International projectionist (Jan-Dec 1949)

VOLUME XXIV SEPTEMBER 1949 NUMBER 9 Lens and Film Factors Affecting Focus OVER and above the special problems peculiar to the projection of colored pictures, certain other focusing difficulties are common to prints of all types. These are (1) poor photographic image, (2) image flutter, and (3) focus drift. Little need be said about out-offocus pictures on the film; the projectionist can do nothing to improve their appearance on the screen. The distributor, ever evading responsibility, would like to have us believe that the condition is also beyond his control; so it avails little to complain about a poor print when the fault lies with either the camera crew or the processing laboratory. Image flutter is a rapid in-and-out-offocus effect caused by buckled and warped film. In rare instances frameembossed film gives a somewhat similar fluttery image. Buckling is caused by the heat to which the film is subjected in the projector. Heat shrinks film, and if the heating were uniform, the consequent shrinkage would be uniform. But in passing through the projector gate the perforation margins absorb more heat than the picture part of the film because of their prolonged contact with the hot gate shoes, or runners. The picture portion receives only a momentary flash of radiant heat (infrared), and much of this passes through the film with the visible light instead of being absorbed. As a result, the edges undergo greater shrinkage than any other part of the film. The picture portion is thus buckled out of shape so that it flops in and out of focus during projection. Nitrate (inflammable) film is much more susceptible to the deleterious effects of heat than the more stable high-acetyl acetate (safety) film. Severe buckling By ROBERTA. MITCHELL II and film shrinkage should be relatively rare with the new high-acetyl safety film. The projectionist should not attempt to recondition full-length reels of buckled film. However, short rolls of buckled film — announcement and advertising trailers, for example — may easily be treated in the projection room with more or less success. A buckled acetate film should be wound in a roll having the clear, or base, side of the film facing out. The roll is then placed in a film can, the cover of which is fitted with a blotter lightly moistened with water containing glycerine to prevent drying out (1 teaspoonful of glycerine to half a pint of water). The box is closed tightly and left undisturbed for a week. If at the end of that time traces of buckling remain, the treatment should be repeated. A buckled nitrate film should be wound with the glossy clear side of the film facing out. The roll is placed in a film box having a cover which is fitted with a blotter heavily charged with camphor and also lightly moistened. Dissolve a small cake of camphor in an ounce or two of acetone. Wet the blotter thoroughly with this solution. When the acetone has completely evaporated, the fibers of the blotting paper will be impregnated with the camphor. The dry camphorated blotter may then be lightly sprinkled with the waterglycerine mixture used for safety film. The box is tightly closed and left undisturbed for a week or more. Unfortunately, there is no way to over come the image flutter of buckled film during projection. Increased gate tension fails to relieve the trouble. The annoying and insidious phenomenon known as focus drift is more common than is generally supposed. Prints having focus-drift characteristics require changes in focus during projection. Progressive focus drift is characterized by a practically constant shift of required focus. If a reel having this fault be focused sharply at the beginning, the lens not being subsequently adjusted, the image on the screen will gradually lose its definition and become considerably out of focus by the time the reel nears its end. Irregular focus drift is evidenced by random transient losses of focus which are truly perplexing when the film is a supposedly uniform release print. Composite reels (those made up of trailers or other assorted individual rolls) are well-known examples of irregular focus drift. It is almost impossible to detect the presence of focus drift in a reel of film except by actual projection, but all "kinky" films and those having a strongly set curl at either beginning or end are very likely to offer trouble from this cause. Buckled film, we have seen, is caused by projection conditions. Accordingly, prints fresh from the processing laboratories are never buckled unless grossly mishandled. Focus drift, contrariwise, is often present in prints which are practically brand new! The immediate cause of focus drift is film-curl, a more or less permanent deformation of the film base. Irregularities in frame embossing and variations in film thickness could also cause focus drift, INTERNATIONAL PROJECTIONIST • September 1949