Summary: I bought this as an additional secret weapon for weddings and industrial hand held, low-light shooting. This is the latest version of this lens (there is an older version that is not "DG"). This lens is either loved or hated apparently due to the Sigma curse of poor quality control. I've been lucky with Sigma's so far, but I've had friends that had to send their lenses back. To Sigma's credit, they always seem to return a well adjusted and properly operating lens.
Summary: If want a fast aperture f1.8, wide-angle 20mm, full frame sensor capable, prime lens--you have virtually no other options under $1,000. Quality build, a bit heavy with an unusual auto focus/manual focus mechanism. Feels good holding, easy to control and manually focus. Fast auto focus. OK sharpness wide open @ f1.8. Exceptional sharpness @ f4.0 and up. Low distortion and CA. Great bokeh! Get to know it's limitations and you can excel @ photo taking with this unique lens.
Summary: Very bright lens at this focal length. Surprisingly sharp, even though the edges lose some sharpness. So, it may not be the best lens for film or full-frame digital cameras, but with the crop factor of many digital SLR's, the edge sharpness in negligible. Still, given the price, this is an excellent lens for all formats. One thing that could be better is focusing speed. It's kind of slow, so I recommend manual focusing, especially in low light conditions.
Summary: Wonderful lens... ideal for urban exploration photography with it's sharpness and high contrast -- it really lends itself to black and white photos, just as much as in color. Very solid feel.... the only complaint that I have is that the AF is somewhat slow -- as it isn't yet offered with HSM.
Conclusion: Har sine svakheter, men også sine klare styrker som "spesialist". Alt i alt er jeg veldig godt fornøyd med dette objektivet.
Pros: Svært lyssterk vidvinkel med kort nærgrense gir unike kreative muligheter. Rimelig priset. Ni blader gir pene solstråler stoppet litt ned. Byggekvaliteten virker ok/god, med den typiske matte Sigma-finnishen på de eldre modellene. Dette objektivet fikk en rennessanse da jeg gikk over til fullformat,og jeg liker det bare bedre og bedre. Kan produsere usedvanlig skarpe bilder i sentrum, men er litt soft i kantene. Har en lett varm fargetoning. Jeg bruker det primært til...
Cons: Temmelig uskarpt vidåpent. Får lett "flare" i motlys. 82 mm filter. Til landskap finnes det langt bedre alternativer.