Quality

Longitudinal Chromatic Aberration and focus shift

Lenses with focal ratios of f2.8 or larger are often prone to longitudinal color aberrations (loCA, a.k.a. “axial color” or “bokeh CA”). These show up as magenta coloration in the foreground and greenish hues in the background and are not easily corrected in post-processing. The Sigma clearly shows loCA wide open. And at f2.0 it is still stronger than from the Sony FE 20mm f1.8 G.

Sigma 20mm f1.4 DG DN Art Longitudinal Chromatic Aberration (loCA)

100% crops, from top to bottom: f1.4, f2.0, f2.8; left = foreground, right = background

When stopping down the background become sharper faster than the foreground especially between f2.0 and 2.8 almost resulting in focus shift.

The following real life shot under harsh contra-light conditions shows that the Sigma also produces some purple fringing around high-contrast edges in the focus plane and some spill-over from the extra-bright background into the subjects:

Above: Sigma 20mm f1.4 DG DN Art at f1.4; 100% crop; click image to access 4k version, here for large original

The second image on the samples page of the fountain backlit by the sun also shows some coloration in the air bubbles.

Sharpness and contrast

Let’s have a look at the theoretical performance of the Sigma 20mm f1.4 DG DN Art and compare it to the performance of the Sony FE 20mm f1.8 G and Sigma 24mm f1.4 DG DN Art:

Above: Sigma 20mm f1.4 DG DN Art, Sigma 24mm f1.4 DG DN Art (both at f1.4)

Above: Sony FE 20mm f1.8 G (at f1.8), Sony FE 24mm f1.4 GM (at f1.4)

Out of curiosity I’ve also added Sigma’s older DSLR designs and the f2.0 offerings from Sigma’s Contemporary line:

Above: Sigma 20mm f1.4 DG HSM Art, Sigma 24mm f1.4 DG HSM Art (both at f1.4)

Above: Sigma 20mm f2 DG DN, Sigma 24mm f2 DG DN (both at f2.0)

The MTF charts show the computed contrast-curves at 10 line-pairs/mm and 30 lp/mm without influence of diffraction. Higher values are better (more contrast) and the closer the dotted and solid lines are together the less contrast dependents on the orientation of the test-pattern (less astigmatism). The x-axis displays the distance from the optical axis (=center of the sensor) in mm. I’ll show you the real-life performance at 4 mm (“center”), 13 mm (APS-C-corner), and 20 mm (FF-corner) on a 42MP Sony A7R II body.

From the charts both new Sigmas should be clearly sharper than the older DSLR designs and the Sigma 20mm f1.4 DG DN Art even looks sharper at f1.4 than the Sigma 20mm f2 DG DN at f2.0. Only the Sony FE 20mm f1.8G seems to have an edge up to 8mm image-height – but then it is at f1.8 while the Sigma is at f1.4.

Let’s see how this theoretical performance of the Sigma translates into real life results in the sharpness test based on Siemens-stars. Processing was done in Lightroom 11.4.1/CRAW 14.4.1 from RAW to Adobe Color profile with CA compensation applied. Noise-reduction is set to 0, sharpening to 50/0.5/36/10, with no extra tone, color, or saturation adjustment. White-balance was adjusted to a neutral white and I did some exposure compensation to make the brightness of all crops match. So you will not see light fall-off in the corners.

The following 100% crops show the Sigma 20mm f1.4 DG DN Art from f1.4 down to f11 compared to the Sigma 24mm f1.4 DG DN Art at f1.4 and Sony FE 20mm f1.8 G at f1.8.

Sigma 20mm f1.4 DG DN Art compared; 100% crop from center, APS-C-corner, FF-corner

Above: Sigma 20mm f1.4 DG DN Art at f1.4

Above: Sigma 24mm f1.4 DG DN Art at f1.4

Above: Sigma 20mm f1.4 DG DN Art at f1.7

Above: Sony FE 20mm f1.8 G at f1.8

Above: Sigma 20mm f1.4 DG DN Art at f2.0

Above: Sigma 20mm f1.4 DG DN Art at f2.8

Above: Sigma 20mm f1.4 DG DN Art at f4.0

Above: Sigma 20mm f1.4 DG DN Art at f5.6; also available at f8.0, f11

The Sigma 20mm f1.4 DG DN Art looks quite sharp even at f1.4 in the APS-C image-circle having the advantage in the APS-C-corner over the 24mm f1.4 DG DN Art. But the Sony FE 20mm f1.8 G even looks a bit sharper – especially in the FF-corner. Stopping down to f2.8 reveals a slight focus shift away from the camera as the center gets a bit of red coloration and the FF-corner becomes less sharp than at f2.0. So for ultimate sharpness you might want to re-focus after changing the aperture – especially when the focus-point is off-center. Stopping the lens further down finally extends dof more towards the camera which reduces the effect of focus shift. But it takes until f8.0 to get good sharp FF-corners. The 20mm f1.4 DG DN Art has some field-curvature but it is not too strong wide open at the testing distance.

