[FFmpeg-user] How to select x264 preset when do ffmpeg 2-pass encoding is helpful for quality ?

[littlebat]

Hi,

I read the 2-pass encoding guide at:
https://ffmpeg.org/trac/ffmpeg/wiki/x264EncodingGuide#twopass

I found the guide use commands below as a two-pass example:

ffmpeg -y -i input -c:v libx264 -preset medium -b:v 555k \
-pass 1 -an -f mp4 /dev/null && \
ffmpeg -i input -c:v libx264 -preset medium -b:v 555k \
-pass 2 -c:a libfdk_aac -b:a 128k output.mp4

A question: If a slower preset is helpful for the quality at a
specified bitrate when do a 2-pass encoding with libx264 encoder?

For example: select "ultrafast" in pass 1 and "veryslow" in pass 2, or,
select "ultrafast" for both 2 passes?