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300 PROPOSED AMERICAN STANDARDS
Proposed American Standard
Mounting Threads and Flange Focal Distances for
Lenses on 16 Millimeter and 8 Millimeter
Motion Picture Cameras
Z22.76 Sept. 1949
Page 2 of 2 pages
With some lenses a section of the mount, with a diameter smaller than the root of the thread, necessarily extends closer to the film than is indicated by the above drawing. In those cases, the mechanical clearance in the camera must be determined individually.
In the past, a number of lenses with the 1-inch thread had a "B" dimension of 0.1 87 inch. This is considered to be an obsolete practice.
Past practice has not been entirely consistent so far as the "B" dimension of the 0.625-inch thread is concerned; some existing cameras will not accept a thread longer than 0.115 inch; some lenses have been made with a length of 0.120 or 0.125 inch.
Dimension C: This dimension is defined as the distance from the lens seat to the plane of the best photographic image. It should be determined photographically with panchromatic film and with the camera operating normally. Sometimes a compromise is necessary between best central definition and best over-all definition.
The tolerance acceptable for dimension "C" is dependent on the depth of 'focus and on a decision as to what portion of the depth can be used for the focus tolerance. In some cases, the tolerance is very small. For example/ with a 25-millimeter f/1.4 lens and a 0.001 inch circle of confusion, the depth of focus is 0.0014 inch; only part of this is available for the sum of the lens and camera focusing tolerances.
D;'mens/on D: The values given in the table are to be considered as the maximum diameter of the seat on the lens; the seat on the camera should provide clearance for these diameters.
If any part of the lens mount has a larger diameter, it should be checked for mechanical interference with the camera on which-1 it is to be used. Some lenses with the 1-inch thread have been made with a flange diameter of 1.500 inches.
Note: This standard does not apply to continuous-type motion picture cameras because of the type of optical system employed in these cameras.
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