Performance at long distances

The Siemens-star test-targets are shot at a distance of 45x focal length (i.e. at around 0.9m for 20mm focal length). But performance of lenses also depends on the shooting distance. Therefore I did another series of test-shots of a city around 1 km away. Processing was done in Lightroom 11.4.1/CRAW 14.4.1 from RAW to Adobe Color profile with the Sigma supplied lens-profile reducing distortions and light fall-off which results in a slight overcompensation of vignetting. CA-removal is On, Noise-reduction set to 0, sharpening to 50/0.5/36/10, with no extra tone, color, or saturation adjustment. I used manual focus at the largest aperture and did not change focus for other apertures. All shots were made at base ISO and image stabilization switched off.

The following image shows the complete scene wide open to give you an impression of the angle of view. Following the main image are 100% crops from the center, APS-C-corner, and FF-corner from the Sigma 20mm f1.4 DG DN Art compared to the Sigma 24mm f1.4 DG DN Art shot only minutes apart and the Sony FE 20mm f1.8 G shot at a different day – but with roughly comparably atmospheric conditions. As usual I chose the diagonal with the better FF-corner as almost any lens is a bit decentered.

You can access the large originals but please respect our copyright and only use those images for personal use.

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 24mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 20mm f1.4 DG DN Art at f2.0

Above: Sony FE 20mm f1.8 G at f2.0; click image for 4k version, large original available at f1.8, f2.0, f2.8, f4.0, f5.6, f8.0, f11

Above: Sigma 20mm f1.4 DG DN Art at f2.8

Above: Sigma 20mm f1.4 DG DN Art at f4.0

Above: Sigma 20mm f1.4 DG DN Art at f5.6; also available at f8.0

Wide open the Sigma 20mm f1.4 DG DN Art shows very good sharpness in the center but gets a bit softer towards the corners – although it is still sharper than the 24mm f1.4 DG DN Art. Stopping down to f2.0 lifts acuity visibly in the APS-C image-circle and also reduces vignetting in the FF-corner strongly. The Sony FE 20mm f1.8 G in comparison is still sharper across the full-frame sensor. Stopping the Sigma 20mm f1.4 DG DN Art down to f2.8 further improves acuity in the APS-C image-circle which indicates that focus shift is not an issue at longer distances. But the FF-corner stays soft until f5.6 or f8.0.

Vignetting and distortions

To make it easier to see light fall-off in the corners of a full-frame sensor I’ve arranged a series of shots with the Sigma 20mm f1.4 DG DN Art at different apertures. All images were shot as JPGs and developed to the same brightness in the center.

Above: Sigma 20mm f1.4 DG DN Art with shading compensation Off (top) or Auto (bottom) at f1.4, f2.0, f2.8

The sample images above show that with shading compensation on Auto the extreme corners are lifted about 1 EV at f1.4 which makes vignetting of the Sigma pretty inconspicuous. Adobe’s RAW converter automatically applies shading compensation as it was set in camera – but you cannot alter the setting in postprocessing. Btw: If you have shading compensation on Auto and apply the lens profile supplied by Adobe you might be overcompensating. In this case reduce the amount of vignetting correction from Adobe’s lens profile.

Regarding distortions: Adobe’s RAW converter ignores what was set in camera and always corrects them. Looking at JPGs with distortion compensation Off the lens has visible barrel-distortions with a slight mustachio effect at a shooting distance of around 1m

Distortions of Sigma 20mm f1.4 DG DN Art: JPG with distortion compensation Off (top), RAW with lens-profile from CRAW (bottom)

The lens profile corrects distortions pretty well – at this distance. But the closer you focus the more extreme the barrel distortions become. Even pushing compensation to 200% does not fully get rid of them at minimum object distance. The Sony FE 20mm f1.8 G is much better corrected in close-up shooting.

Rendering of point-light sources at night-shots

Night-shots pose a different challenge for lenses as the contrast is even higher than under bright sun and point-light sources can reveal some weaknesses such as coma, haloing and colour-aberrations that do not show up as prominently in other test-shots. The 100% crops below the main image show the effect of coma in the FF-corner of the Sigma at various apertures:

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 20mm f1.4 DG DN Art; 100% crops from the FF-corner at f1.4 (left), f2.0 (middle), f2.8 (right)

The Sigma 20mm f1.4 DG DN Art shows very good performance here with only little coma at f1.4 and f2.0. Looking at the large original there’s a bit of haloing and purple fringing around street lights at f1.4. But the effect is minimal and completely gone at f2.0. And if you want to know how this translates into shooting the starry sky have a look at the last sample image of Ursa Major on the next page.

