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

Proposed American Standard Aperture Calibration of Motion Picture Lenses PH22.90 see American Standard Z38. 2. 1-1947, page 11. One-third of a stop represents a logarithmic illumination ratio equal to 0.1, which is the transmittance of a neutral density of 0.1. The ratio of successive circular stop diameters is equal to \/2 — 1.123. 8. Symbols 8.1 Lenses calibrated on the f system should bear the designation f/ or f: followed by the numerals (see American Standard 238.4.71943). 8.2 Lenses calibrated on the T-stop system should bear the designation T or T— followed by the numerals. 9. Accuracy of Marking (f System) 9.1 The maximum opening of a lens on the f system shall be marked with an accuracy of 12 per cent of area, or — 6 per cent of diameter.* 9.2 NOTE: Since in most factories a blanket calibration is generally used for the f apertures of a complete run of lenses of the same type, the smaller openings may be in error by * Z38.4.4-1942 the engraved focal length of lenses for still picture photography must be within ± 4 per cent of its true value, and in Z38. 4.7-1 943 the measured diameter of the maximum entering beam shall be at least 95 per cent of the quotient obtained by dividing the engraved focal length by the engraved f number. Thus by combining these tolerances we find that the diameter of the maximum lens aperture may be in error by as much as 9 .per cent. This represents an error in area of 18 per cent, or one-quarter of a stop, which is felt to be unnecessarily large for the maximum aperture. The proposed tolerance on aperture marking for motion picture objective lenses allows less latitude than that provided for still picture camera lenses by Sectional Committee Z38 (Photography), because of the stricter requirements in cinematography on the same continuous length of film using different lenses. p. 5 of 10 pp. — 25 per cent of area, or — 12 per cent of diameter (one-third of a stop), particularly in short-focus lenses. These figures are based on the assumption that the iris will always be closed down to the desired aperture and not opened up from a smaller aperture, to eliminate backlash effects. 10. Accuracy of Marking (T System) 10.1 Since each lens is individually calibrated, an accuracy of one-sixth of a stop (10 per cent in illumination or 5 per cent in diameter) becomes entirely possible throughout the whole range of the diaphragm scale. This is assuming that the diaphragm is always closed down to the desired aperture and not opened up from a smaller aperture, to eliminate backlash effects. 10.2 Alternatively, the manufacturer should be prepared to guarantee this accuracy even though each stop marking may not be individually determined. 10.3 It may be of interest to indicate the approximate magnitude of this tolerance. Since 5 per cent in diameter corresponds to 5 per cent in f number, a lens of aperture nominally f/2 may be anywhere between f/1.90 and f/2.10. A lens nominally f/4.5 may lie between f/4.28 and f/4.72; and a nominal f/8 may lie anywhere between f/7.6 and f/8. 4. 11. Measurement of t Apertures (Distant Object) 11.1 The procedure for measuring the f number of any lens having a circular diaphragm aperture is described in American Standard Z38.4.20-1948, paragraph 3. 11.2 If the entrance pupil is noncircular, it is necessary to measure its area. This may be done conveniently by mounting a point source NOT APPROVED October 1952 Journal of the SMPTE Vol. 59 343