Originally posted by FozzFoster I don't know much about anything.. but I'm pretty sure that anything that wide is gunna have some soft corners...
It's certainly true that wrangling wide-angle light rays onto a sensor can be a bit challenging.
That said, some lens makers manage to a decent job of it. The Samyang/Rokinon 14/2.8 has a good reputation (if you get a non-decentered copy) and the IRIX 15/2.4 is also decently regarded. The Sigma 12-24 DG is quite good at 12 (and insanely wide!) but is softer at 24. And the old Pentax M20/4 is fairly sharp corner to corner.
On the other hand, I've got a Rikenon 24/2.8 that's a OK to the edges but a horror show in the extreme corners.
One minor issue is the Pentax's in-body image stabilization adds to the soft-corner challenge. Whereas a camera with a fixed sensor needs a lens that provides good image quality over a 43.3 mm circle, a camera with IBIS needs a lens that provides good image quality over a 47.4 mm circle because that's how far the sensor might wander.