An Instrument for Photometric Calibration of Lens Iris Scales
An instrument for calibrating iris scales of photographic lenses is described. The iris scales are calibrated in T stops based on the photometric transmission parallel to the axis. This follows the procedure proposed by Daily.1 The new instrument employs measuring and comparison of optical paths which receive alternate light pulses from a single incandescent source. Calibrated attenuation is provided in the comparison beam so that the two systems may be balanced to give a null output from the photomultiplier cell which is used as a null detector. The null-balance principle makes the unit extremely stable and the sensitivity is sufficient to make accurate measurements on iris openings as small as 0.031 in., which corresponds to 1 in. T 32. Data are given on transmission of several lens types, and on the accuracy of the instrument, and a proposal is made for changing over to the new system.
- Print ISSN
- 0097-5834
- Published
- 1947-08
- Content type
- Original Research
- DOI
- 10.5594/J17822