities, and an in- 
tentional oversimplification as an initial approach. 
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