Measuring the Effective Illumination of Photographic Objectives

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 1/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 I0 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.

Print ISSN

0097-5834

Published

1930-05

Content type

Original Research

DOI

10.5594/J13063