Testing: 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 new Nikon is no exception: it shows quite some loCA at f1.8 and f2.8.
The test also revealed that there is no focus shift to speak of at distances of 1m and farther away. For comparison have a look at the Nikon 20mm f1.8G which shows less loCA, Nikon Z 24mm f1.8S, and Sigma 20mm f1.4 Art (shown at f1.4/f2.0/f2.8).
In the test above the coloration from loCA does not seem much stronger than from the Nikon Z 24mm f1.8 S. But in real life shots I found the effect more annoying on the 20mm prime lens than on its 24mm sibling. See the following shots where the Nikon Z 20mm f1.8 S produces some purple fringing around high-contrast edges in the focal plane and green outlining around background subjects:
Sharpness and contrast
Let’s have a look at the theoretical performance of the new Nikon Z 20mm f1.8 S first and compare it to the performance of the Nikon AF-S 20mm f1.8G and two other alternatives from Sigma and Nikon:
These MTF charts show the computed lens-performance wide open without influence of diffraction. Higher values are better (more contrast) and the closer the solid and dotted lines are together the less astigmatism (= resolution depends on the orientation of the test-pattern) the lens has. 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 (DX-corner), and 20 mm (FX-corner) on a 45MP Nikon Z7 body.
From the charts the new Nikon Z 20mm f1.8 S should have a clear advantage over the Nikon AF-S 20mm f1.8G and the Sigma 20mm f1.4 Art. Both older lenses show a distinct drop in resolution of fine details at 10mm image height and then again towards the extreme corner of a full-frame sensor. Plus overall contrast outside the DX image-circle is greatly reduced.
Compared to the Nikon Z 24mm f1.8 S the Z 20mm f1.8 S should have a slightly better resolution at the center.
Let’s see how this theoretical performance of Nikon’s new Z Nikkor translates into real life results in the sharpness test based on Siemens-stars. Processing was done in Lightroom 9.2/CRAW 12.2 from RAW to Adobe Color profile with the built-in lens profile 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 Nikon Z 20mm f1.8 S from f1.8 down to f11 compared to the Nikon AF-S 20mm f1.8G and Sigma 20mm f1.4 Art at f1.8 plus the Nikon Z 14-30mm f4.0 S at f4.0. The Sigma Art was shot on a 36MP Nikon D810. Processing is comparable and the 11% lower linear resolution of the 36MP sensor is just a little less challenging for the Sigma. Still, you can get a good impression on how it performs.
The Nikon Z 20mm f1.8 S is the clear winner here. And although its FX-corner is a bit soft it still has the higher contrast and better resolution than the other two lenses. But it shows a little magenta coloration in the center crop from loCA.
Let’s see how performance develops when stopped down and compare the new Z Nikkor to the Nikon Z 14-30mm f4.0 S at 20mm f4.0.
The FX-corner of the Z 20mm f1.8 S gains some acuity when stopped down to f2.8 or f4.0. That the Nikon Z 14-30mm f4.0 S is only a little softer outside the center than the new prime lens is testament to the very good performance of Nikon’s ultra-wide zoom lens.
Here is how the new Z Nikkor develops when stopped further down:
Not much to be seen here except for diffraction clearly softening performance at f11. Btw.: The Nikon Z 20mm f1.8 S shows very little field curvature.
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 present another series of test-shots of a city around 1 km away. The following images were taken with the 45MP Nikon Z7 camera with lens profile in camera set to normal/on. The images are RAW files developed in Lightroom 9.2/CRAW 12.2 to Adobe Color profile, Noise Reduction=OFF, sharpening=50/0.5/36/10 , with no extra tone, color, or saturation adjustment. I used autofocus at the largest aperture and did not change focus for other apertures. All shots were made at ISO 64 and image stabilization switched off. As usual I have selected the diagonal that provided the better corner results as the Nikon Z 20mm f1.8 S was a bit decentered.
The main image shows the complete scene wide open to give you an impression of the angle of view and to judge vignetting of the Nikon Z 20mm f1.8 S. You can access the respective shots up to f11 via the links beneath the main image. Following the main image are 100% crops from the center, DX-corner, and FX-corner down to f11 compared to the Nikon AF-S 20mm f1.8G and Sigma 20mm f1.4 Art (shot on a 36MP Nikon D810) at f1.8 and the Nikon Z 14-30mm f4.0 S at 20mm f4.0. The zoom lens and the Sigma were shot at other days but under similar atmospheric conditions.
