Sigma 24-70mm f/2.8 EX DG IF HSM - £640/$900
Sharpness is good, but drops off a little at the 50mm zoom setting and could be better in the corners at the wide-angle end at f/2.8. There's plenty of contrast and distortions are fairly low.
Vignetting is less pronounced than with the Tamron 28-75mm lens on a full-frame body and negligible on a DX camera. Overall, it's a high-quality lens that only costs about half the price of the competing Nikon.
Sharpness test
Very good overall, but there's a slight dip in sharpness at focal lengths around the 50mm mark.
Lab test at f/8
Sharpness at wide angle: 2298
Sharpness at mid range: 2175
Sharpness at telephoto: 1926
Fringing test
There's not much fringing throughout the zoom range but the lens can't quite match the mighty Nikon 24-70mm.
Lab test at f/8
Fringing at wide angle: 0.31
Fringing at mid range: 0.23
Fringing at telephoto: 0.24
Distortion test
A little barrel distortion at wide-angle and mid-zoom lengths, switching to fairly low pin-cushion at the long end.
Lab test at f/8
Distortion at wide angle: -2.17
Distortion at mid range: -0.55
Distortion at telephoto: 0.71
Image quality verdict
The Sigma 24-70mm offers excellent image quality on both full-frame and DX-format cameras, making it a great value option.