But how does it compare to 120 film?
First off, what you should know is that we’re obviously not the first ones to think about this. For years, photographers shot with 70mm film with cameras designed to shoot 120 film using special camera backs.
Oddly enough, the film sizes were more or less the same. According to a Wikipedia entry:
That is all well and good, but then you start to look at graphs. While IMAX 70mm is larger on one end, 120 film is larger on the other end depending on the format you’re working with. For the photographers out there who don’t understand how 120 film works, you have a single roll of film. The film is a massive piece and it’s split up to be shot into various formats. 645 is the smallest and 6×9 is the largest conventional size. You can go bigger, but most photographers don’t.
For some professional medium-format cameras, those used in school portraiture for example, long-roll film magazines were available. Most of these accommodated rolls of film that were 100 ft (30.5 m) long and 70 mm wide, sometimes with perforations, sometimes without. Some cameras, such as the Hasselblad, could be equipped with film magazines holding 15 foot rolls of double perforated 70mm film passed between two cassettes. 70 mm was a standard roll film width for many decades, last used as late as the 1960s for 116 and 616 size roll films. It was also used for aerial photo-mapping, and it is still used by large format cinema systems such as IMAX.
70mm film used in still cameras, like Mamiya and Hasselblad, and 70mm print film used in IMAX projectors have the same gauge or height as 120 film. With 70mm cine projector film, the perforations are inset by 2.5mm to make room for the old-style optical sound tracks; a standard established by Todd-AO in the 1950s. IMAX cameras use 65mm film, which have perforations and pitch that match-up to the 70mm film used in IMAX projectors.