Nikon Z 20mm f/1.8 S

Size comparisons

Lens photo 1 Lens photo 2

Top left: Nikon Z 20mm f/1.8 S and Nikon AF-S 20mm f/1.8G.
Top right: Nikon Z 20mm f/1.8 S and Nikon AF-S 20mm f/1.8G with the FTZ adapter mounted.

Lens photo 3 Lens photo 4
Top left: Nikon Z 20mm f/1.8 S with the fully retracted Nikon Z 24-70 mm f/4 S.
Top right: Nikon Z 20mm f/1.8 S mounted on the Nikon Z6.

Introduction

In 2020, Nikon announced the Nikon Z 20mm f/1.8 S lens, calling it a fast, ultra-wide prime for landscapes, environmental portraits and interior shooting. The S-line lens is produced for Nikon's mirrorless cameras with their short flange focal distance of 16 mm (Nikon's F-mount flange focal distance is 46.50 mm) and is said to offer "superior performance and resolution". I purchased this lens mainly for astrophotography work in combination with the Nikon Z6 camera.

The Nikon Z 20mm f/1.8 S is the sibling of the often praised Nikon AF-S 20mm f/1.8G ED so lets see how they compare in specifications.

Specifications: Nikon Z 20mm f/1.8 S: Nikon AF-S 20mm f/1.8G ED:
Lens construction: 14 elements in 11 groups 13 elements in 11 groups
ED glass elements: 3 2
Aspherical elements: 3 2
Diaphragm blades: 9 7
Nano crystal coat: Yes Yes
Closest focusing: 0.2 m / 0.66 ft 0.2 m / 0.66 ft
Maximum Reproduction Ratio: 0.19x 0.23x
Angle of View (FX): 94° 94°
Filter: 77 mm 77 mm
Hood: HB-95 HB-72
Dimensions: 3.4 x 4.3 in (84.5 mm x 108.5 mm) 3.2 x 3.1 in (82.5 mm x 80.5 mm)
Weight: 17.9 oz. (505 g) 12.6 oz. (355 g)
Appearance: 2020 2014

The new Nikon Z 20mm f/1.8 S is about one inch longer than the Nikon AF-S 20mm f/1.8G. The Z lens might not be stowed away in that side pocket in which the G lens just fits nicely. With the sun hood mounted, the Z lens puts 533 g on the scale, while the older G lens weighs 378 g - a difference which can be felt. However, if one wishes to mount the older G lens on a mirrorless camera, an FTZ adapter is needed and with the FTZ mounted, the G lens is actually heavier (the combination weighs 536 g with the sun hood attached) and taller than the Z lens.

The Nikon Z 20mm f/1.8 S balances well with the Nikon Z6 and the combination is still light enough to be carried around all day (comparable to the Z6 / Nikon Z 24-70mm f/4 S combination). On a side note, the portable iOptron Skyguider Pro star tracker mount still easily handles the combination, tracking the stars accurately for several minutes.

The Modulation Transfer Functions (MTF) and the optical construction details for both lenses can be seen below:

MTF Lens construction
Top left: MTF chart for the Nikon Z 20mm f/1.8 S lens.
Top right: Optical construction for the Nikon Z 20mm f/1.8 S lens (yellow: ED glass, blue: Aspherical lens elements).
Image © 2020 Nikon Corporation (source)

MTF Lens construction
Top left: MTF chart for the Nikon AF-S 20mm f/1.8G lens.
Top right: Optical construction for the Nikon AF-S 20mm f/1.8G lens (yellow: ED glass, blue: Aspherical lens elements).
Image © 2020 Nikon Corporation (source)

The horizontal axis of the MTF diagrams shows the distance from image center to the image corner measured in millimeters while the vertical axis displays contrast values for the lens when the aperture is wide open (f/1.8). The red 10 lines/mm curves S10 and M10 are a measure for contrast reproduction, while the blue 30 lines/mm curves S30 and M30 are a measure for the resolution of the lens. The higher and straighter the lines, the better the lens. There are two red curves for the 10 lines/mm measurement (S10, M10) and two blue curves for the 30 lines/mm measurement (S30, M30). They are called sagittal and meridonial curves. Sagittal curves are obtained by using a test chart with line pairs which are parallel to the sensor diagonal. Meridonial curves are obtained by using a test chart with line pairs which are perpendicular to the sensor diagonal.

Both lenses display an excellent contrast reproduction in image center (values above 0.9 or 90 percent are considered to be very good). While the Z lens contrast reproduction stays excellent across the whole image field, the contrast reproduction of the G lens shows some weaknesses in the image corners. Sharpness values for the Z lens are considerably better than for the G lens (in image center but especially in the corners), indicating less field curvature. In addition, the meridonial and sagittal curves are impressively close together for the Z lens, indicating very little astigmatism (especially important in astrophotography). However, MTF diagrams only tell us part of the story since all kinds of aberrations are at play and Nikon's graphs are only obtained at the widest aperture of the lens.

Optical performance

Astrophotography deals with pinpoint light sources and thus puts any lens that is developed for daytime photography to the acid test. In order to deliver excellent results in night sky photography, a lens not only has to be well-corrected for chromatic aberration but also for coma - and all this at the widest possible aperture (to collect as much light as possible) and at the infinity setting.

