The Sigma 17-40mm F1.8 DC Art ($919) is the first standard angle zoom lens for select APS-C mirrorless systems to open to an f/1.8 aperture, in effect giving crop sensor cameras the same background-blurring power as a full-frame, and catching more light for more reliable focus and cleaner photos when snapping pictures in dimly-lit interiors. The lens delivers on its promises, rewarding photographers with clear images with softened, albeit sometimes busy, backgrounds, and a downright reasonable size given its aperture and angle of view. It competes against a different set of lenses in each mount, but proves to be a unique tool for creative photography.

Design: Prime Aperture In a Zoom Lens

The 17-40mm F1.8 DC Art is a novel lens for mirrorless cameras, but it is not the first time Sigma has tried its hand at this type of zoom. Way back in 2013, it released the 18-35mm F1.8 DC HSM Art, which was compatible with the DSLRs of the era. I considered it a revolutionary photographic tool on launch, especially since the cameras of the time struggled to net clean images at higher ISO sensitivity settings and delivered more consistent focus with ample light entering the lens.

(Credit: Jim Fisher)

Mirrorless cameras use newer sensor tech and perform better in dim light, both in terms of focus and picture quality. So, it's the extra background-blurring power that will push you to pick this lens over a smaller, more affordable alternative like the Sigma 18-50mm F2.8 DC DN Contemporary. At 4.6 by 2.9 inches (HD) and 18.9 ounces, the 17-40mm pairs well with gripped APS-C mirrorless cameras like the Canon EOS R7 I used during testing.

The exterior is mostly made from Sigma's thermally stable composite (TSC) polycarbonate, with metal at the mount. Gaskets inside protect the lens from dust and splashes, and a protective fluorine coat effectively repels moisture and grease. Water beads off the glass without leaving a mark, and it's easy to wipe away a stray fingerprint with just a microfiber cloth. If you want to add extra protection, the lens works with 67mm protective and creative filters.

(Credit: Jim Fisher)

The lens comes with the expected accessories: front and rear caps, a cloth pouch, and a reversible petal hood with a push-button lock. Flare wasn't a major problem with the optics, but it's still a good idea to use the hood, as it provides physical protection to the front element.

Mounts: Available for Canon, Fuji, L-Mount, and Sony Cams

In addition to Canon RF, Sigma makes the lens for Fujifilm X, L-Mount Alliance, and Sony E camera systems. The Canon version (pictured throughout) includes a flexible control ring that clicks as you turn it, while the others have an aperture ring that swaps between clicked and smooth operation.

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Canon owners don't have a lot of alternatives to consider. The aforementioned Sigma 18-50mm F2.8 DC DN Contemporary is the only other upmarket zoom made in RF-S mount, so you'll mull between the two if you're eying an upgrade from either the RF-S 18-45mm F4.5-6.3 IS STM or 18-150mm F3.5-6.3 IS STM kit lens.

Fujifilm has a much stronger catalog of first-party lenses, including the XF 16-55mm F2.8 R LM WR II and XF 16-80mm F4 R OIS WR. Sony has one serious contender, the E 16-55mm F2.8 G in its catalog. The Sigma 18-50mm F2.8 Contemporary and Tamron 17-70mm F2.8 Di III-A VC RXD represent the third-party options for both Fuji and Sony. The Tamron is our Editors' Choice for both systems because of its ample zoom range, stabilized f/2.8 optics, weather sealing, and value price ($699 for Sony, $799 for Fuji).

(Credit: Jim Fisher)

Sigma also offers the 17-40mm for L-Mount cameras, though I don't think it'll sell many copies. Leica made a few APS-C L-mount bodies a decade ago, but they've been out of production since it discontinued the Leica CL. All L-mount cameras made today are full-frame, so the 17-40mm is only of interest if you intend to use it in a Super 35 video mode.

Controls and Focus: Speedy for Candid Moments

The 17-40mm F1.8 Art's controls vary slightly depending on the lens mount. The Fuji, Sony, and L-Mount versions include an aperture ring with third-stop markings from f/1.8-16. The ring switches between clicked and smooth adjustment with the flick of a switch. It has an "A" position to return aperture control to the camera and a lock switch that restricts the ring to either A or manual adjustment when engaged.

(Credit: Jim Fisher)

The Canon version uses an unmarked, always-clicked control ring instead and omits the aperture lock function because of it. The ring supports one of more than a dozen functions set via the camera menu. I like using it for EV compensation, but aperture, ISO, and white balance are also popular choices.

All versions of the lens include discrete zoom and focus rings, an AF/MF toggle switch, and two copies of a configurable function button. The button function is set via your camera menu, and options vary from model to model, but understand that both copies of the button are always mapped to the same function. The control surface is repeated so it's in the same relative position regardless of whether you're holding the camera in landscape or portrait orientation.

