Sigma 17-70mm f/2.8-4 DC Macro OS HSM - £350/$470
This Sigma bridges the gap between fast constant-aperture lenses and those with a more generous zoom. Its maximum available apertures are about a stop larger than with most kit lenses, ranging from f/2.8 at 17mm to f/4 at 70mm.
The zoom range is quite impressive, too, equivalent to 25.5-105mm. The lens also features four-stop Optical Stabilization and only costs marginally more than the Tamron 17-50mm VC.
The HSM (Hyper-Sonic Motor) autofocus isn't a ring-type system, so uses a motor and gear wheels, but it's a lot quieter and a bit faster than the Tamron lenses' more basic electric motors. The focus ring rotates during autofocus, so take care not to foul its action.
You get plenty of sharpness and contrast, though not quite as much as with the similar Nikon 16-85mm VR. Barrel distortion at 17mm switches to pin-cushion at 35mm, which then stays at a similar level up to 70mm. Even so, the amounts of these distortions aren't too severe, especially considering that the overall zoom range is bigger than average.
Sharpness test
Corner-to-corner sharpness is great for a lens of this price, but the zoom isn't on a par with the best in the group.
Lab test at f/8
Sharpness at wide angle: 2345
Sharpness at mid range: 2143
Sharpness at telephoto: 2134
Fringing test
Fringing is well contained and only really noticeable around high-contrast edges towards the corners of the frame.
Lab test at f/8
Fringing at wide angle: 0.43
Fringing at mid range: 0.21
Fringing at telephoto: 0.12
Distortion test
Barrel and pin-cushion distortion is restrained, especially considering the above-average zoom range of this lens.
Lab test at f/8
Distortion at wide angle: -2.97
Distortion at mid range: 1.26
Distortion at telephoto: 1.28
Image quality verdict
Overall image quality lags a little behind the more expensive lenses in the group, but the Sigma is still very good value at the price.
