Errors in Calibration of the f Number

The present system of marking the diaphragm stops in terms of the geometric f number is subject to serious deficiencies so far as uniform performance for lenses set at the same marked stop opening is concerned. Decisions regarding the proper exposure time to use at a selected stop opening may be in error by ± 10 per cent for a lens whose surfaces do not have antireflection coatings, and by even greater amounts for a lens whose surfaces do have antireflection coatings. These errors arise from differences in the reflection and absorption losses in the lens elements themselves, departures of the measured from the nominal focal length, and departures of the measured diaphragm openings from the nominal diaphragm openings. — A method is described whereby a lens can be calibrated by a light meter in terms of an ideal lens so that the variation in axial illumination in the focal plane need not exceed ± 2 per cent in using different lenses set to the same calibrated stop opening.

Print ISSN

0097-5834

Published

1948-09

Content type

Original Research

DOI

10.5594/J11752