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A film shot on IMAX format is shot on 70mm film instead of 35mm. It is also shot at 48fps instead of 24fps.
Ofcourse most films that run in IMAX theaters are just upconverted regular movies. So you just get a bigger screen.
However most theatres have changed to digital projection using 2K projectors. 2048x1080, pretty much blu-ray resolution. Ofcourse the movie you see in the cinema has a lot less compression. They run at max 250mbps, compared to blu-ray's max sustained 40mbps for picture.
IMAX digital cinema currently uses 4K projectors, 4096x2060.
The 70mm prints easily offer 8k resolution which is the next step for digital IMAX projectors. Just as 4k is better for 35mm. A film shot on 35mm and mastered in 4k for an IMAX screen will look sharper then in a regular cinema using 2k projectors.
Best quality is still watching a 70mm film on a 70mm projector, no compression, full resolution.
