Raylon wrote in post #11962455
From what I've read the Sigma is actually sharper in the center than the Canon at 2.8. I've been deciding between the tamron non-vc, sigma, and canon. I think I've finally decided on the Sigma.
That is correct, the Sigma is sharper in the center and similar at the edges.
Apart from my own findings, which by the way was one reason I sold my 17-55 f2.8 IS for the EX lens, Lenstip has a good review: http://www.lenstip.com …EX_DC_OS_HSM_Summary.html
"Reading our test you can see very clearly that the Sigma entered the segment of more expensive fast zoom lenses, aimed at ambitious amateur photographers or even professionals working with smaller sensors, very aggressively indeed. The performance of the tested lens is sometimes really impressive. In the frame centre the Sigma behaves better than more expensive constructions of Canon and Nikon. At the edge it is worse than the Nikkor but it can compete on equal terms with the Canon. In fact it doesn’t lag behind the competitors in any category and in some it fares definitely better. Sigma lately has started to emphasize the fact that its aim is not only to produce cheap and worse substitutes of brand name lenses but also to reach a similar price-quality segment as its brand name competitors. The newest models, like the 1.4/50 or the Sigma 17-50 mm f/2.8 tested here, are perfect examples of this strategy.
The Sigma 17–50 mm is not a flawless device but neither are so its most serious competitors which are more expensive. The Nikkor overall seems to be an optically better lens but it is much more pricey than the Sigma and doesn’t feature image stabilization. The optics of the Sigma and the Canon are very similar – both boast silent and quick autofocus mechanism and stabilization. Sigma’s advantage consists of a better warranty and a bit lower price.
For the Sony Alfa system reflex cameras users the Sigma 17-50 mm can be a very interesting suggestion because Sony doesn’t offer such a lens in its line-up so the competition is definitely less fierce. It won’t be so easy in the case of Pentax because the 16-50 mm f/2.8 model from that company tempts with a wider angle of view although it is optically worse and more expensive than the Sigma.
To sum up the Sigma 17–50 mm f/2.8 EX DC OS HSM will find many satisfied users for sure. It would be difficult not to be satisfied when you can take photos as good as or even better than the brand name competitors using a cheaper lens."
After having used over 30 copies of the Canon 17-55 since its 2006 release, and owning 2 for the past 2.5 years, I've moved to the Sigma 17-50 because I think it is actually a better lens. Canon-only users will not be persuaded, but those with an open mind will find the Sigma very impressive.