Kelvin, lenses are usually designed to be sharpest at frame centre, and aberrations of some kind almost always creep in towards the edges/corners.
I’ve experimented with my 70-200mm with is more tolerant in some respects because it is the f/4 version.
The problem with “nice” lenses is they are heavy and this risks introducing tilt between the lens and sensor. I have yet to find an adapter I’m really happy with.
I’ve heard good things about the Samyang 135mm but never tried one myself.
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