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Report of
Lens Calibration Subcommittee
I. INTRODUCTION
THIS SUBCOMMITTEE of the Standards Committee was appointed primarily to establish a standard method of photometrically calibrating diaphragm openings for motion picture camera lenses as distinguished from the well-known / system of aperture markings. The demand for a photometric type of aperture calibration ("T stop") is becoming increasingly felt, and it has the advantage that diaphragms of any shape, pentagonal, scalloped, or irregular, can be correctly labeled with as much ease as a circular aperture. The presence or absence of antireflection coatings is automatically included in the calibration, and so also are factory variations in the focal length and in the iris mechanism. The illumination on the film in the center of the field will therefore be the same for all lenses at the same T stop, assuming that the object is a uniform plane surface perpendicular to the lens axis. It is implicit, also, that each lens shall be individually calibrated if the photometric method is used.
Lens speeds for distant objects only are considered in this report; the corresponding problem for lenses intended to be used with near objects will be discussed later.
II. THEORY
Suppose we have a well-corrected lens satisfying the sine condition, and used with a very distant object. Then if at some given diaphragm opening the area of the entrance pupil is A and the focal length is /, the illumination at the center of the field will be E, where
E = tBA/f*. (1)
In (1) t is the transmittance of the system (emerging flux divided by entering flux), and B is the brightness of the object in candles per square unit.
For the special case of a lens having a circular aperture,
hence E ~ 4(/ number)' (3)
where / number is equal to l/(2 sin 0). (4)
368 OCTOBER, 1949 JOURNAL OF THE SMPE VOLUME 53