Sigma 24-70mm f/2.8 EX DG IF HSM - £640/$900
This beefy Sigma optic is a little broader than Nikon's 24-70mm full-frame lens, but is slightly lighter and nearly 20mm shorter. Build quality and finish are excellent, with silky smooth zoom and focus rings. The latter gives the additional benefit of full-time override in autofocus mode, thanks to a super-fast and blissfully quiet ring-type ultrasonic system.
The constant f/2.8 aperture is nice to have, but one thing that's lacking is Sigma's OS (Optical Stabilization), which is featured on the smaller 17-50mm f/2.8 lens for DX cameras. Filters will also be expensive, because at 82mm this lens has the widest diameter thread in the group.
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.
