Design

The 10-20mm is typical Sigma in design and build, and features the company's familiar matte black 'zen' finish. The lens feels solid and durable, with a metal mount and an overall impression of quality to the materials and construction; it also appears rather more rugged than Tamron's SP AF 10-24mm F3.5-4.5 Di-II. The layout is entirely conventional, with the focus ring to the front and the zoom ring to the rear, and a focus mode selection switch on the side of the barrel (Canon and Sigma mount versions).

On the camera

As is typical for a wideangle lens, this lens is rather broad and stubby, and as the length scarcely changes on zooming, it maintains its balance well at all focal lengths. It's perhaps most at home on slightly larger mid-range DSLRs, but is still sufficiently compact that it doesn't feel out of place on smaller cameras (such as the EOS 450D (above right). The zoom and focus rings fall readily to hand, and are perfectly easy to operate when shooting.

It's worth pointing out that this lens isn't terribly compatible with the on-board flashes found on DSLRs; most of these only cover an angle of view equivalent to using an 18mm lens. At wider angles, the flash will give uneven frame coverage with darkening towards the corners, coupled with shadowing from the lens itself in the lower centre of the frame. This is absolutely normal for a wideangle zoom; if you really want to use this lens with flash you'll need to invest in a suitable external unit.

Autofocus

The Canon, Nikon, Sigma and Four Thirds mount versions of this lens use Sigma's ultrasonic-type HyperSonic motor for focusing , and in real-world use it works very well, acquiring focus rapidly and near-silently. This particular system is also of the ring-type, which offers several advantages; the manual focus ring does not rotate during autofocus, and focus can also be tweaked manually with the lens set to AF. This is certainly a major plus for this lens over its competitors (in this class only Canon and Nikon's more expensive offerings feature something similar). The HSM motor also enables the lens to focus on Nikon's entry-level DSLR bodies (D40 / D40X / D60).

Sony and Pentax users, however, have to make do with screw-drive focusing driven from the camera body, however it's worth bearing in mind that the advantages offered by HSM are arguably rather less important on a wideangle lens than on a telephoto. As always, it must also be noted that focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels.

Lens body elements

	The lens comes in versions for Canon, Nikon, Pentax and Sony DSLRs. All of our images on this page show the Canon EF version, but note that the test data was acquired using a Nikon mount lens.
	The filter thread is 77mm. It does not rotate on autofocusing, which should please filter users. Further good news is that normal 8mm-thick polarizers can be used perfectly happily without vignetting, and the slim-mount type is not required.
	The petal-type lens hood is provided as standard, and fits positively onto the bayonet mount on the front of the lens. It's made of thick black plastic and features ribbed moldings on the inside to minimize reflections of stray light into the lens. In trademark Sigma fashion, a series of markings on the outside of the hood aid alignment for mounting.
	Unlike the Tamron 10-24mm F3.5-4.5 we reviewed recently, the lens hood reverses neatly to give a reasonably compact package. With its 95mm diameter, it should fit happily in most bags.
	The zoom ring has a 13mm wide ribbed rubber grip, and the action is smooth and precise. It rotates 50 degrees clockwise from ultrawide to wide; the 'right' way for Nikon, Pentax and Sony users, but opposite to Canon and Olympus lenses. The front element extends just 4mm on zooming from 10 to 20mm.
	The 12mm wide focus ring rotates 120 degrees clockwise from infinity to 0.5m, this time matching Canon, Sony and Olympus lenses but opposite to Nikon and Pentax designs. It does not rotate during autofocus, and again the action is smooth and precise. The HSM version of the lens features full-time manual focusing, allowing the user to tweak focus even in AF mode.
	A basic distance scale is marked in feet and meters, and the focus ring travels slightly past the infinity position. There's no depth of field scale, however. This window is absent from the non-HSM Pentax and Sony versions, replaced by a distance scale on the (now mechanically coupled) focus ring.
	A large, positive switch on the side of the lens barrel is used for changing the focus mode between auto and manual on Canon and Sigma mount variants.

Reported aperture vs focal length

Here we show the maximum and minimum apertures reported by the camera at the marked focal lengths.