International projectionist (Jan-Dec 1958)

'Focus-Drift' By JAMES J. FINN A few pert observations relative to a prime projection problem concerning which there has been too little observation and thought and much too much talk not founded upon the realities. FOCUS-DRIFT is a prime projection problem today, as reflected by the steadily increasing number of complaints by projectionists anent their inability to achieve proper focus of the screen image. Heat from the arc, radically increased in recent years, is the relentless enemy of good projection, with the lens, the projector parts, and the film base itself being the victims thereof. Of late, however, projectionists have lent ready acceptance to the notion that the combination of magnetic striping and acetate film base is the chief culprit, and that of the two the acetate base itself is the more blameworthy. This ready acceptance of a proposition which, while given widespread expression within the craft, is not sup NO SHUTTER POSITIVE BUCKLE 0 NEGATIVE BUCKLE POSITIVE BUCKLE 0 NEGATIVE BUCKLE POSITIVE BUCKLE 0 NEGATIVE buckle: ported by the facts violates good projection practice in that it precludes serious thought about other aspects of the problem and induces inertia in devising means to solve it. As a noted statesman once said: "Let's have a look at the record [facts]." 'Rigidity' in Film Bases While acetate film is slightly less rigid than nitrate, the differences are small and, actually, both lose about the same percent in rigidity as the temperature rises. While it is true that acetate film softens when warmed, we must remember that nitrate also softens. When nitrate was used for motion picture film, it was also used for the production of combs, dresser sets, etc.; all of these were formed by warming nitrate and pressing it or blowing it into molds where it behaved exactly like the acetate plastics that are marketed today. Extensive tests have not indicated that the presence of magnetic striping results in any increased absorption of heat by the film base. First, it is the film trap itself which receives the direct heat of the arclamp and which contacts the larger area of film. This, of course, touches the emulsion side on which there are no stripes. The film trap door is heated only by conduction through the mechanism and 2-BLADE 60° 5HUTT Illustrative of the radical changes in projector mechanisms and, therefore, projection practice within the past decade is this graphic example of just one factor which affects "true" focus (if such be possible) — film behavior in the projector gate. Film normally enters the gate with a slight "positive buckle" bulging toward the lens. Emulsion begins to expand relative to the film base the moment arc radiation impinges upon the film, buckling negatively toward the lamp. INDOOR THEATRE PROJECTION Summary Indicating Changes During Last Ten Years LAMPHOUSES 1948 1953 Total Screen Lumens. Top Houses 14-21,500/33-46,000 PROJECTOR MECHANISM Shutter Transmission 55% 67% SCREENS Average Width (For Top 10% of Indoor Theatres) 25 ft. 55 ft. LENSES Aperture: F:2.0 F:1.7 Average Focal Length (For Top 10%) 5 in. 2.25 in. Depth of Focus (From Tables) 0.010 in. 0.0038 in. by some stray light which has already passed through the film. Accordingly, it reaches a lower temperature than the film trap itself. Furthermore, in an unstriped print the tension pads will be in direct contact with the film base; whatever heat is conducted from pads to base will be greater in this instance than if the two were separated by a magnetic stripe. In other words, the conductivity of the magnetic stripe is many times less than that of steel pads and, therefore, the stripe tends to reduce very slightly the heating of the film base. The only possibility noted for the presence of the stripe to affect focus quality is that the tension pads contact only the surface of stripes 1 and 3 and do not touch the film over the full pad width as they would for unstriped prints. This means that the film is held a little less rigidly if it should be made to buckle toward the projection lens. Since during normal projection, however, the motion is toward the arclamp, this slight difference in holding area has no effect. The Second 'Villian' — the Lens There have been noted slight changes in focus from the heating of the projection lens. Rather than changes in the glass curvature, however, much greater importance should be attached to the expansion of the projector frame as its temperature is increased and to the expansion of the lens barrel. These two motions move the lens farther away from the film and also separate the lens elements, both of which must then be (Continued on page 22) 10 INTERNATIONAL PROJECTIONIST JUNE 1958