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Proposed American Standard
Aperture Calibration of Motion Picture Lenses
PH22.90
p. 7 of 10 pp.
preferably at other apertures also. For this purpose the 3-mm (or IV-z-mm) hole shall be used first at the center of the field, and then moved outwards until its rim is touching the top and side limits of the camera gate. This distance is shown in Table I.
Table
Gate, Mm
35(16.03 x 22.05)
16 ( 7.47 X 10.41)
8 ( 3.51 x 4.80)
Radial Shift of Hole, Mm
11.5
4.5
2.0
13.3 Extended-Source Method of T-Stop Calibration (distant object).
13.3.1 This method of lens calibration has been described by Gardner13 and Sachtleben,9 the underlying theory being given by McRae.4 It is based on filling the lens with light from an extended uniform source, and placing a metal plate in the focal plane of the lens with a 3-mm hole (or 1.5-mm for 8-mm film) at its center. The light flux passing through the hole is measured by a photocell arrangement. This flux is then compared with the flux from the same source passing through the same hole from an open circular aperture of such a size and at such a distance from the plate that it subtends the desired angle 6 referred to in equation (2) above. The greatest care is necessary to ensure that the extended source is really uniform, and also constant throughout the measurements. The open circular aperture is used as the "ideal lens with 100 per cent transmittance" referred to in Section 5.2.
13.3.2 It should be noted that this procedure measures the T-stop Aperture Ratio of the lens directly, regardless of whether or not the lens is aplanatic.
13.3.3 In practice, the photocell reading for each whole T-stop number is first determined
for a series of open apertures, at a fixed distance from the plate. The lens is then substituted for the open aperture with the 3-mm hole accurately in its focal plane, and the iris of the lens is closed down until the photocell meter reading produced by the lens is equal to each of the successive open-hole readings. The full T-stop positions are then marked on the diaphragm ring of the lens. The intermediate third-of-a-stop positions may be found with sufficient accuracy by inserting a neutral density of 0.1 or 0.2 behind each open aperture in turn and noting the corresponding photocell readings.
13.3.4 The following table of aperture diameters may be useful. They are based on a distance of 50 mm from aperture to plate. (It is important to remember the difference between sine and tangent, and that the aperture diameter is not found merely by dividing 50 mm by the T number.)
Table II
Value of 0 =
Cosec~l Diameter of
Desired (2 X T number), Aperture =
T Number Degrees 1 00 tan f). mm
0.5
0.71
1.00
1.41
2.00
2.83
4.00
5.66
8.00
11.31
16.00
22.63
32.00
90 45 30
20.708
14.478
10.183
7.181
5.072
3.583
2.533
1.791
1.266
0.895
oo
100
57.74
37.80
25.82
17.96
12.60
8.88
6.26
4.42
3.12
2.21
1.56
13.3.5 A single set of apertures is sufficient to calibrate lenses of all focal lengths, since the only factor involved is sin 9, and that is
NOT APPROVED
October 1952 Journal of the SMPTE Vol. 59
345