Film Distortions and their Effect upon Projection Quality

Abstract

The five most generally recognized types of film distortion are discussed. These consist of curl, spokiness, embossing, flute or long edges, and buckle or short edges. — Curl has come to be an accepted fact and is ordinarily without importance in projection except when it becomes excessive. — Spokiness, sometimes called square rolls or hexagonal rolls, is a phenomenon observed when film with a high degree of curl is wound with insufficient tension to keep the roll perfectly round. Poor screen quality in the case of 16-mm films has sometimes been associated with this defect. — Embossing is due to differential shrinkage or hardening of the emulsion caused by local absorption of heat in the dense portion of the picture. Careful tests have failed to show any effect upon the screen such as in-and-out of focus due to image embossing. Measurements of the magnitudes of the distortions show that these are ordinarily much less than the depth of focus of the lens. — Flute, or long edges, is more often seen with safety film than with nitrate film. It is generally caused by a stretching of the edges by recessed rolls, by shrinking the center of the film with high-temperature arcs on projection, or by exposing the roll to excessively high humidities, which causes swelling at the edges. Laboratory tests as well as field experience indicate that fluted edges very rarely cause distortion of the images on the screen. — Buckle, or short edges, is believed to be the most serious type of film distortion. It is caused by greater loss of moisture or solvent from the edges of the film than from the center. This leaves a fullness of the center resulting in an “oil-can” effect when film passes through the projector, thus producing pictures that go in-and-out of focus on the screen. — Buckle trouble may result from storing rolls of film in packages that are easily permeable to moisture vapor but it may be avoided by the use of impermeable packaging materials.