A Quick Comparison of the Nikon 20mm f/1.8Z and the Nikon 14-24mm f/2.8Z
In this post of my series on Milky Way Photography I compare two of Nikon’s best astrophotography lenses. The first lens, the Nikon 14-24mm f/2.8 Z, is an upgrade in almost every way from the previous F-mount version. It is more compact, lighter, and better optically than the previous version. The second lens, the Nikon 20mm f/1.8Z, is the mirrorless version of the previous Nikon 20mm f/1.8G F-mount lens. The new mirrorless version is, however, noticeably larger, being a little over an inch longer and 0.2″ larger in diameter. The new mirrorless lens is also heavier coming in at 17.8 oz vs. 12.6 oz for the F-mount lens. Optically the new Z-mount lens is, however, an improvement over its predecessor.
Both the 14-24mm f/2.8Z and the the 20mm f/1.8Z perform quite well for Milky Way photography. The 14-24mm, being a zoom, has the flexibility of allowing for a wider range of compositions. The 20mm, on the other hand, is simply a great focal length for Milky Way photography and usually all I ever need. I have found that ultra-wide focal lengths like 14mm are a little harder to use as they introduce a lot of perspective distortion. Anything narrower than 20mm tends to be a bit limiting in field and also requires shorter exposure times or tracking to avoid noticeable star trailing. The biggest advantage, however, of the 20mm over the 14-24mm zoom in this case is that it is 1-1/3 stops faster.
Having the extra light gathering ability of the 20mm permits shooting at lower ISOs or shorter exposure times. Below in figures 1 and 2 are two images shot using a Nikon Z6II with each lens. The first, in figure 1, is the Nikon 14-24mm f/2.8Z set at 20mm f/2.8 at ISO6400 for 20s. The second, in figure 2, is the 20mm f/2.8 set at f/2.0 at ISO6400 for 10s. I chose f/2.0 on the 20mm rather than wide-open at f/1.8 just to make the difference between the two lenses an even stop. Exposures should thus be identical within the limits of their respective light transmission characteristics.
Both images were untracked and taken from a single fixed position on a tripod with a 5s delay to minimize potential of camera shake. The white balance (WB) for each camera was set to “Daylight” making it the default as-shot setting when the raw NEF files were imported into Capture One. All images were exported to jpg files from Nikon raw NEF files using Capture One. The only adjustments made were +1 EV exposure for both and some slight WB adjustment on each to match. Each image can be enlarged by left-clicking to get a better view. Once enlarged, double clicking zooms in even closer.
At this level the images look pretty much identical and are for the most part indistinguishable. A closer look into the centers is shown in Figures 3 and 4. Each is an approximate 6:1 crop, which is about 3.2″ enlarged from the total 20.2″ image.
The image on the right taken with the 20mm f/1.8 is noticeably better. This is mainly due to the reduced 10s exposure time of the faster 20mm lens resulting in less star trailing. The reduced exposure time also makes the overall image look clearer which I attribute to less subject movement (in this case the relative movement of the sky!). Optically, however, I feel both lenses are quite comparable. Clicking in on each helps see the difference more clearly.
Below in figures 5 and 6 are crops from the upper right corners to specifically show star clarity and lens aberrations.
Both lenses clearly show some sagittal astigmatism. The amount, though, is relatively small and reasonably close between the lenses. The nice thing is that the 20mm f/1.8 shows such good performance at such a wide aperture of f/2.0.
The bottom line is that both lenses perform extremely well for Milky Way astrophotography. The 14-24mm f/2.8 Z zoom is more flexible, like having multiple reasonably fast primes in one lens. The 20mm f/1.8 Z, on the other hand, is faster making it possible to use shorter exposure times for less star trailing and thus improved image quality. Optically, both are excellent and essentially comparable. If I were using a tracker or felt that I might want wider focal lengths, the zoom would easily be my first choice. Otherwise for fixed positions where the 20mm focal length is suitable, I feel the 20mm is a better choice.