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1949 AIR COOLING OF FILM 647
(4) Focus Drift
As the projection intensity is increased still further, it is observed that the projectionist must now refocus occasionally during the projection of a single reel of film. This appearance of focus drift begins somewhere in the region 0.35 to 0.40 mean net watt per square millimeter. Depending upon the circumstances, there may appear to be a rather sudden change in focus with the transition from scene to scene, even though there is no splice in the film at this transition. Or it may appear that there is a gradual loss in sharpness of the projected image obtained at the start of the roll, and this must be corrected by occasional focus resharpening.
At intensities where this phenomenon occurs, we believe the only satisfactory solution is more constant attention by the projectionist. The influence of this effect can be minimized, however, by the technique of the initial focusing. Every projectionist has observed that in the setting of the projection lens there is some leeway over which the projected image remains approximately equally sharp. It is apparent that if the lens is set at the mid-point of this tolerance, slight film motion in either direction may not exceed the depth of focus of the projection lens; if, however, the lens should be focused at the limit of its tolerance, film motion in one direction will be obvious immediately as a softening of the picture detail. If the projectionist acquires the habit of setting his lens at the approximate mid-point of this depth of focus, the necessary focus corrections will be minimized.
This focus drift is apparently the result of a gradual variation in the amount of negative drift of the film at various points in the roll, induced perhaps by changes in moisture content, degree of exposure to air, and other intangible variables of film handling. When the amount of negative drift is small, variations in the amount pass unnoticed, but with the increase in magnitude of negative drift at higher intensities, some of these variations may be large enough to exceed the 0.002to 0.004-inch tolerance in lens position for maximum screen sharpness.
(5) Image Flutter
Beyond the limit of radiant-flux density at which film can be projected without image difficulty, the first warning is a softening of the focus — which we have called "image flutter." The picture is still reasonably sharp upon the screen and it might well be that to the theater audience no lack of quality would be apparent. However,