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

Proposed American Standard Aperture Calibration of Motion Picture Lenses PH22.90 p. 6 of 10 pp. 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 measured 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 towards 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 f Apertures (Near Object) 12.1 To measure the Effective / number of a lens when used with a near object, it is necessary to determine the angle 6 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 0 to be determined. 12.2 The Effective f number is defined by l/(2 sin 6); 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 13.1.1 Since T-stops are based on a measurement of the illumination produced by the lens at the center of the field, it is first neces sary 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 shall approximate that of ordinary panchromatic emulsion. t 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 * Specifically a tungsten filament lamp operating between 2900 and 3200 degrees Kelvin. t A suitable cell is one having an S-3 surface, combined with a Corning 9780 glass filter about 2.5 mm thick. NOT APPROVED 344 October 1952 Journal of the SMPTE Vol. 59