Journal of the Society of Motion Picture Engineers (1930-1949)

Proposed American Standard Aperture Calibration of Motion Picture Lenses PH22.90 such a position that the little image of the source falls wholly within the standard aperture. The meter reading should then remain the same no matter whether the lens is in or out of the beam. A second plate with a 3-mm aperture should be placed over the comparison aperture while the lens is in place to stop any stray light which may be reflected from the interior of the lens. 13.4.4 It should be noted particularly that if this method is used, the focal length of the lens must be measured separately, and a suitable set of open apertures constructed for use with it. However, by suitable devices, one single set of fixed apertures may be used for all lenses, as described by Townsley.14 13.4.5 It should also be noted that this procedure measures f number as the ratio of f/D, and the measurement is thus influenced by the state of correction of the lens in regard to spherical aberration and sine condition. 13.4.6 The corner-to-center ratio at any desired aperture can be conveniently determined by simply rotating the lens through the desired field angle (/> and comparing the photocell reading with its value for the lens axis. The light-flux ratio can then be read off a calibration curve for the photocell system, and converted to the desired corner-to-center illumination ratio by multiplying it by cos3<£. (Note that this procedure will be correct only in the absence of distortion, but no motion picture lens is likely to have enough distortion to cause any significant error.) 13.5 T-Stop Calibration at Finite Magnification. 13.5.1 To use the extended source method (see Section 13.3), it is only necessary to mount the metal plate at the desired image distance from the lens instead of placing it in p. 9 of 10 pp. the focal plane. The open apertures used for comparison must be calculated to have an opening corresponding to the desired Equivalent f number multiplied by (1 + m). This is because we are really comparing the illuminance given by the lens with the Effective f number of the open hole, but the engraving must be done at each standard step of the Equivalent f number (see Section 1 2.2.) 1 3.5.2 The collimated source method cannot be used to calibrate a lens at finite magnification. References General 1 . A. C. Hardy, "The distribution of light in optical systems," J. Frank. Inst., vol. 208, pp. 773-791, Dec. 1929. 2. A. C. Hardy and F. Perrin, "Principles of Optics," McGraw-Hill, New York, 1932, p. 411. 3. L. C. Martin, "Applied Optics," vol. 2, Pitman, London, 1932, p. 210. 4. D. B. McRae, "The measurement of transmission and contrast in optical instruments," J. Opt. Soc. Amer., vol. 33, pp. 229-243, Apr. 1943. Lens Calibration 5. G. W. Moffitt, "Determining photographic absorption of lenses," J. Opt. Soc. Amer., vol. 4, pp. 83-90, May 1920. 6. J. Hrdlicka, "Measuring the effective illumination of photographic objectives," Jour. SMPE, vol. 14, pp. 531-553, May 1930. 7. D. B. Clark and G. Laube, "Twentieth Century camera and accessories," Jour. SMPE, vol. 36, pp. 50-64, Jan. 1941; also U.S. Patent 2,334,906 (filed Sept. 1940, issued Nov. 1943). 8. E. W. Silvertooth, "Stop calibration of photographic objectives," Jour. SMPE, vol. 39, pp. 119-122, Aug. 1942. 9. L. T. Sachtleben, "Method of calibrating lenses," U.S. Patent 2,419,421 (filed May 1944, issued April, 1947). (Note: This patent is held by RCA, which has expressed willingness to grant a paid-op license for a NOT APPROVED October 1952 Journal of the SMPTE Vol. 59 347