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I created two videos of the same scene with two different cameras. The issue however is that camera 1 captured everything in 23.85 fps and camera 2 everything in 24 fps.
Since I need to synchronise both videos exactly, it's quite essential that the frame rates are the same. I'm however not sure whether to decrease the 24 fps video to 23.85 fps or increase the 23.85 fps video to 24 fps. Thanks in advance for your advice!
I think the answer depends upon what frame rate you want to deliver your video at.
In general it is better to shoot at the delivery frame rate.
I agree they probably won't go far out of sync if the clips are only short (a minute or so).
If you do need to adjust them, I would frame rate adjust whichever clip is going to be on screen for less time in the edit (or is less important to you).
If your scene isn’t extremely long, I don’t think you have to worry about. Both frame rates are close enough so it shouldn’t go out of sync. I think changing frame rate might have other side effects you don’t want. Create a Multicam clip and see if the clips are out of sync at the end of the scene. I would expect you can’t tell. In case they really go out of sync you can nudge the clip by a frame after a cut you make.
The scene you were shooting was captured by both cameras simultaneously. If you start playback of both clips at the same instant—or more likely; put them into one video and take care to get them perfectly aligned by synchronizing audio—then no matter how long they are, they will naturally play back with perfect synchronization.
The fact that one camera was taking exactly 24 pictures per second, and the other was taking almost 24 frames per second is immaterial—both clips will play back at the rate they were captured. Frames in = frames out. So everything will stay perfectly synchronized. The reason I'm belaboring the above is because of their inevitable counterpoint: anything you do to change the playback frame rate will disturb the automatic synchronization.
Playback of these clips will require precisely showing the correct frame of one video or the other or both, even when you are far into the video. At, say… exactly 10 minutes into the video, camera 1's clip will need to show frame [24×60×10=] 14000; whereas camera 2's clip will need to show frame [23.85×60×10=] 14310. The reason for belaboring this crazy math is merely this: the math obnoxiousness is again a task best left to the software.
So consider… what are your actual needs?
If you somehow actually don't need content synchronization, just change clip 1 to play back at 24 (set playback speed to 100.6289%) OR set clip 2 to 23.85 fps (playback speed 99.375%). Now your frames will be exactly aligned—though of course note that they will not be showing the same videoed moment in time.
If, for some (rare!) reason, you need to 1. stay synchronized, and 2. keep the clips as separate files, and 3. force playback to not just have subject-synchronization but also actually have exact frame-to-frame alignment between both clips, so both files have the same new native framerate, you'll have to render one clip to the other's framerate. This is an interpolated, lossy process, and will slightly degrade quality on the altered clip.
If you simply need both clips to stay in exact sync over the course of the final video(s), so your overlays or cuts back and forth always match between cameras and faithfully show what was happening at that time—do nothing. Every video editor reads and respects each input clip's framerate. After aligning them near the beginning, hopefully using a sharp audio peak, they'll just stay aligned. When your project is done and it comes time to do the hairy 23.85 vs 24 fps math, the software will take care of it and keep the original subject in sync, even though the frames secretly aren't.
