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I was told the IMAX 70mm 15perf film is simply a regular 70mm film turned sideways. Is this true? According to the following image, it does not make any sense as the frame is that much larger than that of a 8perf one. enter image description here

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I was told the IMAX 70mm 15perf film is simply a regular 70mm film turned sideways. Is this true?

Yes, this is true. A film strip is just a ribbon of celluloid with light sensitive chemicals on it. Prior to exposure, the entire ribbon of film has no "frames" - it is just made with perforation holes so that the gears and pins can move it through the camera and hold it steady for each image exposure - 24 times a second.

The factor which determines the exposed area is the lens and the aperture. In an IMAX camera, the film is exposed horizontally and the left to right image area takes up 15-perfs. The camera then moves the entire film strip horizontally a length of 15-perfs, holds it in place, then exposes another "frame". In an 8-perf, or 5-perf camera, the film is exposed vertically and the top to bottom image area takes up 8- or 5-perfs. The camera then moves the entire film strip vertically (from the unexposed part of the film reel, through the gate and onto the exposed part of the take-up reel), folds the film in place, then exposes another "frame". Prior to this, there are no frames on the film. Just light sensitive chemicals on the emulsion side of the film ribbon.

Maybe it would help to look at a 15-perf and 8-perf side by side with the images rotated so they match visually?

enter image description hereenter image description here

Does IMAX have it's own type of 70mm film?

The film stock, emulsion and film base may be the very same. The difference is in the lensing and the transportation of the film through the camera and projector. 15-perforation 70MM film frames are transported horizontally, whereas 8- and 5-perforation film frames are transported vertically.

Roughly speaking from the image in your question, the 70mm film frame is ~12 perforations across the image. The frame size is the difference in total image area of (12 * 15) vs. (12 * 8), or 180 vs 96 - almost double.

Accommodating such a large image area is no small undertaking. At 24fps the film moves at 3.8 miles per hour and a platter is between 4 to 6 feet in diameter for a 2 hour movie. For more information, see the IMAX wiki article.

enter image description here

Also, note the differences in the images - the 5-perf shows significantly less image above the car. The 8-perf and the 15-perf are anamorphic, i.e. squeezed onto the frame when shot and stretched onto the movie screen when projected.

enter image description here

Per your comment, yes, with a fixed 70mm film strip's width, a 15-perf image has 3 times more image area than a 5-perf image (and this whether the image is looked at vertically or horizontally.)

  • There still must be something I'm missing. I would have expected for a certain film format to be of 15x12 so that if you turn it sideways you get 12x15. The second frame from the left will never produce 2 times it's frame size does not matter how many time I flip it. – JustAGuy May 13 '17 at 22:22
  • I'll just add that according to wiki the avg 70mm film is 5 perf tall. If you turn it sideways you will need 3 frames to fill a single IMAX frame. en.wikipedia.org/wiki/70_mm_film#Uses_of_70.C2.A0mm – JustAGuy May 13 '17 at 22:33
  • @gilfalko see edits. Film stock is not like a digital container expecting a set amount of data per frame. Prior to exposure, the film reel has no frames, just perf holes that fit the gears and registration (holding) pins which move the film through the camera and hold it still while being exposed. – Mr. Kennedy May 13 '17 at 23:03
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    NOW it makes sense. Kudos to you my friend. – JustAGuy May 13 '17 at 23:12

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