Lenses

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Cons: Loss of sharpness in the corners 12mm, some (but natural) cornershading, slow

Pros: Extremly wide, quiet sharp for the enormous perspective (specially from 14-22mm and 24mm tacksharp), nice colours (bit warm tipical Sigma), NO distortion, good design and buildings.

Summary: Simply a great lens! Only the Samyang 14 f2.8 comes close, but without electronics.

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Optical quality
Value for money
Overall

Cons: Some CA at wider end, vignetting ans sometimes very odd flaring

Pros: Very good distortion handling, reasonably fast AF, Extreme view angle, well built.

Summary: Overall, great lens for architecture, especially good for interior shooting. Also, nice lens for landscapes but you have to be careful about flaring. IMHO, my copy is sharp enough, I am not expecting to be sharp as some prime, but for the widest UWA zoom is absolutely perfect. For the price of used one I believe that this is one the best choices for anyone who needs extreme width. Also, it's good lens for photographers who are using both, FF and crop sensor, on crop it acts like 18-36 which is also very wide. From my point of view, my copy works even better on full frame.

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Overall

Cons: Not as sharp as I would like, but still very reasonable. Small aperature makes thin depth of field impossible.

Pros: Very low distortions considering the wide angle. Full frame capability. Good contrast which makes images seem to appear sharper.

Summary: I bought this lens so that I can still use it when I change to a full frame camera. I could wish for it to be a bit sharper but the lens has nice contrast making it have the appearance of greater sharpness than might be the case. It's certainly sharp enough and borders on DX anyway don't lag much behind the centre. There aren't many choices for full frame ultrawide Nikon compatible zooms other than the Nikon 14-24 f2.8 which I have only briefly tried. The Nikon will be sharper but is also very bulky, twice the price and almost twice the weight (600g vs 1000g). The Sigma is therefore a good compromise for a keen amateur who won't be making his income from the lens. The size and weight of the Sigma also means it's more likely to travel with me. With that in mind, I'm still very satisfied with my purchase. The very low level of distortion is its strength. The large bulbous front element has never been an issue. It's also very well built for the price.

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Cons: Its not cheap, a bit soft from f4.5-7.1 and strong fullframe-cornershading, some CA´s on APS-C and more on fullframe, the frontlens very exposed, no filters.

Pros: Nearly NO distorsion (the guy before is completly wrong with his comment!!!), GIGANTIC ultrawideangel, quite sharp fronm f7.1 as well in the corners. Very good buildings and beautifull color-rendition.

Summary: A very very wideeeeee lens, fantastic perspectives. Quite usefull even at fullframe from f7.1 (before soft). Its a very special item but not without flaws. Very useful for architectism, no distorsion! If you work with Fullframe and loving ultrawide you MUST buy it!

Build quality
Optical quality
Value for money
Overall

Cons: Soft. Poor contrast. Heavy distortion.

Pros: Extremely good wide angle capacity

Summary: If the wide focal length is more important than image quality it is worth its price. Nothing for pixel peepers.