The near focussing limit of each lens corresponds to a magnification of 
approximately 0.10 X. The lenses have been selected from the best of the 
latest generation of computer-designed view camera lenses. All provide 
images of superb quality when stopped down to the normal operating apertures 
of f/22 to f/32 employed in close-range photogrammetry. The lenses are 
mounted in self-cocking Prontor Professional shutters with speeds ranging 
from 1 second to 1/125 second. 
For each lens cone calibrations of radial and decentering distortion 
are provided by GSI for the two extremes of the focussing range. The first 
calibration is with the lens focussed at infinity and is performed by means 
of a stellar SMAC (Simultaneous Multiframe Analytical Calibration) reduction 
described on pp. 268, 269 of the Fourth Edition of the Manual of Photogram- 
metry published by the American Society of Photogrammetry. This calibration 
employs measurements of several hundred stellar images and is accurate to 
within *lyum over the entire format. It also provides accurate values for 
the principal distance c and the coordinates Xp» yp Of the principal point. 
The second calibration of radial and decentering distortion is produced by 
the analytical plumb line method (Manual of Photogrammetry, p.273) from a 
frame exposed at a nominal photographic scale of 1:10. This corresponds 
closely to the near focussing limit of the lens. In STARS software appro- 
priate corrections for distortion corresponding to any intermediate dis- 
tances are computed from the results of the two separate calibrations in 
accordance with formulas developed in Brown (1972). 
The focussing mount of each lens serves also as a precise micrometer 
with graduations on the drum corresponding to intervals of 10um. Each full 
turn of the drum advances the focus by one millimeter. The zero point 
corresponds to the calibrated principal distance c for focus at infinity. 
The focussing mount thus permits changes Ac in principal distance accompany- 
ing changes in focus to be monitored with extreme accuracy. In practice, 
one would generally precompute the value of Ac required for the desired 
focussing distance and would then set this value on the micrometer drum of 
the focussing mount. 
A lens cone of the client's choice is included in the basic purchase 
price of the CRC-1. Additional cones may be purchased separately at any 
time. However, the camera must be returned to GSI for the calibration of 
the cone as mounted in the camera. 
2.4 ROLL RING 
The body of the CRC-1 is mounted in a graduated roll ring incorporating 
a pair of precise Kaydon Reali-Slim bearings of 12 inch inner diameter. The 
roll ring permits the execution in a smooth and easy manner of the rotation 
of the camera about its axis. Performance of such a rotation (the so-called 
roll-maneuver) is essential to the process of self-calibration which is 
implemented in STARS software and is explained in Brown (1972). The pre- 
cision of the rotation of the camera about its axis is such that the center 
of projection of the lens remains essentially stationary as the camera is 
rolled. This means that when two or more exposures are taken from a given 
station at different roll angles, a common center of projection can be 
assigned to such exposures. STARS software has provisions for imposing 
the constraint of coincident projective centers. This constraint can 
significantly strengthen the photogrammetric adjustment. 
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