So why fool around with a me too lens?
As Sigma already makes a 12-24 zoom (I owned a ver1) it seems to me that Canon could do Nikon one better and make a 12-24 f4.
My Sigma was sharp in the center at f8 and smaller and sharp-ish at the extreme edges only at f13. I have heard reports that the ver2 is much better.
If Canon did this they would trump Nikon in the UWA zoom range and if we have any justice it would be as sharp as the new 24-70II.
Sigma 12-24 is S**T in the corners! No point in getting the extra 2mm focal length when it is so difficult to construct a 12mm lens, and when corner performance is as bad as it is on the Sigma, completely useless!. Also before Canon tryes to make a 12mm lens they need to prove they can compete with the rest on 14mm!! And forget about f/4, we need f/2.8 just like the Nikon and the Samyang. Canon need to release a 14-24 2.8 lens, no point in making a new 14mm alone, when Nikons 14-24 is sharper and optically superior @14mm to Canons prime 14mm lens. Also you can not use filters on Canons 14mm, so it basically has no advantage what-so-ever over the Nikon 14-24! I have both the Samyang 14mm and the Nikon 14-24, which I am using on my 1DX/5d3/5D2, and I have been using the Canon 14mm 2.8 v2 a lot, so I know Canon need to get a grip!