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with my DSLR, I used to shoot in 60fps and then interpret footage as 23.976 so slow it down. Now that I have moved to mirrorless (Sony) in PAL I can only shoot in 25 fps or 50 fps. If I shoot in 50fps and slow it down to 23.976 fps, will it be the same or will my footage look glichy?

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The recommendation from Adobe for slowing down motion is to use Optical Flow. If you put a 50 fps onto a 23.976 fps timeline, select the clip and set Speed/Duration to 47.952% with optical flow, then you should get a good quality result. And of course you are on your own for sound, because slowing things down 2x is not going to make any sense for the audio you recorded.

Shooting at 25 fps for a 23.976 timeline could be a 4.1% slowdown (in which case you might want to pitch shift your audio so you can keep it) or a re-interpretation (where you drop/optical flow frames) in which case you can keep your audio as is and mate it to the retimed video.

If you find yourself doing lots of 25->23.976 conversions, you may want to seriously rethink the idea of using a PAL camera do make NTSC deliverables and instead buy a camera that is NTSC-friendly.

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  • Note that Optical Flow isn't magic. Sometimes it produces strange artefacts where it guesses the motion incorrectly, in which case it's better to use frame blending. – stib Mar 1 '17 at 1:52
  • Absolutely true. But it's a lot easier for optical flow to go from 50 fps to 23.976 than the other way around. – Michael Tiemann Mar 1 '17 at 13:29
  • @MichaelTiemann what if I shoot 100fps instead? How much do I have to slow it down to in order to get nice slow motion effect? Will 100fps reduce video quality? – rikket Mar 1 '17 at 18:55
  • "nice" is in the eye of the beholder. I like 96-120 fps (4:1 or 5:1), but 60 fps down to 24 (2.5:1) has its place. Here's a dog playing in snow, slowed down 5:1: youtube.com/watch?v=fZBgizsnpy4 – Michael Tiemann Mar 2 '17 at 3:32
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    The speed you need to shoot totally depends on your subject. Someone doing a somersault, 4:1 slowdown should give good results, a bullet hitting a watermelon, you might want 40:1 slowdown. – stib Mar 2 '17 at 10:58
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If you're doing an Interpret Footage on the clip it won't look glitchy, but you won't get quite the same slow-down factor.

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