How does a dual-format (35/70) projector work, and how is the changeover made between formats?
(courtesy David Richards daverich@netcom.com) These comments apply to the Century projector. There are two significant differences between a 35/70 projector and a standard 35mm projector. First of all, it must acommodate two gauges (widths) of film. This mainly impacts the gate. Typically, the gate is easily removable. Whereas the 35mm projector is restricted to accepting a 35mm gate, the 35/70 projector comes with two gates, one for each gauge of film. These gates are precision machined to slide onto dovetails on the frame, and should not be interchanged between projectors. The gates are stamped with the frame serial number to prevent mix-ups. The second difference is the frame pitch. Standard frame pitch for 35mm film is 4 perforations, or .748″. 70mm film uses the same perforation pitch, but 5 perfs per frame, or .935″. Both must advance at 24 frames per sec. There are two possible ways to accomodate the faster linear speed of 70mm. One would be to simply turn the sprockets faster,
