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374 LENS-CALIBRATION REPORT October
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 1.5mm 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 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 (2) above. The greatest care is necessary to ensure that the extended source is really uniform.
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.
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 8 = Cosec -1 Diameter of
(2 X T7 Number), Aperture = 100 tan 0,
Desired T Number Degrees Mm
0.5
90
00
0.71
45
100
1.00
30
57.74
1.41
20.708
37.80
2.00
14.478
25.82
2.83
10.183
17.96
4.00
7.181
12.60
5.66
5.072
8.88
8.00
3.583
6.26
11.31
2.533
4.42
16.00
1.791
3.12
22.63
1.260
2.21
32.00
0.895
1.56