I currently have the Sigma 12-24 and love it; easily one of the best lens purchases I ever made and I've really gotten into wide angle compositions. Some of the photos taken with this lens have been very commercially successful as stock. It's a full-frame lens and thus on a 1.6 crop camera is super sharp and has amazingly little distortion, even wide open.
But... the Sigma is a little slow, at f4.5-5.6, and it can't take regular filters due to the bulbous front element. Flare is a problem for the same reason. The ability to use a polarizer is definitely something I miss. The Sigma has a slot for adding gel filters behind the rear element... but this really isn't useful.
Enter the Canon 10-22 EFS, which came out after I bought the Sigma. It seems to be equally sharp, is faster at f3.5-4.5, and will take 77mm filters (same fit as the 24-105 f4L, which is a bonus).
Thus the questions:
1) Should I just be happy with the Sigma and not waste money buying essentially the same zoom range? Anyone make this transition?
2) I read that the 10-22 contains L glass. Is this true?
The 'L' designation stands for "Luxury", and really only means the lens is expensive. The 10-22 is not designated 'L', even though it is fairly expensive.
I like my 10-22 very much. It has one defect I can see, which is completely correctable digitally: It makes varying amounts of chromatic aberration (red/cyan color fringing), which seems greatest at 10mm.
Image quality is good, things stay sharp to the corners, and I like that it is completely rectilinear (i.e., straight lines stay straight). If you shoot into the sun or very nearly so you may get some lens flare, but it is not overly prone to it.
Thanks -- a friend who has the 10-22 stopped by on the way back from Yosemite, and I have some RAW files that he took there... they look great! The Sigma suffers from some chromatic aberration at the frame edge when wide open, but it's not bad at all and correctable in post processing.
Did you mean "*not* overly prone" when talking about flare?
I've owned the 10-22. I sent it to my father when I switched to the 5D. I definitely liked the 10-22: it's a good lens for the money if you must have zoom and autofocus. (If neither of those characteristics are so important to you, you might want to look into the impending Zeiss ZE lenses.)
I never found the 10-22 to be especially flare-prone, but it's still a very wide angle lens and you should plan to pick up the hood which, last I checked, was an extra cost for the 10-22.
Do expect significant loss of sharpness in the corners.
Do expect significant chromatic aberrations, particularly in the corners.
Do expect vignetting.
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
well i absolutely love that sigma lens, i use it on my 1d mark III. used to have the canon 16-35mm 2.8L and it BEATS that lens IMHO. much less/almost no CA or barrel distortion, while the canon had both. im an architectural photographer, so i dont mind the lower fstops.
its also built as good as any L lens i own. im preatty happy with the AF too (on the sigma). and the sigma is half the canon at $700. the sigma is prone to flare due the front element, but the flare spots are always really small and easy to clone out.
Canon 10-22 EFS vs. Sigma 12-24 EX DG HSM 
Linear Mode
