Sigma 20mm f/1.4 DG HSM Art

Sigma 20mm f/1.4 Art Review

by William Brawley | Posted 11/06/2014

Fans of ultra-wide optics should take note of Sigma's latest ƒ/1.4 Art-series lens, as the new 20mm ƒ/1.4 DG HSM Art is their widest one yet and thus a prime choice for landscapes as well as astrophotography. Designed for full-frame DSLRs, the Sigma 20mm ƒ/1.4 Art offers a 94.5-degree field of view for a nice, ultra-wide perspective, and its impressive close-focusing capability should allow for some dramatic compositions. The lens is compatible with APS-C cameras, as well, offering an approximately 30mm effective field of view.

The Sigma 20mm ƒ/1.4 DG HSM Art features a prominent, bulbous front element, as is common on such lenses, and the petal-shaped lens hood is therefore built in -- there are no filter threads on the front, nor a gel-filter holder on the rear. The lens is offered in Canon EF, Nikon F, Sigma SA and Sony E mounts for an estimated retail price of $900.

Sharpness
As we've seen on Sigma's previous Art-series primes, sharpness is the name of the game for this 20mm version -- this lens is sharp! On a full-frame camera, even at ƒ/1.4, the Sigma 20mm ƒ/1.4 impresses with excellent center sharpness. Not surprisingly, corners display noticeable softness at ƒ/1.4, but a good portion of the frame, otherwise, remains sharp. Stopping down a bit, especially to around ƒ/2.8, really sharpens things up around the edges. The lens remains very sharp throughout the aperture range, even into the smaller apertures; however, we do see slight diffraction-related softness appear at ƒ/16.

Sharpness on a sub-frame camera follows a very similar pattern, though corner softness is greatly reduced. Wide-open, we see excellent sharpness with only slight corner softness. As before, stopping down sharpens things up a bit, and between ƒ/2.8 - ƒ/5.6, images are practically tack sharp across the entire frame. Diffraction-related softening, again, is minor and only begins to really set at ƒ/16 -- the minimum aperture available on this lens.

Chromatic Aberration
Thanks to the Sigma's optical configuration that includes dual "F" Low Dispersion elements and five Special Low Dispersion elements, the lens combats chromatic aberration very well considering its ultra-wide-angle focal length. On both full- and sub-frame cameras, CA is very minimal, even wide-open, with only subtle purple and green coloration on high-contrast edges in the corners. We still see a hint of fringing stopping down to ƒ/8 on our VFA test chart images, but overall it's very minor.

Shading (''Vignetting'')
Given the ultra-wide, ultra-bright nature of the Sigma 20mm ƒ/1.4 lens, there's bound to be some vignetting, and that's certainly the case here with this lens. On a full-frame camera, vignetting is, in fact, quite severe at the wider apertures. At ƒ/1.4, vignetting is literally off the charts at just around 1.5EVs of light falloff. Stopping down will reduce vignetting, as expected, though it's still exhibits over 0.75EVs of corner shading at ƒ/2.0. By the time you stop down to ƒ/4, however, vignetting is significantly reduced to around 0.25EVs and drops slightly under that -- but not much further -- as you stop down further.

Like corner softness, vignetting is much less of an issue on sub-frame cameras. We calculated only around 0.4EVs of light falloff at ƒ/1.4 on our APS-C test camera. By ƒ/2.8 and onwards, vignetting is at a negligible amount, around 0.09EVs and less, for the remaining aperture values.

Distortion
Despite its ultra-wide focal length, the Sigma 20mm is surprisingly well-corrected for geometric distortion. On a full-frame camera, we only measured about +0.3% barrel distortion on average, though in the far edges, it gets up to around +0.5%. On our sub-frame camera, distortion is even less prominent at only +0.09% barrel distortion and about +0.2% at the max. Overall, there's very little barrel distortion with both sensor sizes and to a degree that's very easily correctable in post-processing.

Focusing Operation
Like the rest of Sigma's current Global Vision lenses, the new 20mm ƒ/1.4 Art utilizes their HSM (Hyper Sonic Motor) drive to power autofocus. As such, the lens focuses very quickly, at well under one second to slew through the full focus range. Manual focus is also available, both with full-time MF override of the AF system or in complete manual focus mode using the AF/MF switch on the side of the lens. The large focusing ring only has about 120-degrees of focus throw, but feels very nice nonetheless with a direct, mechanical connection to the focus mechanism (in other words, not an electronic, focus-by-wire system). The lens includes a small focusing window with distance markings and depth of field indicators for ƒ/8 and ƒ/16. The focus ring has soft stops at either end of the focusing scale, but the ring will otherwise continue to rotate.

