Constant quality (CRF 20)

From FFmpeg Compression, End to End · CRF vs. target bitrate

ffmpeg -i input.mov -c:v libx264 -crf 20 -preset medium -c:a aac -b:a 128k output-crf20.mp4

Size varies with content. This is what you want for archival and VOD masters.

h264 rate-control crf

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