Journal of the Society of Motion Picture Engineers (1930-1949)

The emulsion, being more opaque than the base, absorbs energy, expands and becomes the outer or convex surface of the bulge. The magnitude of the deformation produced varies continuously during the frame cycle and by an amount which is more than sufficient to affect sharpness of image focus. The emulsion side of 35mm film is toward the light source; and hence the film tends to move toward the light, away from the lens, while it is in the aperture. In accordance with the accepted terminology, the deformation is called negative when the emulsion side is convex, and conversely, positive when the emulsion side is concave. Flat film is considered to have zero deformation. The film upon entering the projector gate is not necessarily flat, but may have a slightly positive curl, the magnitude of which depends to some extent on the age and condition of the film. It appears that there is some shrinkage of both emulsion and base, the emulsion shrinking more than the base, so that the resulting curl is slightly positive. Typical positive displacement at the center of cold 35mm film as it enters the gate is between zero and 0.010 in. Instantly upon registration of the film frame with the aperture, the shutter uncovers the light for the first exposure of the cycle. Light energy is absorbed in the emulsion and transformed into heat. The expanding emulsion causes the exposed portion of the film frame to move from its initial zero or positive position, shifting it to a negative position, and causing it to take a somewhat spherical shape. There is a constantly increasing deformation during the first exposure, and a constantly changing distance of the emulsion surface with respect to the lens. Upon interception of the light by the flicker blade, further movement of the film surface toward the light source comes to a halt. With no light on the film, heat absorption by the film cannot take place. Instead, there is a loss of heat which causes the film to recede slightly toward the zero plane. At the start of the second exposure, the film surface stands somewhere between zero and its former maximum negative position. During the second exposure, the film continues its excursion negatively, first rapidly, then leveling off. At the end of this exposure the film reaches a more negative position than at the end of the first exposure. Figure 1 shows the correlation between the movement at the center of the film surface and particular instants in the frame cycle. The same movement occurs at points off the center of the film surface, though to a smaller degree. A significant effect which may be noted is that the center of the film, which is in motion during the entire cycle, travels through and beyond the acceptable focus limits defined by the depth of the focus of the projection lens. The projectionist, whose eye just cannot follow this rapid sequence of events (48 times per second), has to pick a "best average focus" position of the projection lens, somewhere between the maximum positive and maximum negative of the two exposure periods. If he judges the focus at the center of the screen, he picks a "best average focus" position near the maximum negative buckle of the first exposure. The remaining, earlier part of this exposure produces only a poor and undefined image on the screen. A portion of the second exposure, also, is beyond the limit of good image definition on the screen, and good optical performance can take place only during that part of the exposure in which the film displacement line lies within the depth-of-focus range. The "best average focus" thus obtained gives the best attainable image at the center of the screen. Actually, in practice, the projectionist may choose a slightly less negative lens position, which is a compromise to gain relatively fair overall definition across the whole screen. Even this compromise results in a fairly large percentage of "out-of-focus" time during a cycle. Willy Borberg: Reducing Film Buckle 95