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200
BRITISH KINEMATOGRAPHY
Vol. 17, NO. 6
the summation of errors in the perforation of this number of films, and in the registration of the several films in cameras, printers, and projectors.
Instability of Film.
Such a degree of accuracy would be far from easy of attainment if we were dealing with a comparatively rigid unvarying substance such as steel. But apart from the low tensile strength of film as compared with metals, both base and emulsion are exceedingly hygroscopic, and vary considerably in dimension according to the humidity.9 Furthermore, the different characteristics of the base and emulsion cause the film to behave in the manner of a bi-metal strip, resulting often in an inherent buckle. Yet another complication is that in the manufacture of film stock, those strips near the edges of the wide roll are apt to have a lateral curl or " weave. "
Either buckle or weave is capable of displacing the film from the focal plane of the optical system, or in the case of a contact printer, out of contact with the second film, so causing loss of critical sharpness. In the majority of devices, other than certain of thtfse to be later described, the film is kept flat in the gate simply by spring pressure on the four margins of the picture, aided in the case of contact printers by the use of a curved gate, which naturally prevents lateral buckle.
Intermittent Motion Devices.
Earlier cameras and printers made use of simple feeding claws ; in these mechanisms, as in the Maltese cross system, the film is braked frictionally, and
A
ft
VN
'
1
1
v\
J
Fig. 2. Comparison of film perforations.
A, negative ; 8, positive ; C, proposed
combination type.
is guided laterally by its edges.10 To attain a higher standard of accuracy, the film must be controlled solely by its perforations ; it must be under positive control during the whole period of movement and exposure ; and the registering means must be close to the picture aperture, in order that variations in the pitch of perforations shall have the least effect upon film registration. Consequently, feeding claws are now supplemented by register pins.
One objection to such a mechanism is that the moving register pins must necessarily have some working tolerance in their guides, which may lead to slight unsteadiness. Furthermore, the register pins must obviously be a good fit in the film perforations ; if they are slightly too tight, their movement — part of which takes place during the exposure period — may cause the film to move slightly out of the focal plane, an effect known as " breathing.'' These defects are for many applications endured, because motions of this type can be constructed to operate very silently.11
However, it is better engineering practice to provide stationary register pins. The first intermittent motion device which embodied such a feature was designed by an engineer to whom the industry owes much, the late Arthur S. Newman. A device of similar principle is to-day widely used in all types of apparatus where noise is of no consequence. In both these mechanisms, the film is guided in a lifting gate, which lifts it to engage with the feeding claws, and during exposure clamps it upon the pressure pad and in engagement with the fixed register pins.