4) Lens Sharpness, Contrast and Color Rendition
Being a modern Nikon prime, the 28mm f/1.8G performs reasonably well on high-resolution cameras. Below is how the lens tested out on a Nikon D810, using Imatest software:
As you can see, the Nikon 28mm f/1.8G starts out pretty sharp wide open, even in the corners. The corners improve significantly stopped down to f/4 and get very close to the center sharpness, although the mid-frame does look worse in comparison. This is due to field curvature issue the lens exhibits – sharpness starts out good in the center, gets a little worse in the mid-frame and gets good again in the corners.
5) Focus Shift and Field Curvature
The Nikon 28mm f/1.8G lens was somewhat of a pain to test, because it has several optical problems. First, it has the previously mentioned focus shift issue. While focus shift is normal in fast aperture prime lenses, the bad news is that this lens has a combination of focus shift and field curvature. The donut-shaped field curvature is fairly evident at all wide apertures, all the way to f/8 (as evidenced by the above chart, which shows weaker mid-frame performance).
Note: to make sure that I was not dealing with sample variation, I tested a total of three samples of the Nikon 28mm f/1.8G – all three showed the above optical issues, with slightly varying results.
What does this mean for field use? Since cameras acquire focus with lenses at their maximum aperture (in this case f/1.8), stopping down the lens to smaller apertures like f/4 might put your focused subject slightly out of focus, moving the focus plane back – result of focus shift. In addition, the area that appears sharp in the center will not distribute evenly across the frame – result of field curvature. This is generally not a huge problem for landscape photography (most of these issues are gone by f/8), but could definitely be problematic for other uses.
Take a look at this sample image that shows focus shifting from the center to back when going from f/1.8 to f/2.8:
Keep in mind that focus shift actually gets worse at f/4 and f/5.6 when compared to f/1.8.
Despite the fact that the Nikon 28mm f/1.8G is a wide-angle lens, the shallow depth of field at f/1.8 yields relatively pleasant out of focus areas. Although the quality of bokeh is not as great as on some Nikon portrait lenses, it is still not bad when compared to the Nikon 24mm f/1.4G:
As you can see, wide open, the bokeh on the Nikon 28mm f/1.8G looks less busy than on the 24mm f/1.4G. The outer edges of background highlights seem to be smoother and less defined. Stopped down to f/2.8, both lenses look about the same. However, if you take pictures of bright highlights at night, you will notice onion-shaped bokeh – result of the use of two aspherical lens elements. Personally, I like the rounded 7-blade diaphragm, which yields more pleasant out of focus areas than the straight 9-bladed diaphragm used on older Nikkor lenses.
Here are a couple of image samples that demonstrate the bokeh qualities of the lens:
Most prime lenses vignette heavily when shot wide open and the same is true for the Nikon 28mm f/1.8G, so no surprises here. The good news is that as you stop down to f/2.8, vignetting decreases significantly. At f/4.0 vignetting is decreased to about half a stop in the extreme corners. The bad news is that from there on it stays about the same, at about half a stop, no matter how much you stop down. Take a look at lens vignetting at different apertures (the below numbers are computed as a mean value from all four corners of the frame):
And here is a graph that shows the spread of light falloff across the image frame, when shot wide open at f/1.8:
Fortunately, these vignetting issues can be quickly corrected in Adobe Photoshop or Lightroom, so it is that big of an issue.
8) Ghosting and Flare
Thanks to Nikon’s proprietary Nano crystal coat and a clever optical design, the Nikon 28mm f/1.8G handles ghosting and flare as good as other modern Nikon primes. Take a look at the below comparison samples:
One important thing to note here, is the sun star characteristic of both lenses here. The Nikon 24mm f/1.4G is known to produce busy-looking sun stars (thanks to the 9-blade diaphragm), while the Nikon 28mm f/1.8G has less points in comparison, which in my opinion, typically looks better in images.
As expected from a wide-angle lens, there is some visible barrel distortion on the Nikon 28mm f/1.8G. Imatest measured 1.25% barrel distortion, which is a little worse than the 0.9% barrel distortion exhibited by the bigger Nikon 28mm f/1.4E. It is really nothing to worry about and the problem can be easily fixed in Lightroom using the Lens Correction sub-module (the Nikon 28mm f/1.8G is supported by the latest version of Lightroom).
10) Chromatic Aberration
The Nikon 28mm f/1.8G exhibits lateral chromatic aberration in the range of 1.75px wide open to 2.07px at f/8 in the extreme corners. Here are the results from Imatest:
I would not worry much about lateral chromatic aberration, as it can be easily fixed with post-processing software.
When it comes to longitudinal chromatic aberration (LoCA), the Nikon 28mm f/1.8G handles it fairly well. Take a look at the below image that shows a small amount of LoCA:
Stopping down the lens to f/2.8 and beyond completely gets rid of LoCA.
Let’s now move on to lens comparisons.