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

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 percent in illumination or 5 percent 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. 1 0.3 It may be of interest to indicate the approximate magnitude of this tolerance. Since 5 percent in diameter corresponds to 5 percent 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 i/7.6 and f/8.4. 1 1 . Measurement of f 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 Methods of Designating and Measuring Apertures and Related Quantities Pertaining to Photographic Lenses, 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 of light, such as a small hole in front of a lamp bulb or a 2-watt zirconium lamp, at the rear focal point of the lens, and allowing the light beam which emerges from the front of the lens to fall upon a piece of photographic material. After processing, the recorded area is meas Page 5 of 8 pages ured with a planimeter and applied in equation (7). If the lens is too small for this procedure to be employed, it may be placed in a suitable telecentric projector working at a known magnification (a workshop profile projector is suitable), the back of the test lens being towqrds the source of light. The entrance pupil then will be projected onto the screen of the projector at a known magnification, whence its area can be determined with a planimeter. 12. Measurement of i Apertures (Near Object) 12.1 To measure the Effective f number of a lens when used with a near object, it is necessary to determine the angle 9 in equation (9). This may be done by using a point source of light at the correct axial object position, and measuring the diameter of the emerging beam at two widely separated planes a known distance apart. A simple computation will enable the semicone-angle 9 to be determined. 12.2 The Effective f number is defined by l/(2 sin 9); and the Equivalent f number for engraving on the lens barrel will then be equal to the Effective f number divided by (1 + m), where m is the image magnification. (See section 4.4 above.) 13. Photometric Calibration of a Lens 13.1 General Requirements 13.1.1 Since T-stops are based on a measurement of the illumination produced by the lens at the center of the field, it is first necessary to define the latter term. For the purpose of illumination or flux measurements, the term "center of the field" shall be taken to mean any area within a central circle approximately 3 mm in diameter for 35mm or 16mm frames, or 1.5 mm in diameter for 8mm frames. 13.1.2 The light used in making the determination shall be white,* and the sensitivity characteristic of the photoelectric receiver * Specifically, a tungsten filament lamp operating between 2900 and 3200 K. PH22.90-1953 February 1954 Journal of the SMPTE Vol. 62 179