It's not often that an independently manufactured lens costs more than some camera manufacturers' own-brand rivals, but this Sigma is a bit special. Unlike the Canon and Nikon lenses on test, the Sigma 17-50mm boasts a fast, constant aperture of f/2.8 that remains fixed throughout the zoom range.
Street price : around £575
Read the full Sigma 17-50mm f/2.8 EX DC OS HSM review
At all focal lengths, the Sigma optic shows good sharpness at the centre and middle frame, but there's a slight softening towards the edges.
At all focal lengths, there's a little sign of fringing at the centre frame, but towards the edges there's a mild influence of blue and red.
At 17mm and 21mm there's slight barrel distortion, and above this, pinch distortion is only minimal and barely visible.
Image test verdict
Sharpness at all focal lengths, and especially at the centre of the frame, is very good. Fringing and distortion are also minimal.