Full Aperture - Sharpness in the center of the frame at at all focal lengths is very good, and although corners are slightly soft, they’re much better than I expected. Corner darkening or vignetting is slight for this sort of lens - less than observed with most lenses with the Canon EOS 5D.

Canon 10-22mm EF-S at 10mm f 3.5
Some darkening in the corners but not extreme.

10mm f3.5 center

 10mm f 3.5 top left corner

 22mm f 4.5 center

 22mm f 4.5 top left corner