Quote Originally Posted by JBrunner View Post
I don't believe it has anything to do with diffraction, nor do i think the effective stop due to bellows factor is doing anything to create increased diffraction. [...]
Perhaps not the culprit here, no.

Often ignored, but diffraction increases every time you stop a lens down, no matter what image scale it is set to.
So much so that with roughly every two stops you close a lens down, resolution* is halved.
That's quite something.

*maximum achievable resolution - there are more limiting factors, like motion blur, or lens aberrations, which often are (even) worse than diffraction.