Following in the footsteps of Sigma’s full-frame 12-24mm DG zoom, the 10-20mm DC 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 focussing 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 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.