Tokina 12-24 on 5D
Tried my Tokina 12-24 II on a borrowed 5D II recently.
The smallest focal length I found usefull was 18mm., or maybe even 17mm, but sure not less than that because of heavy vignetting. Corner sharpness was not great but acceptable to me when stopped down.
I guess 18mm is not bad either. But yeah, depends on how the corners come out.