Please respect our copyright and only use those images for personal use.
Again the new Nikon Z 20mm f1.8 S is the best of the three lenses although its lead in the FX-corner over the Nikon AF-S 20mm f1.8G is minimal.
Let’s see how the new Z Nikkor develops when stopped further down and compare it against the Nikon Z 14-30mm f4.0 S at 20mm f4.0:
Stopping the new Z-Nikkor down to f2.8 reduces vignetting considerably. But except for a slightly stronger vignetting of the Nikon Z 14-30mm f4.0 S at f4.0 there is no discernible difference in performance between both lenses under these test conditions.
Following is the new Z Nikkor stopped further down:
The Nikon Z 20mm f1.8 S renders the DX image-circle very sharp already at f1.8 with some softening toward the FX-corner.
Vignetting and distortions
With Nikon’s Z cameras you can select the degree of Vignette Control in camera between High, Normal, Low, and Off and Distortion Control On or Off. Adobe’s CRAW converter as of version 12.2 applies the Vignette Control as set in camera but treats Distortion Control always as On (regardless of setting). Plus you cannot change these settings in CRAW while Nikon’s Capture NX-D software allows to change the settings when developing RAWs.
To make it easier to see light fall-off in the corners of a full-frame sensor I’ve arranged a series of three shots with the Nikon Z 20mm f1.8 S at different apertures. All images were developed to the same brightness in the center and the lens profile applied, Vignette Control in camera was set to Normal (the default). This lifts the extreme corners by about 3/4 to 1 EV at f1.8:
The sample images above show that even with the lens profile applied vignetting remains very visible at f1.8. It’s slightly stronger at f2.8 and f4.0 than from the F Nikkor:
Regarding distortions: The new Z 20mm f1.8 S shows some barrel distortions with a slight mustachio effect but this is well corrected through Nikon’s lens profile:
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 FX-corner at various apertures:
The Nikon Z 20mm f1.8 S has very little coma wide open, much better than the F Nikkor. But there is some magenta haloing around the street lights when shot wide open:
The effect is mostly gone at f2.8.
Rendering of out-of-focus point-light sources
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 44% crops below the main image show the Bokeh balls at the center, DX-corner, and FX-corner at the largest aperture:
When shot wide open the Z 20mm f1.8 S shows only a very moderate cat’s eye effect toward the FX-corner. There are some mild onion rings and also a little green outlining due to longitudinal CAs.
Let’s see how this analysis of out-of-focus point-light sources translates into Bokeh-performance shooting a book-shelf:
Despite the Sigma Art having an f1.4 focal ratio it cannot pull ahead of the Nikon Z 20mm f1.8 S as it is too nervous in the background. Nikon’s new lens manages to balance foreground and background Bokeh nicely with only a bit of nervousness and a smooth transition in the middle-ground.
The Nikon Z 20mm f1.8 S goes down to 1:4.6 magnification with manual focusing. Its performance is pretty good once you stop down to f2.8. The following image was shot at 1:4.8 magnification where the area of sharp focus is just 115 x 173mm. The crops shown below are from 0mm, 7mm, and 17mm off the center of the sensor respectively:
This is clearly better than the Nikon 20mm f1.8G or the Nikon Z 24mm f1.8 S can do.
Flare, ghosting, and sun-stars
Catching a strong light-source shining directly into the lens is always a risky business: it could produce strange colourful 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 new Nikon Z 20mm f1.8 S for these artefacts 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. See an examples at f11 where these effects normally show up most prominently. 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:
The Nikon Z 20mm f1.8 S is very clear of flare and ghosting artefacts. And outside these artefacts the new lens renders a very deep black, so there’s little veiling glare. The lens also produces only a mild flare when the light-source is just outside the corner.
All-in-all the Nikon Z 20mm f1.8 S let’s you shoot confidently under adverse contra-light situations.
Sunstars are pretty weak from the Nikon Z 20mm f1.8 S. See the examples below:
For comparison have a look at the Nikon Z 24mm f1.8 S.
Next check out my sample images!Check prices on the Nikon Z 20mm f1.8 S at B&H, Adorama or WEX. Alternatively get yourself a copy of my In Camera book or treat me to a coffee! Thanks!