For some time I've known that the Movietone ratio was dropped fairly early, but couldn't find much information other than articles referring to the obvious 4x3 image of the screen.
However, after doing a more thorough search, I came up with this informative article which will be of interest to those projecting prints from 1929-1931 (the adoption of the Academy ratio):
From "Movie Age", November 9, 1929
STUDIOS SEEK TO AID TOWARDS BETTER PROJECTION GOAL
Hollywood motion picture studios are now composing all vital elements in sound-on-film pictures within an area of 0.620 by 0.825 inches through continuing to photograph the whole frame. This is in accordance with specifications recently recommended by the Academy of Motion Pictures Arts and Sciences Technicians' Branch acting jointly with the Technical Bureau of the Association of Motion Picture Producers, the American Society of Cinematographers, the Pacific Coast Section of the Society of Motion Picture Engineers and the California Chapter of the American Projection Society.
Theaters which restore the full screen image from sound-on-film pictures have been notified that to secure the maximum image size in 3 by 4 proportion they should use projector apertures whose size would be 0.600 by 0.800 inches on the basis of projection on the level, the horizontal center of the aperture coinciding with the horizontal center of the S.M.P.E standard aperture.
Too Many Aperture Sizes
The recommendations and action by the studios followed the revelation through a nationwide survey that theatres are using a wide variety of aperture sizes in projecting sound-on-film pictures. It was also found that an increasing number of theatres are restoring the full screen proportion through the use of a smaller aperture, lenses of one-half inch shorter focal length, and various recentering devices. As only two studios were composing to allow for this the result that in many theatres part of the heads and feet of characters were cut off in projection. The recommendations of the technical societies are designed to correct this serious condition and were chosen as the best mean of the projector aperture sizes among a number of large theatre chains.
Studios which are now marking the ground glasses of their cameras to conform to the recommended practice are: Paramount-Famous-Lasky, Metro-Goldwyn Mayer, United Artists, Pathe, Universal, RKO, Tiffany-Stahl, Mack Sennett, Darmour, Educational; the Fox Studio markings are the same width but allow .04 inches more height.
Leader Lengths Vary
Committees representing the motion picture technical organizations in Hollywood are also studying the problems of standard release print practice and screen illumination under the sponsorship of the Academy.
The committee on Co-ordination of Release Prints for Theatres is composed of:
S. J. Twining, chairman, manger Sound Track Laboratory, Paramount-Famous-Lasky Sutdio; A. J. Guerin, manager, Bennett Film Laboratories; M. H. Brower, president, Los Angeles Film Board of Trade; Sidney Burton, president, Los Angeles Chapter, American Projectionist Society; Gerald Rackett, director, Technical Bureau Association of Motion Picture Producers.
It is the intention of this committee to draw up tentative recommendations and secure comments from various sources before putting through a final recommendation for adoption by the studios. In this preliminary, your co-operation is cordially invited. The very great need of some standardization here was forcibly revealed when it was found that the length of the different parts of leader varied as much as twenty feet among studios and that operators throughout the country are mutilating the film in all sorts of ways to provide change over cues.
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