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MEASURING THE EFFECTIVE ILLUMINATION OF PHOTOGRAPHIC OBJECTIVES*
J. HRDLICKA
The photo-chemical action of light on the photographic plate depends on the light intensity and on the length of exposure, other conditions being constant. The brightness of the photographic image is proportional to the square of the ratio of the radius of the entrance pupil to the focal distance, or to the square of the relative aperture, which latter may be expressed by l/n, where n = f/d (d is the diameter of the entrance pupil). The illumination afforded by a photographic objective is determined by its relative aperture, on purely geometric considerations. In this expression of the relative aperture, there is no factor expressing the light losses in the lens which decrease the effective illumination of objectives; for example, the losses caused by reflection at the lens surfaces, by absorption in the glass, and by absorption in the cementing material. If we designate by J0 and I the intensities of illumination before and after passing through the objective, the true, effective brightness of a photographic objective is more accurately given by the expression.
To determine 1/w, the usual methods of measuring focal distance and diameter of entrance pupil may be employed. To determine the value of the correction factor, I0/I, I have resorted to the following method:
Arrange in optical alignment on an optical bench, a point source of light, the objective to be examined, a diaphragm whose diameter is smaller than the maximum opening of the objective, and the sensitometer. A small hole, illuminated from behind by a lamp, was used as a point source of illumination. This source was placed at the focus of the objective to be examined, after the manner of a collimator. The diaphragm was placed directly in front of the
* Photo-Revue, 41 (Sept., 1929), p. 267. Translated from the French by William Vaughn.
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