I think using this lens as a portrait lens or something might be perfectly fine.

In my use case : a small Fuji body and the 40mm macro as a glorified film scanner, it is a pretty anoying issue because it is suffisent to mess with the focus, in an unpredictable way.

Scanning is already a delicate subject as you certainly know.

Your precision about how the rotation should not affect focus makes sense. But I see the focus assist highlights slightly fade or enhance just by going to the free play. Could it be insignificant, like an artefact of digital treatment of the focus assist? Or the sensor getting slightly different lighting (like some kind of polarization)?

Thanks for the pointers to big forums, I purposely avoided them to see how Lemmy communities would respond to such a specific subject. Not disappointed that’s great I received many mindful replies.