Yourlensreview - Lens - Sigma 18-50 F/2.8 EX DC Macro

Good build quality, 2.8 all the way. Sharp. Despite the absence of USM/HSM the focus is fast and relatively quiet. Only flares when the sun is in, or close to, the frame. CA is quite well controled, contrary to its predecessor, the non-macro. Good contrast. Low vigneting.

A little soft at the extreme outer edges at 18mm @ 2.8, stopping down to 4-5.6 deals with this perfectly though. Beyond approximately 21mm this problem is no issue. Also moderate barrel distortion at 18mm.

This lens is a very good alternative to the much more expensive genuine brand alternatives. It is not a perfect lens, but its flaws are rarely, if ever, a problem. The 'weak' spot is at 18mm, where there is slight softness in the corners at maximum aperture, however this is not very pronounced and in real life only seldom visible. At smaller apertures the lens is impressively sharp across the whole frame. Beyond 21 mm this lens performs extremely well in every respect and is very hard to beat.

My copy of this lens has a Canon mount and is used on a Canon EOS 400D.