Originally posted by pres589 20-40 on FF would be, what, like a 12-26 on crop? Seems extremely versatile to me as long as you are okay with carrying a lens with some weight and bulk to it. Something just wider than 43mm on full and then out to ultra-wide-ish on the wide end sounds awesome.
FWIW 13.33 - 26.67 mm on a 1.5x APS-C DSLR.
My issue with such a UWA lens is the foreground to background and geometric distortions. I went back through my recent work with this lens and I shot it mostly in the 29-40mm range. I have the DFA 24-70 as well so that explains why it's rarely used now. If I need a wider view, I find stitching together images into a pano works much better.