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EVOLUTION OP PROFESSIONAL CAMERA
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nisms. The design and construction of these demanded the greatest care because of the necessity that the speed of the fingers at the start and end of each stroke be as great as possible and because care had to be taken that the withdrawing of the fingers from the perforation would not begin until the film was in an absolute state of rest, in order to avoid undue stress and consequent wear of the film perforations.
Fig. 4 Shuttle and leaves of Bell & Howell Intermittent Movement. A — Pilot Registering Pin. E. — Mounting Plate. F — Back Register Leaf Work Arm. G — Film Feeding Finger. H— Shuttle Guide Holder. I — Front Register Leaf. J — Back Register Leaf. K — Hand Operating Register Leaves Button. A clearance of less than .0005 inch is left between register leaves and either surface of film, in order to avoid friction upon the film surfaces. For inserting film between register leaves are disengaged from the pilot pins by moving button K.
Once the film was moved into its photographic position, it was found necessary to devise means by which it could be made to stabilize itself in perfect register at the camera aperture and remain in such exact position throughout the period of exposure.
In the early cameras, and in some cameras still manufactured today, this stabilizing effect is secured by holding the film laterally between two solid guide rails and by applying a pressure upon its back surface.
This method presents, however, some very serious inconveniences. Because of the friction generated between the film a,nd the walls of the film channel in which it is running, there is constant danger through the almost unavoidable filtering of dust or dirt particles into the film channel and the producing of abrasion markings and scratches upon the film surface. There is also the danger of creating "static markings" which would often be present when the film is used under unfavorable temperature conditions.