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Here is a shot of the inside of a Panavision 65mm camera.

enter image description here

As you can see the film makes a number of turns. Why is this done? It seems like it would only make noise and scratch the strip.

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    Mostly I believe it's about maintaining an even tension on the film so the film was exposed evenly....back when everything wasn't digital or course. – Paulie_D Aug 12 '16 at 19:24
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    In the limiting case of no friction at the turns, tension would be uniform along the strip. Introducing more turns and friction would seem to make that problem worse, not better. Got any details for that claim? – spraff Aug 12 '16 at 19:26
  • None whatsoever, just my impression. – Paulie_D Aug 12 '16 at 19:28
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One of the reasons is to create a "loop", but this "loop" is probably a different definition of the word than you're thinking.

Film comes off the reel smoothly. Film being exposed to light from the lens has to stay stationary while it's being exposed. To accomplish that, the film strip kind of bulges upwards above the lens, then gets yanked suddenly into place in front of the lens while the aperture opens for an instant. That "yanking" into position gets rid of the bulge ("loop") of film. While the aperture is open, that bulge ("loop") grows again. The aperture closes, and the film is suddenly yanked forward into position again, getting rid of the "loop" that had just formed.

Of course the film "below" the lens will have a loop in it too, after the still section of the film strip is pulled past its position directly behind the lens.

So that's the complicated part, in the middle of the film strip's path. The rest of the turns most likely have to do with getting film from the reel, positioned farther behind the lens, up to a place where the loop can be formed, and without the film becoming detached from the rotating sprockets that are moving it smoothly forward. The same explanation applies to the film after it's been exposed behind the lens: the curves in its path are there to keep it hooked onto the sprockets so that it can be smoothly led back to the receiving reel.

  • Thanks, that's enlightening. Is there a similar process during projection? I often wondered why the image doesn't look like it's racing downwards, unless the lights are on for only the tiniest fraction of the time. – spraff Aug 12 '16 at 20:20
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    @spraff ... yes the film is illuminated only when the frame is in the correct position - but its not illuminated for a tiny amount of the time, its actually more like 50%. The film is actually stationary for the majority of the time, moved by a mechanism called a shuttle - and illuminated by a syncronized mechanism called a shutter which illuminates the film for about 50% of the time – iandotkelly Aug 12 '16 at 20:31
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    The loop is also important because to replay the sound correctly the film has to move smoothly at that part of the mechanism - but for the visual replay, it has to be moved by the shuttle yanking the film over a fraction of the time. So the loop is there to allow that disconnect between the replay requirements of sound and visuals. – iandotkelly Aug 12 '16 at 20:35
  • @iandotkelly I forgot all about sound synch! Thanks for mentioning that. (Film school was a loooong time ago.) – BrettFromLA Aug 12 '16 at 22:32
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The intermittent mechanism or intermittent movement is the device by which film is regularly advanced and then held in place for a brief duration of time in a movie camera or movie projector. This is in contrast to a continuous mechanism, whereby the film is constantly in motion and the image is held steady by optical or electronic methods. The reason the intermittent mechanism "works" for the viewer is because of a phenomenon called persistence of vision.

https://en.wikipedia.org/wiki/Intermittent_mechanism

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