PHOTOGRAMMETRIC ENGINEERING 
9 
wide range of speeds makes possible the use of the camera under extreme light 
conditions, subject of course to the limitations of image motion. The camera, 
having a fixed//6.3 aperture, forces the photographer to operate at the highest 
possible shutter speed. This combination was selected to minimize the loss of 
resolution due to higher image velocities present in current high speed aircraft. 
However, this combination is not without cost, since operating the metrogon 
lens at the//6.3 aperture does result in some loss of resolution. In order to obtain 
the highest resolution possible, //8 and //11 Waterhouse stops have been fur 
nished with the camera. 1 hese units can be easily attached to the shutter and 
installed in the lens cone. With this combination available, the photo officer can 
analyze the problem and select the aperture that will yield the best aerial pho 
tography under a given set of conditions. 
The last and undoubtedly the most important part of the camera to be dis 
cussed is the inner lens cone. This is the “heart” of the camera, for it is in this 
unit that the precision of the T-ll camera is contained. This unit, when once 
assembled in its final form and calibrated, is never disassembled again under 
normal conditions. Should the need arise to disassemble it, this will be done only 
at the place of manufacture or at a camera repair depot having camera calibra 
tion facilities. After reassembly, the lens cone would be calibrated again and, if 
found satisfactory, would be returned to service. 
At this point the design and construction of the lens cone should be examined 
very carefully. The optical parts supplied by the lens manufacturer have been 
hand-selected from a large group of lenses for specific radial distortion character 
istics. This is required in order to match the distortion characteristics of the map 
compilation equipment. During the lens manufacturing process every caution is 
exercised to keep the tangential distortion to an absolute minimum and, when 
completed, the lens is marked to indicate the direction of the tangential distor 
tion vector. This information is utilized by the camera manufacturer in mount 
ing the lens in the lens cone. The tangential distortion vector is placed in the 
lens cone to correspond to the direction of film transport which normally is in 
the line of flight. The reason for orienting the vector in this manner is to mini 
mize the effect of the distortion in control extension by stereo-photograinmetric 
means. After the camera manufacturer receives the lens, it is assigned to a par 
ticular lens cone casting. This casting is designed as one integral unit with the 
lens attached to one end and the fiducial markers attached to the other, which 
is the focal plane. This casting is machined to the focal length of the lens. During 
this process great care and close tolerances are maintained in order to keep the 
lens elements properly centered, to minimize the tangential distortion, and to 
keep the lens mounting surfaces parallel to the focal plane. Once the machining 
of the lens mounting surfaces is completed, the lenses are mounted in the casting 
and are pinned in such a way as to make impossible the interchange of lens 
elements. 
The next major operation on the lens cone is the installation of the fiducial 
and film shrinkage markers. These are temporarily attached by means of screws 
and hand machined to form a film positioning plane flat to + 5/10,000 of an inch. 
The lens cone is then placed on the lens calibration equipment to photographi 
cally check and record the radial distortion of the lens. This process yields the 
distortion curve, the proper position of the fiducial markers, and the calibrated 
focal length of the lens. Directly following the calibration, the fiducial markers 
are repositioned as dictated by the calibration data, and permanently pinned 
to the lens cone. The lens cone is now a precision instrument. However, the 
assembly is not complete,