Comments about SIGMA 18-50MM F2.8 EX DC AF CANON:

[...]I had done a ton of research before I purchased. At the same time I also bought a Tamron 17-50 from another retailer. The two arrived at the same time. I tested both lenses thoroughly with my Canon 40D. The Sigma 18-50 Macro came out the winner, it was a bit sharper and more consistent regarding IQ at all focal lengths. Built quality is very nice, I personally like the coating of the lens housing, some people might not. I like the fact the finally Sigma decided to go with the center pinch type lenscap which you can put on and take off even with the hood on the lens.Center sharpness is excellent, edge sharpness is decent at wide open and sharp stopped down. Colors are rendered very nicely. There is some CA but nothing serious. The Tamron wasn't bad either. I assume I got a better copy of the Sigma and a mediocre copy of the Tamron. The two lenses are very similar and I think variations between good or bad copies would determine which is better.Focusing is adequate, decent speed, not too noisy, and mostly accurate. However, I had to send the lens to Sigma for calibration. It was front focusing at 50mm at close shooting distances. I decided to send it in for calibration instead of exchanging it at the retailer because otherwise I liked the image quality, and many Sigma lenses have front focus problems anyway, which can be corrected by warranty focus calibration by the service department at Sigma.The lens has not come back yet, I just sent it in a few days ago. Hopefully the focus will be spot on.[$] I think it's a good value (but keeping my fingers crossed for the focus calibration).