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CONTINUOUS AIR FROM FRONT JE1
DEPTH OF FOCUS RANGE OF LENS
I 1
f I f 1 I
CONTINUOUS AIR FROM REAR JET Fig. 3. Film displacement due to buckling at center of frame — Continuous Air.
(a) The air from the front jet was modulated by means of a rotary valve driven from the shutter shaft.
(b) The air from the rear jet was not modulated and the steady stream of air from this jet was used to force the film toward the lens, thus partly neutralizing the internal forces, which tend to make the film take a deep negative buckle under the influence of light.
(c) The correcting air pulses from the joint jet were timed so that the resultant forces from both front and rear jets opposed the cyclicly varying buckle forces. The motion of each film frame on the optical axis could thus be controlled.
Figure 5 illustrates the timing of the jets and shows that the position of the film frame can be held steady within fairly close limits. It should be noted that the excursions of the film frame surface can be confined to the depth-offocus range of the lens. Good optical
performance is thus attained over virtually the entire frame cycle.
Figure 5 also shows that a negative displacement of approximately 0.012 in. is allowed to exist at the center of the frame. The question may be asked, "Why is the process not carried beyond this point so as to bring the displacement to zero?" There are two reasons for not doing so. The first, as pointed out by Kolb, is concerned with the performance of the projection lens. In most projection lenses the focal plane of field is not truly a plane, but rather a curved surface. For best performance in this respect, the film is allowed to approximate this surface. The second reason is that flat film seems to be somewhat flaccid under the influence of air flow, as compared to film which is bowed to even a slight degree.
Since the film can be kept within the depth-of-focus limit of the projection lens during nearly the entire time of the two
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August 1952 Journal of the SMPTE Vol. 59