Some of the most expensive Nikon ED lenses deliver mediocre results under the night sky: While they deliver sharp images across the field for a wide range of apertures and are generally well-corrected for chromatic aberrations, coma (UFO-like shape of the stars) shows its ugly head in the corners of the image at wide apertures. Ironically, some of the cheaper (mostly manual) third-party lenses available for the Nikon F mount have the least coma: Lenses from Samyang (equivalent brands: Bower, Rokinon) like the Samyang 14mm f/2.8 IF ED UMC, the Samyang 24mm f/1.4 ED AS UMC or the Samyang 14mm f/2.4 SP. There's a caveat, however: Quality-control for these lenses is rather poor. In recent years, a lot of astrophotographers have switched to or added Sigma Art lenses (e.g. the Sigma 14mm f/1.8 DG HSM), which show better quality-control and better overall sharpness but coma can still be a problem at the widest apertures.

Though there are many Nikon lenses which perform well at f/5.6 or f/4, only a few deliver acceptable star shapes in the image corner at f/2.8. And f/2.8 is the aperture where the fun begins: A lot of astrophotographers put their camera on a tripod and take short exposures - short enough so that the stars remain pinpoints and don't show star trailing (due to the Earth's rotation). With a 20mm lens and a full frame camera, exposure times longer than 10 seconds will lead to star trailing (to be more precise: if images are analyzed at 100% or printed large, stars near the celestial equator are already a tiny bit elongated with 10 seconds exposure time, while stars close to the celestial poles will be almost point-like; for small prints or for HD and 4K timelapse videos, longer exposure times can be accepted). The longer the focal length of the lens, the shorter the allowed exposure time. As a rule of thumb, if 200 is devided by the focal length of the lens, the maximum exposure time can be calculated ("200 rule"). E.g. if a 14 mm lens is used, an exposure time of 200/14=14 seconds will produce almost pinpoint stars when images are scrutinized at 100%. In order to get enough signal, a wide aperture and a high ISO value are used and images are "stacked" to minimize image noise. If the lens has to be closed to f/5.6 due to a bad coma performance, there is just not much light hitting the sensor.

Exposure time comparison Image stack of 20 images
Top left: The star Atair close to the celestial equator photographed with the Nikon Z6 and the Nikon Z 20mm f/1.8 S with different exposure times from 30 s to 10 s. The camera was mounted on a tripod with no star tracking. At 10 s, stars are almost pinpoints ("200 rule"). Longer exposure times lead to increasing star trailing.
Top right: Stack of twenty 10 second images taken with the Nikon Z6 and the Nikon Z 20mm f/1.8 S at f/1.8 and ISO 6400. The images were stacked with the Sequator software. The camera was mounted on a tripod with no star tracking. A few clouds decided to make a visit.

The astrophotography technique just described is one of two distinct methods. The other technique uses some sort of star tracking mount. Star trackers like the iOptron Skyguider Pro or Skywatcher's Star Adventurer have become very popular among astrophotographers who like to keep it simple and portable. Since the star tracker makes the camera follow the apparent motion of the stars, much longer exposure times are possible (limited mainly by the accuracy of the tracking mount). In order to collect as much light as possible, the widest useful aperture of the lens is still a deciding factor.

The Nikon AF-S 14-24mm f/2.8G is one of the best Nikon lenses for astrophotography. If set to 14 mm focal length, the lens can be used wide open at f/2.8. Coma is well-controlled though I usually close it to f/3.5 or f/4 for even better results when on a star tracking mount. At longer focal lengths, the lens doesn't perform equally well. Bright stars turn into conspicuous elongated objects in the image corners. In 2019, I purchased a Nikon AF-S 20mm f/1.8G lens just for astrophotography. This lens shows quite ugly coma in the image corners at f/1.8 and f/2, but closing it down to f/2.8 eliminates coma effectively (beating the Nikon 14-24mm f/2.8G at 20mm focal length).

In search for an even better performance I purchased the Nikon Z 20mm f/1.8 S lens and made some comparisons with the Nikon AF-S 20mm f/1.8G under the night sky.

The new Nikon Z 20 mm f/1.8 S shows a bit of chromatic aberration across the field at f/1.8 and surprisingly little coma. The quality at f/1.8 is very acceptable. In small prints, coma is hard to detect. In very large prints a bit of coma will be visible. At f/2, chromatic aberration decreases a little bit while coma stays more or less the same (at a pretty moderate level). At f/2.8 coma and chromatic aberration performance are very good if not excellent. The 20mm Z lens sets new standards for astrophotography.

Astrophotography test images with the Nikon Z 20mm f/1.8 S and the Nikon AF-S 20mm f/1.8G for coma performance analysis (full resolution JPG sample images, opening in a new window, 25 MB file size each):

Nikon Z 20 mm f/1.8 S at f/1.8 Nikon AF-S 20 mm f/1.8G at f/1.8
Nikon Z 20 mm f/1.8 S at f/2.0 Nikon AF-S 20 mm f/1.8G at f/2.0
Nikon Z 20 mm f/1.8 S at f/2.8 Nikon AF-S 20 mm f/1.8G at f/2.8

Sample image details:
Camera: Nikon Z6.
Exposure time: 3 minutes.
ISO: 1600
Guiding: iOptron Skyguider Pro

The brightness differences of the three images (due to the different aperture used) were corrected in Adobe Photoshop (-1.3 EV for the f/1.8 image, -1 EV for the f/2 image and 0 EV for the f/2.8 image).






Crop positions

Crop positions.

Nikon Z 20mm f/1.8 S external reviews

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The cache numbers in parenthesis next to the links lead to cached pdf files (just in case the original links don't work anymore). The files usually only represent parts of the original contents from August 2020.

Image taken with the Nikon Z 20mm f/1.8 S

Milky Way photo
Image above: The Milky-Way, photographed on August 9, 2020 with the Nikon Z 20mm f/1.8 S and a Nikon Z6. 24 min exposure time at f/2.8 and ISO 1600 on an iOptron Skyguider Pro tracking mount. Stack of 8 frames with 3 min exposure time each. Processing: Sequator and Adobe Photoshop CC.