Converting from x264 to x265 of equal quality

English Multimedia
1

I am trying to convert some of my x264 MP4 to x265 MKV.

ffmpeg -i file.mp4 -c:v libx265 -c:a copy -vtag hvc1 file.mkv
With this I got a file size about 50% of the original.
However, it seem the result was not of the same quality. It had a much lower bitrate.
Although I could not see any clear difference when playing.

I found a ffmpeg flag I could use to set quality when encoding:
ffmpeg -i file.mp4 -c:v libx265 -c:a copy -crf 18 -vtag hvc1 file.mkv

From what I could find mentioned was that -crf 18 means of equal quality, and that 14-18 mean lossless.
However I got a file size with x265 that was over twice the size of the x264 file.
Should it not be possible to convert to x265 and get same quality as x264, but with smaller file size?

2

I vaguely remember that -crf 28 is the default and corresponds to -crf 23 or around that number for x264. Try something closer to default. Like 26. And maybe add -preset fast.

This is close to what I had in my notes a couple of years ago:

ffmpeg -i file.mp4 -preset fast -c:v libx265 -c:a copy -crf 26 -tag:v hvc1 file.mkv