The motion picture projectionist (Nov 1929-Oct 1930)

12 Motion Picture Projectionist October, 1930 Proposed New Standard Focal Lengths By Wilbur R. Rayton Scientific Bureau, Bausch & Lomb Optical Company * IN the early days of motion pictures, projection objectives were supplied in a series of focal lengths which differed from each other by only an eighth of an inch. The burden of making and carrying in stock so great a variety of focal lengths became oppressive for both the manufacturer, and the dealer and, in consequence, the Society at its Chicago meeting in 19171 adopted as standard a series of focal lengths in which the interval between numbers was a quarter of an inch. This practice has been followed up to the present day. The only further attention given by the Society to the subject of projection lens focal lengths consisted in adopting a standard permissible tolerance of plus or minus 1 per cent.2 At the present time the popular numbers include the range from 4.0 inches to 8.0 inches in focal length. A variation of 0.25 inches in 4.0 inches amounts to 6.25 per cent; in an 8.0 focal length the same change is only half as great per cent or 3.125 per cent. This range of focal lengths is required not so much because of any corresponding range on screen sizes as to accommodate a wide variation in projection distance. A lens of 4.0 inches focal length will project a 20-foot picture at a distance of about 90 feet. If a change is desired it must be either a 3.75 or a 4.25 inch lens if we take the shortest available focus on either side. The corresponding change in screen size will be about 1.25 feet. If the projection distance is 180 feet it will require an 8.0 inch lens to give the same size picture, and the nearest focal length on either side will permit a variation of picture size of about 0.6 feet. The question arises as to why a minimum variation of picture size which is satisfactory when dealing with a 4.0 inch lens should not be satisfactory if a lens is required of double the focal length. In fact, why should the series of focal lengths be based on an arithmetical increment at all except for convenience in remembering what lenses are available? It would seem much more sensible to make the series of focal lengths a geometrical series in which each focal length differed from the one before it by a fixed ratio. Under any circumstances this suggestion would merit consideration, but at the present time a new com *A paper read at the Washington, May, 1930, meeting of the S. M. P. E. 1Report by Committee on Optics, Trans. S. M. P. E., No. 4 (1917). 2Report of Committee on Standards. May, 1923, Trans. S. M. P. E., No. 16 (1923). p. 314. plication is introduced by the deplorable state of affairs introduced by sound motion pictures. Sound Adds to Problems It is difficult to imagine just how the solidly entrenched ranks of directors, cinematographers, and exhibitors ever permitted themselves to be manipulated into the position in which they now are as a result of the manner in which the sound-onfilm process was developed. The appropriation of that relatively small area on the film for the sound record HARRY C. GRIFFIN It is with deep regret that the General Office reports the passing of one of the most honored and popular members of the International Alliance, Brother Harry C. Griffin, Secretary of Detroit, Mich., Local No. 38. Our beloved member passed away at a local Detroit hospital on August 19th, following an operation. Since his initiation into the Detroit organization in November, 1907, Brother Griffin has played a most active part is the progress of his own local union, as well as a wholehearted activity in the affairs of the Alliance. His loss to the Eighth District will be severely felt and his constructive influence sorely missed. At the recent Los Angeles Convention he was re-elected to the office of Delegate to American Federation of Labor Conventions. It is also recalled that Brother Griffin acted in the capacity of Chairman of the Joint Detroit Committee at the 1926 Convention, during which time, in his customary unselfish manner, he spared himself no inconvenience and made every sacrifice that the Delegates might receive every consideration to make their sojourn in Detroit a most pleasant one. We feel that each and every member of the Alliance, with whom Brother Griffin has come in contact, joins with the official family in extending our sincere condolence and profound sympathy to the bereaved relatives of the deceased.— I. A. General Bulletin, No. 254. has played havoc with photography and projection. From the projectionist's standpoint, the attempt to project from the complete sound-on-film picture requires a movable mask at the screen to hide the empty space otherwise plainly visible and makes the projected picture so nearly square as to be decidedly displeasing. If, on the other hand, a mask is employed in the aperture in the projector to restore the original ratio of height to width either a smaller picture results or a projection, lens of shorter focal length must be used. If the latter expedient is adopted we can restore the picture size, to be sure, but the center of it no longer coincides with the center of the screen and the lens must be shifted in a direction perpendicular to its axis to restore the center of the picture to its desired place on the screen. This violates all good optical practice and is responsible for some pretty bad projection. Ignoring the troubles of the studio, it is easy to see that the sound-onfilm process has not filled the life of the projectionist with joy and it is to some degree surprising he has not made a more effective protest. Film and Disc Projection To add to his troubles, the careful projectionist who tries to find combinations of focal lengths which will give pictures of the same size from Vitaphone and Movietone or Photophone film finds that in only a few cases is it possible. His film apertures differ in a definite ratio, but available focal lengths differ in an arithmetical progression. The difference in film apertures is 11 per cent., and if projection lenses could always be found which differ in focal length by just this amount he could always project pictures of at least the same size. A series of focal lengths in which the interval was as great as 11 per cent, would be totally unsatisfactory because of too great change in picture size in passing from one lens to Table I Proposed Series of Focal Lengths, Interval 5.5 Per Cent. 3.00 in. 4.46 in. 6.62 in. 3.17 in. 4.72 in. 7.01 in. 3.36 in. 4.99 in. 7.42 in. 3.55 in. 5.28 in. 7.85 in. 3.76 in. 5.59 in. 8.31 in. 3.98 in. 5.92 in. 4.21 in. 6.26 in.