Sigma 10-20mm 1:4-5.6 EX DC HSM review

The Sigma 10-20mm F4-5.6 EX DC HSM is, on paper at least, a very attractive option for APS-C users looking for an ultra-wideangle zoom. The standout selling points are undoubtedly that 10mm wideangle view and the HSM focusing motor, which when coupled with Sigma's traditionally keen pricing, makes for a compelling overall package. In the flesh that promise is to a great extent realized; optically the lens is a solid if not outstanding performer, build quality is up to Sigma's usual standards, and the focusing is indeed fast, silent and accurate. So far, so good.

Delve a little deeper, though, and problems start to appear. Distortion at wideangle, with its hefty barrel effect localized towards the corners of the frame, can be highly detrimental for architectural shots, and to compound the issue it's difficult to correct in software. But the bigger question mark hangs over the lens's sharpness, and in particular its inconsistency through the zoom range and across the frame. It's fine at 10mm and between 17 and 20mm, where stopped down to F8 or F11 results are firmly within the 'green zone' on our widget (corresponding to a critically sharp print of at least 12" x 8" / 30 x 20 cm). But in the intermediate range it's simply soft across much of the frame, even at F11. This is compounded by curvature of field effects which can lead to some odd real-world results, as objects which would be expected to fall within the depth of field can sometimes be rendered unexpectedly soft. As a result, extra care in focusing is advisable, especially when shooting at relatively close distances - in such instances it's best to use off-centre focus points (or even better, manual focus in live view) to make sure the subject is properly sharp.

Comparison to other wideangle zooms we've tested recently suggests the Sigma's class-leading days may be numbered. On APS-C, it simply can't match the biting central sharpness of the Tamron 10-24mm F3.5-4.5 Di-II in the 10-18mm range (for which, we suspect, users will mainly be buying this type of lens), and has more problematic distortion characteristics. However in a typical case of swings and roundabouts, the Sigma shows superior sharpness when shot wide open, and is better in the corners of the frame at all settings; it also has lower chromatic aberration, is better built and uses a superior focusing system. On Four Thirds, the new Olympus Zuiko Digital ED 9-18mm F4-5.6 offers a wider angle of view and markedly superior optics in a lighter and more compact package; so while it's more highly priced, on balance it's a better buy for most users.

So overall the Sigma 10-20mm F4-5.6 EX DC HSM is a solid, well-built lens with good operational characteristics, which justifies its popularity with generally decent imaging results. While its long-running unique selling point of being the only third-party 10mm zoom has now disappeared, and it faces strong competition from Tamron's 10-24mm, the different characteristics of the two lenses means it's still a good option for APS-C users looking for the widest of ultra-wideangle zooms. It may no longer necessarily be the leader of the pack, but it's still well worth considering.

Andy Westlake

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