Bought a 15mm Canon fisheye on eBay and giving it a test run. One thing I noticed, there's a little vignetting wide open and it's less sharp right in the corners but it is clinically sharp as a knife! If anything, it could even be too good. Everybody is into retro filters and making things look less than digitally perfect but even this extreme wide angle has limited colour fringing in high contrast areas and blows other lenses out of the water!
elephant's trunk