Journal of the Society of Motion Picture and Television Engineers (1950-1954)

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shall approximate that of ordinary panchromatic emulsion.f It is considered that these factors are not at all critical and no closer specification than this is necessary. Obviously, errors will arise if the lens has a strongly selective transmission, but such lenses would be undesirable for other reasons. 13.1.3 The incident light shall fill a circular field whose angular diameter is no more than 10 degrees in excess of the diagonal of the intended angular field of the lens itself. During measurement, the light shall traverse the lens in the direction ordinarily employed in photography. 13.1.4 The lens should be carefully examined before calibration to ensure that there are no shiny regions in the barrel which would lead to flare or unwanted stray light, since this would vitiate the measurements badly. The lens surfaces should be clean. 13.2 Corner-to-Center Ratio. Having calibrated the stop markings of the lens on the T system by one of the methods to be described, the observer may, if desired, determine in addition the ratio of corner illumination to center illumination, at full aperture and preferably at other apertures also. For this purpose the 3-mm (or ll/a-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 1. Table 1 Gate, Mm Radial Shift of Hole, Mm 35(16.03X22.05) IT. 5 16(7.47X10.41) 4.5 8 ( 3.51 X 4.80) 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.* 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 8mm t A suitable cell is one having an S-3 surface, combined with a Corning 9780 glass filter about 2.5 mm thick. Page 6 of 8 pages 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 8 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 perce.nt 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 filter of 0.1 or 0.2 behind each open aperture in turn and noting the corresponding photocell readings. 13.3.4 Table 2 which lists 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 nof found merely by dividing 50 mm by the T number.) 13.3.5 A single set of apertures is sufficient to calibrate lenses of all focal lengths, since the only factor involved is sin 0, and that is fixed by the aperture used. The apertures should be bevelled to a sharp edge, and well blackened on both sides. 13.3.6 The extended source should be uniformly bright over its useful area to within ± 3 percent. (This can be tested with a suitable PH22.90-1953 180 February 1954 Journal of the SMPTE Vol. 62