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.
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|
|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|
|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)|
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:
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.
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.
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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.