Group Test: Independent 10-24mm zoom lenses

Tokina SD 12-24mm (IF) DX AT-X Pro

Street price (October 2009): £450

With a fixed f/4 maximum aperture and an AT-X Pro designation it would be reasonable to expect a lot of the Tokina SD 12-14mm (IF) DX AT-X Pro lens - and its on-the-street price is likely to be well below the manufacturer's RRP, suggesting that it could be a real bargain.

The Tokina's design appears a little old-fashioned thanks to a slightly conical profile that contrasts with the modern fashion for more uniform cylindrical forms. It also has a push/pull AF/MF selector that is part of the manual focusing ring rather than a separate switch elsewhere on the barrel.

This does not mean, however, that the Tokina SD 12-24mm is behind the times - it has an internal focusing mechanism that leaves the focusing ring undisturbed in AF mode and also features both special-dispersion glass and aspheric profiles that are de rigueur in lenses of this type. A slight failing on the specifications front is the minimum focal length of 12mm when other APS-C lenses of this type tend to go just a shade shorter.

Similarly, although Tokina's zoom ring offers just the right amount of uniform resistance, the focusing ring does not have quite the same smoothness. There is a window showing the focused distance but there are no depth-of-field markings.

Resolution is at least good and in parts excellent. The Tokina 12-24mm's MTF curves obtained for maximum and intermediate focal lengths are so well matched that they are indistinguishable - the 12mm curve is slightly lower but, apart from a distinct dip at f/4, it remains above the critical 0.25 cycles-per-pixel level right out to f/16.

Significant chromatic aberration is evident at 12mm, caused by a distinct separation of the blue curve, but visually (and in real-world pictures) Tokina's zoom produces some very pleasing images that are virtually free from imperfections.

The fact that the lens looks a little different may not be to everybody's liking, and the fact that it is only offered in Canon and Nikon mounts may disappoint others, but the Tokina handles very nicely.

Sigma 10-20mm EX DC HSM

Street Price (October 2009): £420

Following in the footsteps of Sigma's full-frame 12-24mm DG zoom, the Sigma 10-20mm EX DC HSM version has a lot to live up to. Fortunately, it is not only equal to the challenge but also pulls ahead in several important respects.

Real-world testing reveals that the APS-C lens is visually sharper and freer from chromatic aberrations than the DG version. It is also less expensive and more compact, yet still offers internal focusing with full manual intervention in AF mode. There is a serious loss of sharpness beyond f/22 (not shown on the graph) owing to the tiny size of the hole through which light is then travelling, but there is surely no good reason to use such extreme settings with ultra-wide lenses.

In theory it is possible to fix low-profile filters to the front of the lens but vignetting and ghost images, caused by the extreme off-axis rays, are both potential problems. Flash coverage is also likely to be problematic with ultra-wide angles of view.

Focusing is brisk, thanks to Sigma's Hyper Sonic Motor, and handling is good by virtue of the freedom that comes from having an internal-focusing mechanism. In fact there really is remarkably little to criticise in this lens other than the aforementioned advice to avoid the smallest aperture settings. Provided that the aperture is no smaller than f/16 the lens returns better than 0.25 cycles-per-pixel optimum at all focal lengths.

 It is even possible to use the manual-focusing ring with the petal lens hood reverse-mounted in its stowed position, which may seem minor but that is not always the case and should be taken as another sign of attention to detail.

The only question that remains is why, given this zoom's excellent performance, Sigma has felt the need to announce a fixed-aperture (f/3.5) variant that will inevitably be bigger and more costly. Only time will tell what the new design has to offer, but whatever it is the chances are that if you need an ultra-wide zoom for an APS-C or Four-Thirds body then the current 10-20mm DC is still going to be very hard to beat.