Well, let's at least start some of the test-reporting.
Let me get off with a personal remark: I really was fascinated by this lens. It was the largest, with the ugliest bulging front lens I had ever seen, a monster for an ultra-wide!
(See the ugly duckling in this
shot third from left, almost dwarfing the 18-200mm and the 80-400mm!)
But it had a mission: Full-Format, FF (or FX as Nikon now likes to call it (= pure, unneccessary marketing trickery) or the "Final Frontier" as I'd like to call it. FF is on the agenda now that Nikon switched to reverse with the D3. And they're right: FF is the only
way to get more IQ than with APS-C! Canon has long understood that , and I was quite astonished that it took Nikon so long to understand that some of their customers are looking for the ultimate IQ.
So Sigma is to be applauded for the widest zoom on earth for FF! And why should this not benefit the aspiring APS-C user? You get very
low distortions and very
low light fall-off (tested and proved) on any smaller-than-FF sensor. Plus the lens handled like a charm (as did its smaller brother, the 10-20mm).
So this lens was sitting on my camera a lot of times and you could feel its effort to collect all the light from all the angles and hammer it on the sensor. And I love a lot of photos I made with this lens and almost couldn't make up my mind to send it back (thinking of a special "collectors" place on my shelf for it).
See one of my favs (@ 12mm, f/4.5, 1/1000sec, ISO 200)
So my standard question: "Could you
make great photos with this lens?" has certainly to be answered by a resounding "Yes!"
But ultimately I sent the lens back because of the following reasons:
(1) The overlap with a 18-xxx zoom is too large, only 6mm (between 12 and 18mm) are the real extension over what I could do before. In FF/FX-world that argument would have been invalid, because there is no standard 18-70/105/200mm zoom. There, a 12-24mm would have fit perfectly. Alas, I'm only owning a cropped-sensor cam...
(2) the lens had some serious issues in contra-light that I'll show you in detail
(3) the lens had some prominent color-fringing, that you'll see too in the full report.
: And as lack of distortions and light fall-off are not very high on my must-have list those advantages did not outweigh (1)-(3).
Let me add some comments regarding comparison to the Sigma 10-20mm. This lens has almost the same probs described in (2), but color-fringing is much better controlled and the range of 10-20mm is clearly more attractive for non-FF users.
I'm also very open in saying that the color fringing with the 12-24mm never happend to bother me in one of the real-life shots. But it was so prominent in test-shots, that I believed that it would have nagged me sooner or later. But this may be just a misjudgment on my side coming from looking to closely at too many test shots and slowly beginning to become a pixel-peeper.
What a sad thought