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Almost every time we deal with footage that was shot on an iOS device in a post production workflow, we find it's Variable Frame Rate (aka VFR) rather than Constant Frame Rate (CFR). The hardware seems to be doing timing based on a 600 Hz internal clock, so within the mov the timebase is based on 600 ticks per second. But what I'm seeing on multiple mp4/mov AVC clips shot on an iPad Pro running iOS 15.2.1 is that almost every frame is 25 ticks, but with every 30 seconds one frame will be 26 ticks. This results in an effective frame rate between 23.99865 fps and 23.99880 fps. I would understand a system where most frames are one number of ticks but occasionally one is longer if it resulted in a more common framerate, but that number seems very strange to me.

Does anyone know why this would be a goal? Did someone at Apple just mess up their math, or is there some application where that frame rate would be desirable (instead of, say, 24.00 fps or something closer to the ntsc-film standard of ~23.976 fps)?

UPDATE: This continues to be the case at least through the iPhone 14 Max, running iOS 16.3

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When shooting at 24 frames per second you are better off using a professional camera which can shoot at 24 frames per second. NTSC based field rate equipment has a compromise and it will be 23.976 frames per second. NTSC field rates were know as 30 frames or 60 fields per second but they were actually 29.97 or 59.94. NTSC was the result of adding color to an existing monochrome standard. There's a thorough explanation of how the math for this worked here. The 625 line systems which adopted a lower field rate due to the higher line rate and the need to ensure channel usage was maximized and they just sped up the film shot at 24 fps so it sounds higher pitched and for people like me with Migraine, working with 625i50 editing screen you need external light to cut the flicker effect which ordinary viewers do not notice. So it is a mess and if you want 24 fps then you need a pro camera which can capture it.

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  • But Rich to your point, interestingly because the iPhone's internal timing crystal seems to be exactly 600 Hz to a relatively high degree of precision, on a pure hardware level as far as I can tell it can shoot 24.00fps, it just is set in the software not to do so. This seems to be a simple mistake on Apple's part, confusing clock ticks with frames, but I guess I'm looking for more info from that perspective. Just telling productions not to shoot on phones isn't super practical, sadly May 5, 2023 at 2:54

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