(Credit: Jim Fisher)

The lens uses Sigma's fastest high-speed linear actuator (HLA) motor tech to drive its autofocus. The result is an outstanding focus experience: The lens snaps from near to distant subjects in an instant. With this zoom, you're not likely to miss many candid photo ops due to lagging focus. Manual focus is great, too—the control ring is properly dampened, so there's no slop or wobble when setting focus, and focus response is instant, without any discernible lag.

(Credit: Jim Fisher)

The 17-40mm F1.8 isn't a good lens for macro photography, but it has a fairly decent close focus distance (11 inches) and captures objects at 1:4.8 life-size at 40mm. This type of close focus is useful in an everyday zoom, and matches the macro capability of the Tamron 17-70mm F2.8, which focuses for 1:4.8 magnification at 17mm and 1:5.2 at 70mm.

(Credit: Jim Fisher)

The lens does not include any sort of optical stabilization, which is a real concern as many APS-C cameras lack in-body image stabilization (IBIS). I used it with a camera with 5-axis IBIS, the EOS R7, and enjoyed the flexibility of switching between a bright aperture for short shutter speeds or a smaller one for long exposures. I had good luck holding the lens for as long as 1/4-second with the R7.

(Credit: Jim Fisher)

Image Quality: Crisp Results, But Shows Some False Color

I used Imatest software to check the 17-40mm's resolving power in my lab with the 32.5MP Canon EOS R7. The lens delivers very good center detail (3,400 lines) at its widest aperture, but softens toward the edges. In many instances, the lens's shallow depth of field will obscure loss of contrast around the periphery; a primary reason to use a lens at f/1.8 is to soften backgrounds and isolate a subject after all. If you're using the zoom for landscapes or other scenes where you want more depth of field for sharpness across the frame, use f/4-8; it scores in the excellent range there (4,000 lines at 17mm, 3,500 lines at 40mm).

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The aperture goes down as small as f/16, but you'll encounter optical diffraction if you narrow it that far. Light particles scatter apart (or diffract) as they pass through a very small diaphragm, which softens overall detail. The effect is more noticeable on higher pixel sensors; with the EOS R7, it drops resolution down to the acceptable (2,350 lines) range. There is a silver lining to diffraction; the scatter effect renders small points of light as 22-point starbursts, an effect that can add some oomph to landscape scenes with the sun in frame and nighttime cityscape images with street lights.

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Like most lenses for mirrorless systems, the 17-40mm F1.8 relies on geometric corrections to remove complex barrel and pincushion distortion from photos. If you use your camera for JPG images or video, these corrections are performed transparently, and are shown in real-time in the viewfinder as well as in finished photos. If you use your camera for Raw format images, you'll want to apply a profile to remove the visible distortion it exhibits through the zoom range. The profile also compensates for a vignette that is visible at wider aperture settings in Raw images. Adobe Lightroom Classic does not yet support this lens, but Sigma provided early access to a forthcoming correction profile, and it works perfectly.

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When used at f/1.8, the zoom blurs backgrounds readily, especially at longer focal lengths. The bokeh is pleasingly soft, but not without a few issues that can show up in the blur. Specular highlights are thankfully free of the onion ring effect, a sign that Sigma has taken care to smooth out the surfaces of the four aspheric elements in the 17-40mm's complex 17-element/11-group optical formula. But they show slightly bright edges—the soap bubble effect—along with some false purple-green color in the transitions between focus and blur (longitudinal chromatic aberration, or LoCA).

(Credit: Jim Fisher)

You won't see distracting backgrounds in every scene, but the combination of LoCA and soap bubble highlights lends itself to busier bokeh in scenes with repeating patterns (like foliage or bare tree branches) or many points of bright light (like sunlight reflecting off the surface of water or dewy grass). LoCA is most apparent at f/1.8 and lessens as you narrow the aperture. It's all but gone by f/4, but the lens doesn't blur backgrounds as readily at smaller f-stops.

(Credit: Jim Fisher)

All and all, I'm quite happy with what Sigma has done with this lens. Its optical formula provides prime-quality background blurring power through the zoom range, something that no first-party APS-C zoom has accomplished. Distortion and vignette correction are effective, though a likely factor for the softness the lens shows at its corners at wider apertures, and the images are pleasing all and all. I missed having the extra zoom power I'd get from the Tamron 17-70mm when using the 17-40mm as a walkaround lens, but a shorter range of coverage is a reasonable trade for the brighter optics, especially when you remember that there are no other F1.8 zooms made today for APS-C camera systems.

(Credit: Jim Fisher)