Best zoom lens upgrade for Nikon DSLRs: 8 tested

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

Best zoom lens upgrade for Nikon DSLRs: 8 tested

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

Best zoom lens upgrade for Nikon DSLRs: 8 tested

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

Best zoom lens upgrade for Nikon DSLRs: 8 tested

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

Best zoom lens upgrade for Nikon DSLRs: 8 tested

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.