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370 LENS-CALIBRATION REPORT October
given lens at a given opening. This T number is to be defined as the/ number of an open circular hole, or of a fictitious lens having 100 per cent transmittance and a circular aperture, which would give the same central-image illumination as the actual lens at the specified stop opening (assuming a very distant object). Hence, for a lens with a circular aperture,
T number = (8)
Vt
and for a lens with an entrance pupil of any shape and area A, the corresponding formula is
T number = s*Z.. (9)
2 \ tA
In practice, however, it is expected that the normal procedure will be to re-engrave the iris-diaphragm ring on the lens at a series of definite T numbers, rather than to measure the T number corresponding to each of the existing marked /numbers.
It may be remarked that the T number is a photometrically determined quantity, whereas the / number is a geometrical quantity. Since the T numbers are determined photometrically, they automatically take account of the size and shape of the aperture, the actual focal length of the lens, the lens transmittance, and any internally reflected stray light which may happen to strike the film at the center of the field (such as in a flare spot).
V. STANDARD SERIES OF APERTURE MARKINGS
The iris-aperture ring of a lens shall be marked at every whole stop on either system. The diaphragm ring shall always be turned in the closing direction, and not in the opening direction. The whole stop numbers are accurately 0.71, 1.00, 1.41, 2.00, 2.83, 4.00, 5.66, 8.00, 11.31,16.00,22.63,32.00,
These marks shall be engraved on the lens as follows: 0.7, 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. The maximum aperture of the lens shall be marked with its measured / number or T number, stated to one decimal place. These recommendations follow the American Standard Z38.4.7-1943.
VI. SUBDIVISIONS OF A WHOLE STOP
In addition to the engraved values, each stop may be divided into three subdivisions by dots or marks (not numbered), the dots being at