Sigma 18-35mm F1.8 DC HSM

Prior to the introduction of the Sigma 18-35mm F1.8 DC HSM ($799 direct) lens shutterbugs with APS-C D-SLR cameras?had to make a choice when it came to lens functionality. You can go with a zoom lens and cover a range of focal lengths, but even top-end models only opened up to f/2.8. Or you can opt for a prime lens with a wide aperture, but it doesn't support zooming. Sigma aims to change that with this lens. It's impressively sharp, but it is a bit large compared with slower zooms or fast prime lenses, and its zoom range is modest at 1.9x. It stills earns high marks and our Editors' Choice award; the lens is an accomplishment, and the fact that it doesn't come with a sky-high price tag means that more photographers will actually be able to shoot with it.

The lens is currently available to order in Sigma, Nikon, and Canon mounts, but Sigma also plans on offering it for Pentax and Sony/Minolta cameras. It isn't compatible with full-frame bodies; the image circle only covers an APS-C sensor. This makes it an EF-S lens for Canon shooters, a DX lens for Nikon owners, and a DT lens for Sony D-SLR owners. The 18-35mm zoom range is roughly equivalent to 27-52.5mm in full-frame terms, which captures a wide angle view when zoomed out, and narrows to a standard-angle when zoomed all the way in. It doesn't offer quite the zoom range as the 18-55mm lens that likely came with your D-SLR, but it captures about four times the light on the wide end, and about ?six times as much at the 35mm setting.

In addition to the obvious advantages when shooting in low light, using the lens at f/1.8 will allow you to capture images with a shallow depth of field, but just how shallow the focus is will depend on the focal length and your distance from your subject. When shooting at a wide angle it's difficult to get the bokeh look in your images, unless you are working fairly close to your subject. You'll get a shallower depth of field by backing up and zooming in. When conditions are right, the lens is able to capture photos with a smooth, blurry, out of focus area behind your subject.

The 18-35mm is fairly large. It measures 4.8 by 3.1 inches (HD) and weighs about 1.8 pounds. The barrel is made of a composite material that feels sturdy and is cool to the touch, just like metal. There are two knurled control rings?the zoom ring is close to the base of the lens, and the manual focus ring is just behind the front element. The only control switch is a toggle to change between autofocus and manual focus mode. The filter thread accommodates a 72mm filter, and the lens is able to focus from 11 inches to infinity. A reversible lens hood and carrying case are included. Its size is not out of line when compared with the Pentax SMC DA Star 16-50mm F2.8 ED (IF) SDM lens, which has a narrower maximum aperture but a longer zoom range.

I used Imatest to check the sharpness and distortion characteristics when paired with the Canon EOS Rebel T3i. The lens is impressively sharp at f/1.8, recording 2,383 lines per picture height using a center-weighted test; that's much better than the 1,800 lines we require for an image to be called sharp. Edges are also impressive, scoring an average of 1,868 lines, which would be impressive even for a prime lens. Stopping down to f/2.8 improves the overall score to 2,423 lines with edge performance just shy of 1,900 lines. Resolution at its widest focal length peaks at f/4; the lens manages 2,377 lines there.

Zooming to 24mm improves sharpness. At f/1.8 the lens scores 2,470 lines, and it peaks at f/4 at 2,690 lines. At 28mm the lens manages 2,457 lines at f/1.8 and improves to 2,562 lines by f/4. Zooming all the way in to 35mm delivers 2,399 lines at f/1.8 and peaks at 2,528 lines at f/4. Edge performance when zoomed in a bit is consistently good; better than 1,800 lines at every tested focal length and aperture.

The 18-35mm does show some distortion. At 18mm it records about 1.9 percent barrel distortion, which gives a slight outward curve to images. This is something that you can correct in software if it distracts from a shot. It's just on the cusp of being mild enough to ignore, but a few ticks with Lightroom's distortion slider adjustment tool will correct it. At 24mm the distortion is almost nonexistent, only 0.5 percent pincushion?which makes lines curve slightly inwards. You get a bit more of this, about 1 percent, at 28mm, and 1.2 percent at 35mm.

To put this performance in perspective, compare the Sigma 18-35mm with the Canon EF-S 17-55mm f/2.8 IS USM zoom. That lens is priced at just under $1,200 and features an aperture that is more than an f-stop slower. It does have a slightly longer zoom range and optical stabilization?which is lacking from the Sigma lens. It's also sharp, but not through the entirety of its zoom range. It's great at 17mm, where it notches 2,039 lines, and still good at 35mm where it hits 1,952 lines. Zoomed in at 55mm the score drops to a disappointing 1,516 lines, though it does sharpen up when stopped down. The Canon lens does a better job controlling distortion on the wide end; it shows only 1 percent barrel distortion at 17m, and its 1.6 percent pincushion distortion at 35mm and 55mm is not out of line with the Sigma.

But the Sigma 18-35mm F1.8 DC HSM captures twice as much light as an f/2.8 zoom and delivers more consistent sharpness than Canon's 17-55mm. When you factor in its attractive price, you have a lens that earns our Editors' Choice award. It's a great option for anyone who is looking to upgrade from an 18-55mm kit lens, as you are no longer required to choose between a lens with a very wide aperture or one that zooms. The only real knock on the 18-35mm is its rather modest zoom range. Photographers who are used to an 18-55mm lens may find themselves missing a bit of telephoto reach. But the range it does cover replaces three classic full-frame primes?a 28mm, 35mm, and 50mm?all at f/1.8 maximum aperture. That's an impressive feat for a zoom.

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