I'm looking for an ultrawide in the 17-21mm range. It has to be under $500-600, sharp in corners, will take f2.8 or f4, and I have no problem with manual or automatic (can be an older film lens like a zuiko with adaptation.
One lens that caught my eye is the Canon 20mm 2.8. I don't see it on here very often, but it seems to have reasonable sharpness in picture, but then in reviews, it says otherwise.
Any other ideas?