Thanks for the images Thomas. By the looks of things it seems to be an improvement. How much though over the 17-40mm? Is it worth the upgrade?
My upgrade path was from the close-focusing Sigma 20/1.8 and 28/1.8 lenses. It is worth it thus far for these advantages: one lens instead of two, faster focusing, and weather sealing. The Sigmas work rather well optically when housed for under water use. I have no personal experience with the 17-40.