I read a bunch of reviews/comments, tried several M-mount 28s – Summicron, Skopar, Ultron – and finally ended up with one that works, at least for me, on A7: Konica M-Hexanon 28mm f2.8.

I was most disappointed in the Summicron because it wasn't just soft in the corners, it smeared them at an angle from f2-f4. The CV 28 Skopar vignetted too much to be taken seriously. The Ultron was Ver. 1 (f1.9), which doesn't focus shift like the newer version. It was quite a bit better: not the sharpest knife in the drawer, but acceptable with no serious smearing; just some color vignetting in the blue channel. I could've lived with this one, but the Hexanon was about the same in corner sharpness and noticeably better in the center. And it vignetted without as much blue in the corners. It doesn't smear, though it isn't remarkably sharp at the outer edges. I believe that's the lens itself – not specific to its use on A7 sensor. So this one turned out to be the best compromise among those I tried.

(I didn't try 28 Elmarit Asph because I found it too contrasty on M8/9 – not a good 'sunny day' lens; so I'd traded it for Summicron.)

This was shot at f11, but at larger apertures it also performed best among the four.