Bokeh quality

This test is for the rendering of point-light sources in an out-of-focus background. The circle of confusion that is produced by this test is pretty indicative of Bokeh performance (in the background) and light fall-off. Ideally the out-of-focus image of the point-light is evenly lit and perfectly circular, with no “onion-rings”, and without coloration. Large aperture lenses normally produce an effect known as “cat’s eye” the further away from the optical axis the point-light is projected. This is due to optical vignetting in the lens barrel when light enters the lens from an angle.

The crops below the main image are from the center, APS-C-corner, and FF-corner of the 4k version.

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 100% crops

Above: Sigma 20mm f1.4 DG DN Art at f2.0; click image for 100% crops

Above: Sigma 20mm f1.4 DG DN Art at f2.8; click image for 100% crops

Above: Sigma 20mm f1.4 DG DN Art at f4.0; click image for 100% crops

The diameter of the Bokeh balls in the center is determined by the entrance pupil of the lens which is only 14mm on any 20mm f1.4 lens, 11mm on the Sony FE 20mm f1.8 G and 17mm on the Sigma 24mm f1.4 DG DN Art wide open. So Bokeh balls are always disappointingly small on (ultra-)wide angle lenses – unless you focus very close. But the “beauty” of the balls is still important. The Sigma 20mm f1.4 DG DN Art shows some outlining with a slight green tint. There’s also one inner ring which disappears once the lens is stopped down to f2.0. Compression of the circle towards the corners is not that pronounced although the effect already starts in the APS-C-corner at f1.4. But at f2.0 or f2.8 compression becomes minimal. The aperture blades of the Sigma are well-rounded up until f5.6.

Now let’s see how this analysis of out-of-focus point-light sources translates into Bokeh-performance shooting a book-shelf. The crops of the foreground, the middle-ground, and the background below the main image are resized to make them comparable across all my reviews.

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 24mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sony FE 20mm f1.8 G at f1.8; click image for 4k version, here for large original

The Sony clearly loses in this comparison to both Sigmas – although to be fair it has the slower f1.8 focal ratio and the scene was shot at a slightly different angle. Comparing both Sigmas the 24mm has the softer transition zone albeit with a slightly stronger green cast from loCA. But the foreground and background of both Sigmas look very similar.

Below is another crop (now at 100%) from the same images showing the ruler. Both Sigma lenses again look pretty similar in the transition zone near the plane of focus with little hinting at double contours. Looking at the large originals there’s probably a higher risk of artifacts in the foreground.

Above: Sigma 20mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sigma 24mm f1.4 DG DN Art at f1.4; click image for 4k version, here for large original

Above: Sony FE 20mm f1.8 G at f1.8; click image for 4k version, here for large original

Close-up performance

The following shots were taken with the Sigma 20mm f1.4 DG DN Art close to its maximum magnification of 1:5.0. The crops shown below are from 0mm, 11mm, and 18mm off the center of the sensor respectively, the area of sharp focus is 139 x 208mm. As field curvature was quite strong I focused separately for the center crop and the crop of the eye.

Sigma 20mm f1.4 DG DN Art, 1:5.8 magnification; 100% crops

Above: Sigma 20mm f1.4 DG DN Art at f1.4

Above: Sigma 20mm f1.4 DG DN Art at f4.0

Above: Sigma 20mm f1.4 DG DN Art at f8.0

While the center is already very sharp at f1.4 the Sigma 20mm f1.4 DG DN Art needs stopping down to f8.0 to produce usable results across the full frame.

Flare, glare, ghosting, and sunstars

Catching a strong light-source shining directly into the lens is always a risky business: it could produce strange colorful ghost-images or reduce contrast considerably through flare and glare. The appearance of flare and ghosting depends on factors like the aperture and the angle of the light hitting the lens. So to judge the proclivity of the Sigma 20mm f1.4 DG DN Art for these artifacts I went through a series of well calculated shots against a strong light source to provoke glare and ghosting. The lens hood was mounted in all shots. Following are two of the more extreme example results. The little bright square inset in the upper left shows the respective area with an exposure compensation of +3 EV to make it easier to see which levels of black the lens renders at that point:

Above: Sigma 20mm f1.4 DG DN Art at f11; click image for 4k version or here for +3 EV exposure compensation

Above: Sigma 20mm f1.4 DG DN Art at f1.4 click image for 4k version or here for +3 EV exposure compensation

The Sigma is very clear of flare and ghosting artefacts. And outside these artefacts the lens renders a very deep black, so there’s little veiling glare. The lens also does not produce the typical flare which other lenses show when the light-source is just outside the corner. All-in-all the Sigma 20mm f1.4 DG DN Art let’s you shoot confidently under adverse contra-light situations.

Sunstars of the Sigma 20mm f1.4 DG DN Art are well defined and regular – once you stop down to f11 or beyond.

Next check out my sample images!