5.10 How does a dual-format (35/70) projector work, and how is the changeover made between formats?
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This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
5.10 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, with gearing for example. But this would not work with the
existing geneva movement, and would also throw the shutter timing off.
The way it is actually ac- complished is by using dual sprockets.
There are 3 critical sprockets: the upper feed sprocket, which pulls
film off the reel or platter at a constant speed, the intermittent
sprocket, which advances the film at the gate, and the lower sprocket,
which smooths out the pulsations from the intermittent sprocket once
again. There are additional sprockets in the area of the sound head,
but they do not need to be used for 70mm, as there is a separate
magnetic sound reader for that.
Typically, these sprockets have 16 teeth for 35mm film. Since one
frame is 4 perfs, exactly 4 frames could be wrapped around each
sprocket. Another way of saying this is that each sprocket turns 90
degrees per frame. Since 70mm film requires a 5-perf advance, we can
simply increase the number of sprocket teeth by 5/4, to 20 teeth, and
the speed and intermittent advance distance are increased exactly the
right amount, without changing the Geneva movement, motor, or anything
else. By a happy coincidence, the 70mm film requires both a larger
diameter sprocket, and one with the two sets of teeth further apart to
accommodate the greater width. So, by using stepped sprockets, both
may co-reside on the same shaft. The 35mm film rides in-between the
larger 70mm sprocket flanges.
The only thing remaining is the pads that hold the film against the
sprocket. Since there are two different sprocket diameters, there are
two different places the pads must stop. This is accomplished on the
Century with two different diameter pad rollers, which rotate
individually, the assembly of both of them revolves on a common shaft
with a knob. By turning the knob one way, the 35mm pad roller comes
against the film. By turning the knob the other way, the 70mm pad
roller comes against the film. With 35mm film threaded on the machine,
turning the knob the wrong way does no damage, however, the film will
not be held securely against the sprocket. With 70mm film threaded,
care must be taken, because turning the knob the wrong way will damage
the print.
This combination 35/70 idea, while good in theory, has some drawbacks
in practice. Even with everything set correctly for 70mm, it is
sometimes possible for the base side of the film to touch the 35mm pad
rollers. This can cause base side scratches, which show up as dark
lines about 1/4 of the picture width from each side. Those "in the
know" will remove the 35mm pad rollers when showing a 70mm print, and
replace them with spare 70mm rollers. This allows them to turn the
knob either way without creasing the print, and at the same time
eliminates the risk of base-side scratches.
As a footnote, the lamphouse generally must be readjusted for 70mm as
well, to cover the larger frame area.
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