I am experimenting with ffmpeg and x264 codec. And I have found some unexpected behaviour.

I am encoding video with parameters CRF 23, high profile and changing preset. Documentation says:

for constant quality encoding, you will simply save bitrate by choosing a slower preset.

And I am using CRF, so it should be this particular quote. So how is possible that I have achieved the smallest file for veryfast preset?

medium    256MB
fast      261MB
faster    235MB
veryfast  209MB
superfast 324MB

What am I missing? BTW another comparison have similar results.

1 Answer 1


I was able to find an answer. https://stackoverflow.com/a/14308561/941493.

In the nutshell, CRF calculation is also affected by preset so the same CRF number is not the same with different presets. Based on some developer difference can be mostly ignored.

  • That's a bit of a gotcha. I didn't know that, thanks.
    – stib
    Commented Apr 28, 2020 at 14:35

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