Why your 120 fps clip plays in slow motion on an iPhone
It plays normally for two seconds, then drags, and the sound turns into a hum. The file is fine. Apple is treating it as a slow motion recording.
A customer wrote to us: the video plays normally for the first few seconds, then goes extremely slow, the original sound disappears, and all that is left is a distorted hum. They had only added a caption.
We checked the export and every frame and every sample of sound matched the source. The file was fine. The problem is in how it gets played.
What is going on
Apple Photos and QuickTime on a Mac treat a movie at roughly 85 frames per second or more as a slow motion recording. They play it at normal speed for a moment, slow down the middle, and drag the sound down with it. Unless the file says otherwise, a 120 fps video gets that treatment even when nobody asked for slow motion.
A browser does not follow that convention, which is why the same clip looks perfect in an editor and only misbehaves after it lands in Photos.
What Captik does about it
An export now keeps the source’s frame rate up to 60. Above that it divides by a whole number, so the motion stays even and no frame is invented.
| 120 fps | 60 fps |
| 119.88 fps | 59.94 fps |
| 100 fps | 50 fps |
| 240 fps | 60 fps |
| 60 fps or less | Unchanged |
The desktop plugin is the exception, on purpose: its clip goes back into your editing timeline, so it keeps the source rate and you decide there.
If you see this with any tool
- Check the frame rate of the exported file, not the source
- Export at 30 or 60 if your tool lets you choose
- Film ordinary talking videos at 30 or 60 to begin with, because a phone left in its slow motion mode keeps writing a high frame rate into every clip
Free to try, no card. Any video, any size.