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Proposed American Standard
Aperture Calibration of Motion Picture Lenses
PH22.90
p. 4 of 10 pp.
definite T numbers, rather than to measure the T number corresponding to each of the existing marked f numbers.
5.5 It may be remarked again that the T number is a photometrically determined quantity, whereas the f 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). It is implicit in the T number system of aperture markings that every lens should be individually calibrated.
5.6 For a lens designed to be used at finite magnification, the engraved T number will correspond to the Equivalent f number defined by equation (1 1).
5.7 The procedure for measuring the T number of a lens is given in Section 13.
6. Standard Series of Aperture Markings
6.1 The diaphragm ring of a lens shall be marked at every whole stop on either system. A "whole stop" is taken to represent an interval of double or half the image illuminance, corresponding to a ratio of \/2 or \f0.5 in the diameter of a circular lens aperture. By convention, the series of whole stop numbers to be used are accurately:
0.71, 1.00, 1.41, 2.00, 2.83, 4.00, 5.66, 8.00, 1 1.3, 16.0, 22.6, 32.0
6.2 These marks shall be engraved on the lens as follows: 0.7, 1 , 1 .4, 2, 2.8, 4, 5.6, 8, 1 1 , 16, 22, 32. The maximum aperture of the lens
shall be marked with its measured f number or T number, stated to one decimal place. These recommendations follow American Standard Z38.4.7-1943.
6.3 In setting the lens aperture, it is assumed that the diaphragm ring will always be turned in the closing direction, and not in the opening direction; this is to eliminate backlash effects.
7. Subdivision of a Whole Stop
7.1 If it is desired *o subdivide a "whole stop" interval, we may refer to a fraction S of a stop, defined so as to yield a ratio of image illuminance R equal to 2s or (0.5)s. Then, for any given illuminance-ratio R, the corresponding fraction of a stop will be given by S — (log R)/(log 2) = 3.32 log R. A few typical examples are given in the following table:
Fraction of a Stop (S) Illuminance Ratio (R)
one-tenth
one-sixth
one-quarter
one-third
one-half
two-thirds
three-quarters
a whole stop
.072 or 0.932
.122 or 0.891
.189 or 0.841
.260 or 0.793
.414 or 0.707
1.587 or 0.630
1.682 or 0.594
2.0 or 0.5
7.2 When engraving a lens, each whole stop interval may be divided into three subdivisions by dots or marks (not numbered), the dots being at "thirds of a stop," namely, 0.7, 0.8, 0.9, LQ, 1.13, 1.27, L4, 1.6, 1.8, 2,0, 2.2, 2.5, 2JJ, 3.2, 3.6, 4.0, 4.5, 5.0, 5^6, 6.3, 7.1, 8.0, 9.0, 10.0, 11.3, 12.7, 14.2, 16, 18, 20, 23, 25, 28, 32
7.3 The reason for dividing each stop interval into three parts is so that the lens apertures will agree with the exposure-meter markings stated in American Standard Z52.121944, page 5. The same cube-root-of-two series is used for the Exposure Index of a film,
NOT APPROVED
342
October 1952 Journal of the SMPTE Vol. 59