The Sigma 17-70mm f2.8-4.5 DC is one of Sigma’s mid range lenses in terms of price and is particularly attractive to Canon DSLR users who want to choose a better quality standard zoom than the Canon 18-55 EF-S kit lens, which although not the worst lens in the world, certainly isn’t as good as any Canon DSLR deserves.
Sigma’s DC lens are their equivalent of Canon’s EF-S lenses - In other words they can only be used on digital SLR cameras with the smaller DX size of sensor - not with 35mm film or with the Canon 5D or 1Ds. This lens is aimed at the Pentax / Sony / Canon Digital Rebel / 450D DSLR market. Build quality is quite good - certainly far less plastic and lightweight than the Canon kit lens. It also has a proper metal lens mount and feels much better in use too, balancing well on the smaller DSLR bodies.
Optical performance, based on this short test, is pretty good - certainly much better than most kit lens I’ve used, except the Nikon 18-70. Image quality at 17mm and full aperture is very good with good sharpness right into the corners. Stopping down, even to f4 produces excellent quality, even in the corners. In general, performance is better at the short end of the zoom range - results at 70mm f4.5 were good too - better than I expected - but there is some softness in the corners at full aperture.
Overall this is a really good lens and is certainly an attractive proposition for Canon and Sony users, offering similar performance and specification to the class leading Nikon 18-70 for about the same cost. Optically this easily outperforms the Sigma 18-50 f3.5-5.6 DC or the Canon 18-55 EF-S, so if you’re looking for a high quality standard zoom for a Canon, Sony or Pentax DSLR this is a very good buy. Have a look at the Tamron 17-50 f2.8 too though.