Pros & Cons
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- Sharp from edge to edge.
- Minimal distortion.
- Short minimum focus distance.
- Available for multiple systems.
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- Heavy.
- Dim corners and edges.
- No support for filters.
Sigma 20mm F1.4 DG HSM Art Specs
| 35mm Equivalent (Wide) | 20 |
| Dimensions | 5.1 by 3.6 inches |
| Lens Mount | Canon EF |
| Lens Mount | Nikon F |
| Lens Mount | Sigma SA |
| Stabilization | None |
| Type | Lens |
| Weight | 2.1 |
Sigma's Art series of f/1.4 prime lenses has delivered consistently strong performance, and its latest entry, the 20mm F1.4 DG HSM Art ($899), is no exception. It manages to capture sharp, ultra-wide images, a heck of a lot of light, and focus close so you have the option of blurring the background behind your subject, all without showing significant barrel distortion. It's not quite perfect, as corners and edges are noticeably dimmer than the center, but that's easily rectified using software correction. If you're looking for a stellar ultra-wide prime lens, this is the one to get—it's a clear Editors' Choice.
Design
The 20mm Art ($899.00 at Amazon) is huge for a prime lens—it dwarfs some compact zooms. It measures 5.1 by 3.6 inches (HD) and weighs 2.1 pounds. The hood is integral to the design and can't be removed. It helps to improve contrast, blocking light from hitting the bulbous front element from oblique angles, and is also effective at protecting the front element from stray fingerprints. Because of the curved front element, there is no thread for filters. A slip-on lens cap is included. The lens is finished in black; Sigma sells it for Canon, Nikon, and Sigma SLRs, as well as Leica, Panasonic, and Sigma mirrorless L-mount cameras, and Sony E-mount mirrorless cameras. It is eligible for Sigma's mount conversion service and compatible with the USB Dock ($42.00 at Amazon) for firmware updates and focus adjustments.
The barrel is a mixture of metal and sturdy composite plastic. The manual focus ring sits behind the front element and hood. It's covered in a textured rubber finish and requires just a 90-degree turn to move from the closest focusing point to infinity. You can turn the barrel beyond either point—it's not a hard stop as with pure manual focus lenses—you'll feel a bit of resistance and hear a click when you hit either focus extreme. A toggle on the side switches between manual and automatic focus.
The 20mm focuses on subjects as close as 10.9 inches. That's measured from the image sensor, so the working distance from the front element is more on the order of 4.1 inches. The close focus capability comes into play when shooting at wider apertures—you don't think about a 20mm lens as an optic that can blur backgrounds, but it's certainly possible with the 20mm Art.
Image Quality
I paired the 20mm Art with the full-frame Canon EOS 6D ($1,999.99 at Amazon) . The combination netted excellent results when analyzing our SFRPlus chart using Imatest's standard center-weighted resolution test. At f/1.4 the lens scores 2,333 lines per picture height, better than the 1,800 lines we look for in an image. Performance is strong through most of the frame, but the outer third does lag behind the center. The periphery still scores 1,799 lines, which is a little weak, but by no means a bad result for an ultra-wide lens at f/1.4.
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Fidelity improves as you narrow the aperture. At f/2 the center-weighted score improves to 2,411 lines; edges also improve to 2,010 lines. It gets better at f/2.8 (2,427 lines average, 2,223 lines at the outer third of the frame) and at f/4 (2,640 lines average, 2,414 at the edges). The lens is at its absolute sharpest at f/5.6—it nears 2,700 lines on average, with edges lagging behind the overall score by just 200 lines. Diffraction sets in at f/8, but the lens still shows 2,656 lines there, 2,506 lines at f/11, and 2,336 lines at the smallest f/16 aperture.
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There's very little barrel distortion—about 0.9 percent. That's a nominal amount for a 20mm lens, so straight lines in the frame will appear fairly straight. If you do notice a curve, it's easily removed using Lightroom ($9.99/Month at Adobe) . You will have to deal with perspective distortion—if you're shooting a subject from an askew angle and frame it near the edge or corner of the frame it will appear unnaturally stretched. That's something that can happen with any wide angle lens—try and keep the camera as level as possible to minimize the effect.
If there's criticism to be made of the 20mm Art, it's the uniformity of illumination—corners and edges are dim when compared with the center of the frame, especially at wider apertures. I took photos of a flat gray sky at each full-stop setting and ran the results through Imatest's Uniformity analysis tool.
At f/1.4 the corners show a 4-stop drop in exposure when compared with the center of the frame, and the sides how a 2.5-stop dip. That's significant enough to draw a noticeable vignette around the edges of a photo. If you edit photos in Lightroom or another processing application it is easy enough to brighten them to bring them in line with the rest of the frame. The latest version of Lightroom includes a profile for one-click corrections.
Stopping down to f/2 provides slight improvement—illumination gains a half-stop all around. At f/2.8 the corners are better, showing a -2.5EV drop from the center illumination and a -1.5EV dip at the sides. At f/4 and beyond the corners show a drop of 2 stops and the sides are about 1.5EV dimmer than the center.
Conclusions
Wide-angle lenses are often on the large side, and it's rare that you see one with an f/1.4 aperture. But the Sigma 20mm F1.4 DG HSM Art is a special lens indeed. Not only does it capture a lot of light, it also manages to focus close, shoot images with excellent detail, and minimize barrel distortion. That's quite a feat, and its size and weight are signs of just how much glass is required to do all of it. Dim corners and edges are an issue, but that's a common among wide lenses, and one that software tools can make short work of. The 20mm is a phenomenal performer, and an easy pick as our Editors' Choice.
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