AF Nikkor 20 mm f/2.8 test

This page discusses a test of the Nikon AF Nikkor 20 mm f/2.8. I discussed the lens also on an earlier page. I decided to further test this lens in order to more easily compare it with other legacy Nikon F wideangles, like the AF 14 mm f/2.8.

Test

I tested this lens at f/2.8 (fully open), f/4 to ascertain whether stopping down by one stop makes a significant improvement, and f/11, which is the maximum aperture not visibly affected by diffraction on the test camera (Nikon Z8, 45.7 Mpixel). In most lenses that I tested, f/11 produces an optimal image quality. At f/16, diffraction begins to appear on images shot with this camera. On a 24 Mpixel camera, picture quality at f/16 would probably still be fine.

AF 14
f/2.8
 
AF 20 test
f/11
Figure 1. Test subject shot with 20 mm, reduced.
 

Figure 1 shows the test subject at f/2.8 and f/11. Sunlight intensity changed somewhat during the test, because of variable weather and cloud cover. The camera automatically applied geometry corrections and compensated for vignetting. However, at f/2.8 there remains a visible darkening of broad corner regions, and the picture as a whole is a little underexposed. The dark corners are much narrower and the picture as a whole a little brighter at f/4 (not shown), and at f/5.6 and higher there is virtually no visible vignetting.

AF 14
f/2.8
 
AF 14
f/4
 
AF 14
f/11
Figure 2. 20 mm, 1:1 pixel crops at center of frame.
 

In the center of the frame, the 20 mm performs acceptably at f/2.8. There is virtually no improvement at f/4, but f/5.6 and f/8 produce better results. As expected, the best results are obtained at f/11.

AF 14
f/2.8
 
AF 14
f/4
 
AF 14
f/11
Figure 3. 20 mm, 1:1 pixel crops at left edge.
 

At the left edge of the frame, f/2.8 is moderately worse than at the center, but still usable. There is a small improvement at f/4. At f/11, image quality is essentially the same as in the center.

Conclusions

The AF Nikkor 20 mm f/2.8 is a good extreme wideangle on the Nikon Z8.

Image quality is good (albeit not excellent) in a broad center region already at f/2.8 and f/4. Quality improves already at f/5.6, and f/11 gives excellent results across the whole frame.

Image quality near the edges is acceptable, but not quite good, at f/2.8-f/4. It improves at f/5.6, and at f/11 becomes virtually the same as in the center.

Even after in-camera correction of vignetting, there is a detectable and broad darker area around the frame at f/2.8, which almost disappears at f/4 and is invisible at f/5.6 and higher.