Why does video conversion lower quality so much?

Solution 1:

Read here about the -crf values: https://gist.github.com/steven2358/ba153c642fe2bb1e47485962df07c730 Start with single operations to determine the effect on quality and size.

Every reencoding lowers the quality.

The author in the link above states: The default value is 23, and visually lossless compression corresponds to -crf 18.

You are using 17 instead for even higher quality which comes at the price of additional storage.

I only see a quality loss on top of the shoe. Everthing else is fine. "So much" is not a quantitative measure.

There is a measure for quality realized in a compressed video stream called PSNR.

German link: https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_news_from_rs/199/N199_Video-Bildqualitaet-messen_d.pdf