>>4462580
>he doesn't know
Some veteran Sony shooters like to adapt M mount glass since the times that Sony, having the only mirrorless FF digital bodies on the market at the time, was the only real option. M mount of course having a great flange distance for adapting, with very non-intrusive adapters compared to adapting SLR glass.
Such comparisons interest these users because it helps them make an informed decision about whether a lens will perform reasonably on their Sony camera (or if they ought to get a Kolari mod or some other cope, or not bother at all).
It's induced field curvature caused by Sony's extremely thick sensor cover glass. Most people think it only affects extremely fast wide-angle lenses, but here is a case of it affecting a 90mm f/2 lens, neither wide nor particularly fast.
I hope this has been educational.