Summary: As always, when buying third party lenses there is an element of doubt regarding the quality of the lens itself. The fact that third party lenses are also very competitively priced sometimes adds to the confusion as we all like to believe, "you get what you pay for".
Summary: Purchased this a little over a year ago. The only dissapointing thing was unlike the advertised picture this lens did NOT have the little auto/manual focus cut out switch (Nikon mount). I would love to have it on this lens but I have learned to work around that little shortfall.
Conclusion: Bought one used, mint. Astonishingly wide but this is a lens that begs to be used on a tripod and stopped well down, to f11 at least, if not further, where it does shine with some great results. It's just too slow for general hand-held use (which is not what it's really designed for).
Pros: Very wide on full frame, sharp when stopped down, well built, quite inexpensive for the widest non-fisheye available, decent contrast. Remarkably distortion free if kept upright
Cons: Very slow at f4.5-f5.6, dreadfully soft at wider apertures, some CA, needs a tripod and liveview to get the best out of it - needs using like a large format lens really
Summary: I got amazed of how wide the lens is. Today I took some shots in a place im used to. First, I compared to the 24-70 mm L series from canon. Now the results: 1. Super wide angle 2. Very little chromatic aberrations 3.
Conclusion: This lens has a fairly steep learning curve compared to most. On a FF it vignetts (sp?) if the filter/lenscap holding ring is left on, for example. It took me a while to really appreciate what this can do for me.
Pros: Excellent color and very good sharpness, even at 12mm. VERY nice from 14-24mm. Good low price for the wide view(s). Solidly made.
Conclusion: using this lens on a 5D, and 5DII. Returned my copy to sigma for some decentering. Came back better, but not perfect. The corners are never sharp, even at f11-f16, but the rest of frame is very good.
Pros: Range Distortion generally well controlled. Center sharpness is good Good CA control
Cons: Corners, never completely sharp Slight decentering No possiblity for filters slightly flare prone
Conclusion: I've only had the lens for a day and using with a 5D MKII, but I like it so far. I have a 17-40L, but I wanted something wider for a different perspective....I don't plan to use the lens except for at 12mm and it seems like a quality alternative to a 14mmL considering the price difference!
Summary: This lens is fantastic... Takes brilliant photos, no fish-eye, incredibly wide, just what I wanted. ONLY thing that seems an obvious mistake is that Sigma doesn't have a clear way to mount a CPL (circular polarizing lens) on this baby. I've had to search and search for 'workarounds'.
Conclusion: This was bought as my 16-35L just didn't seem wide enough on the 5DmkII for many shots. Wow! This is an amazing lens that impressed me for the lack of distortion (of course there is some but it's very good) and the lens is useable wide open though it really is best stopped down to f/8 and beyond.
Pros: Sharper than I expected, super wide on full frame that's great fun to play with. Less distortion than I'd expected.