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

American Standard Aperture Calibration of Motion -Picture Lenses Reg. 1/.S. Pat. Of. PH22.90-1953 -UDC 778.5 1. Scope 1.1 The purpose of this standard is to define the f and T numbers used to express the relative aperture of a photographic objective. A second purpose is to establish means for calibrating the diaphragms of objectives in both the f and T systems, with suitable tolerance specifications. 1.1.1 The f number of a lens represents a true geometrical measure of the relative aperture. 1.1.2 The T number is a photometrically determined measure of the relative aperture of a lens adjusted to take proper account of the lens transmittance, so that the illuminance in the center of the lens field will be the same for all lenses at the same T-stop setting. This assumes that the object is a uniform plane diffusing surface perpendicular to the lens axis. 1 .2 It should perhaps be mentioned that the photometric calibration of a lens diaphragm as contemplated by the T system of diaphragm marking established by this specification is only one step in extending the control for the purpose of producing negatives of a desired uniform density. The density of a negative is dependent upon the illumination and reflectance of the object photographed, the correctness of the diaphragm marking, the absorption of the lens, the accuracy of timing of the exposure, the uniformity of the emulsion employed, and complete control of the processing. The application of the T-stop system is designed to improve the control as regards correctness of diaphragm marking and absorption of the lens. The importance and need for this particular control increases as the control of the other factors enumerated is improved. Page 1 of 8 pages 2. Theory 2.1 The illuminance at the center of the image of a uniform plane extended object perpendicular to, and centered on, the lens axis, when the lens has a circular aperture, is given by: E = TT t B sin20 (1) In this formula: E is the illuminance in lumens per unit of area; f is the lens transmittance, expressed as the ratio of emerging flux to entering flux for a beam sufficiently narrow to pass through the lens without obstruction by the lens mount; 8 is the object luminance in candles per square unit; and 0 is the semiangle of the cone subtended by the circular exit pupil of the lens at the point where the lens axis intersects the image plane. 2.2 If the lens can be assumed to be aplanatic, that is, to be free from spherical aberration and to satisfy the sine condition, and if the object is very distant, then the value of sin 0 will be given by: /> Y /o\ sm0-~ (2) where Y is the semidiameter of "the circular entrance pupil of the lens and f is the focal length. The validity of this equation may be seen by reference to Fig. 1 , remembering that, in a lens having the type of correction assumed in this paragraph, the principal planes of Gauss are in reality portions of spheres centered about the axial object and image points, respectively. 2.3 If the lens aperture is not circular, which will often occur when the iris is partly closed, the angle 6 has no meaning. In such a case, we may define the effective diameter, D', of the entrance pupil in terms of its area, A, by: A = 0) Approved December 17, 1953, by the American Standards Association, Incorporated Sponsor: Society of Motion Picture and Television Engineers "Umirrsal Decimal CUssificati. Copyright 1954 by Ihe American Standards Associ: 70 Eati Forty-fifth Street. New York 17. N. Y. Printed in U.S. ASAV£M2St Price, SO Cents February 1954 Journal of the SMPTE Vol. 62 175