We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Proposed American Standard
PH22.90 Aperture Calibration of Motion Picture Lenses
STARTING ABOUT 1940, there has been a rapidly growing need in the motion picture industry for a more accurate expression of the photographic speed of a lens than is afforded by the simple /-number ratio. The Proposed American Standard appearing on the following pages is the product of many years' industrious and patient effort to achieve agreement on a standard photometric method of aperture calibration. It is published here for 6-month trial and criticism. All comments should be sent to Henry Kogel, SMPTE Staff Engineer, prior to April 15, 1953, along with a carbon for R. Kingslake, Chairman of the Optics Committee.
The problem is essentially this: the density of a photographic image depends on (a) the brightness of the subject, (b) the effective speed of the lens, (c) the speed of the film, (d) the exposure time, and (e) processing of the film. In modern motion picture production all these factors except (b) are controlled or known to within a few per cent, but the supposed speed of the lens may be in error by as much as 60 or 70%. This is caused by loss of light through surface reflections or direct absorption in the lens, and occasionally to incorrect marking of the /-number scale.
By August 1947, no less than eight papers on lens calibration had appeared in this JOURNAL.
The Standards Committee, therefore, formed a Subcommittee on Lens Calibration to study the whole subject and to recommend a standard procedure for measuring the effective photographic speed of a lens. In October 1949 the Subcommittee published a report of
their investigations and recommendations, which became the basis of the present proposal. The introduction to the report stated in part: "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 one. The presence or absence of antireflection coatings is automatically accounted for in the calibration, and so also are factory variations in the focal length and in the iris mechanism. 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."
In November 1949 the Subcommittee was given formal status of its own in the creation of the Optics Committee under the chairmanship of Mr. Kingslake. This Committee achieved agreement on the final version of the proposal at its May 3, 1951, meeting and forwarded it to the Standards Committee for processing as an American Standard. The ballot of the Standards Committee on the question of preliminary publication brought forth several negative votes, all of which were based on objections to paragraphs dealing with some of the practical applications of T-stops. These were not fundamental aspects of the proposal and have therefore been eliminated, paving the way for its present publication.
338
October 1952 Journal of the SMPTE Vol. 59