Sigma 17-50mm f/2.8 EX DC OS HSM - £570/$670
At a glance, there's very little to distinguish this lens and the Sigma 17-70mm, which costs much less. As an 'EX' lens, the 17-50mm has an upmarket, textured coating to denote superior build and optical quality, but both lenses are of a similar size and weight, and the controls and onboard markings are nearly identical.
The main difference is that the 17-50mm is a fast constant-aperture f/2.8 lens with a smaller zoom range. As expected, the filter size is larger, at 77mm instead of 72mm.
Disappointingly, the HSM autofocus system isn't ring-type, so the focus ring rotates during autofocus and impairs handling slightly. There's also no full-time manual override. On the plus side, Sigma's four-stop optical stabiliser works very well.
Optical quality is excellent, even at the maximum aperture of f/2.8, and there's impressively low vignetting. The only caveat is that corner sharpness is a little poor at f/2.8, especially at focal lengths of 28-35mm. However, this isn't a problem in practical terms, as you'd usually use the large aperture to reduce depth of field and blur the background.
Sharpness test
Levels of sharpness are impressive right into the corners, even at the maximum f/2.8 aperture.
Lab test at f/8
Sharpness at wide angle: 2562
Sharpness at mid range: 2354
Sharpness at telephoto: 2469
Fringing test
Fairly consistent throughout the zoom range, colour fringing is well controlled and barely noticeable in most shots.
Lab test at f/8
Fringing at wide angle: 0.56
Fringing at mid range: 0.48
Fringing at telephoto: 0.34
Distortion test
Some barrel distortion at 17mm, and this switches to a small amount of pin-cushion by 28mm.
Lab test at f/8
Distortion at wide angle: -2.71
Distortion at mid range: 0.63
Distortion at telephoto: 0.98
Image quality verdict
Overall image quality isn't better than with the Nikon 16-85mm, but this Sigma is a good choice if you want the faster, constant aperture.