Macro
As an ultra-wide angle lens, the Sigma 20mm ƒ/1.4 Art isn't designed for macro photography, with a magnification ratio of just 0.14x (1:7.1). However, the lens does possess rather impressive close-focusing capabilities with a minimum focusing distance of 27.6cm, or just shy of 11 inches. This allows the lens to capture some nice, dramatic close-up wide-angle images.

Build Quality and Handling
The Sigma 20mm ƒ/1.4 Art follows the same design scheme and build quality we've come to expect from their newer Global Vision lenses. The build quality is fantastic. Constructed from their proprietary metal-like Thermally Stable Composite material, the lens feels very solid and is built like a tank.

And a "tank" in more ways than one. The lens is not only well built, but also surprisingly heavy, despite its single-focal length design. At roughly the same size as the Canon 24-70mm ƒ/2.8L II lens, for example, the Sigma 20mm ƒ/1.4 Art, nevertheless, tips the scales at over 2 pounds compared to the Canon lens' 1.8 pounds. Plus, with its large, bulbous front element, the lens feels rather front-heavy, even on larger camera bodies like a Canon 5D Mark III. Comprising most of that weight is the lens' 15 total optical elements. Situated into 11 groups, the lens features one double-sided aspherical element, 2 FLD elements and 5 SLD elements.

The design, like Sigma other Global Vision lenses, is a sleek with a matte black exterior; a large, rubbery-ridged focus ring; and a small patch of ridged plastic along the bottom of the lens barrel for extra grip. The lens does not feature any weather sealing that we're aware of, and there is no gasket seal around the metal lens mount. The aperture diaphragm is comprised of 9, rounded blades that create pleasing out of focus blurring. As mentioned earlier, the lens features a small focus distance window and scale as well as an AF/MF toggle switch on its left side.

The lens is compatible with Sigma's USB configuration dock accessory and is also eligible for Sigma's mount conversion service should you decide to switch camera manufacturers.

Alternatives

As for alternatives, the Sigma 20mm ƒ/1.4 Art stands on its own as this publishing date. It's the world's first ƒ/1.4 ultra-wide-angle lens for full-frame DSLRs. No other manufacturer offers this combination of a 20mm focal length and a bright ƒ/1.4 aperture. However, both Canon and Nikon do offer similar lenses in some form or fashion.

On the Canon side, there are two choices, the EF 20mm ƒ/2.8 USM or the 24mm ƒ/1.4L II. The former offers the same 20mm focal length as the Sigma, but it's not nearly as bright nor as sharp -- though it is much less expensive than the Sigma. On the other end of the spectrum, the 24L II offers the same bright ƒ/1.4 aperture, but at a narrower focal length. The 24mm is slightly less sharp, vignettes more and is much more expensive, but it is weather-sealed.

In the Nikon camp, the alternatives are much the same. On one hand, there's the Nikon 20mm ƒ/1.8G lens, which hits a similar price point ($100 less, in fact) and is quite sharp, but it's not as bright. Similar to Canon, Nikon also offers a much more expensive 24mm ƒ/1.4 lens. Wide-open sharpness isn't as nice as the Sigma's and it's vastly more expensive, but its weather sealing is a nice perk.

Conclusion
Sigma has done it again! At first, the sheer heft and size is a little shocking, especially considering it's a prime lens. However, this shouldn't come as a surprise given Sigma's past Global Vision lenses. Theses lenses haven't focused on being the smallest, or lightest, but rather the best optically. And the new 20mm ƒ/1.4 Art delivers. Images are sharp - very sharp. The lens is also very well corrected for CA and distortion, though vignetting is rather apparent, especially wide-open. The weight and front-heavy design makes it a little bit of a chore as a walk-around lens, but not overly so. All in all, this as yet unrivaled lens is the widest and brightest full-frame ultra-wide lens available, and should certainly make your short list if you're in need of a superior ultra-wide landscape or astrophotography lens.

Product Photos

Sample Photos

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.