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  • Ed Dozier

Nikkor 20mm f/4.0 AI Review

This article is an evaluation of the old manual focus 20mm f/4.0 lens that has been AI-converted. The conversion makes it possible to get automatic exposure on the better Nikon digital cameras, including the D610 with an FX sensor.

The 20mm f/4 was manufactured from 1974 through 1978, back when Nikon was the big man on campus in photography. Mine was purchased in 1975, and virtually never came off of my Nikon F2 when I was back-packing (unless I did some macro shots). Small, light, sharp, tough, elegant, and wide; almost exactly like the ideal woman, except perhaps for the ‘wide’ part.

This 20mm lens is one of the smallest and lightest FX lenses Nikon ever made. At 7.4 ounces, you hardly notice it’s there. It's about 1.4 inches long, like a thick body cap. Cameras like the D7000 series and D610 allow aperture-priority auto-exposure after defining the “non-CPU lens data” for this lens. I absolutely love its field of view (94 degrees) on the D610. The lack of auto-focus using a lens like this isn’t a hardship, since everything is typically in focus all the time. You still get the 3-stage focus indicator inside the viewfinder while manually focusing with the better Nikons.

The 20mm, of course, has the little red dot on the focus scale for infrared focus compensation. Back in the day, Nikon really paid attention to stuff like that. One of my main uses for this lens is infrared, but unfortunately cameras like the D7100 and D610 are essentially useless for infrared (see this article: ). My D7000, however, works perfectly for infrared with this lens (using the Hoya R72 52mm filter).

Manual-focus lenses are actually superior to auto-focus lenses for shooting infrared with a filter like the Hoya R72, since you can’t see through the viewfinder. You frame and focus (and use the lens focus scale red-dot IR shift) before attaching the filter. Those of you who have gone through the pain of framing/pre-focusing a ‘G’ auto-focus lens and then mounting an IR filter know what I’m talking about.

Focusing is still silky-smooth, unchanged since the day it was manufactured. The focus scale is a thing of beauty. I have every reason to believe that this lens will last not just a lifetime, but multiple lifetimes.

Although it works without vignetting, be careful using a polarizer on this lens; the sky will look too un-even because of the wide field of view.

20mm f/4 Nikkor AI-converted on the D610. Sweet.

Resolution Tests

I test lenses using the MTF Mapper software and the recommended resolution charts (printed to A0 size and dry-mounted). The article here explains the software and its use.

As you’ll see, you are going to want to stop down to f/8 or more to get the corners you want. The center is already very good at f/5.6, and only gets better as you stop down. Avoid going beyond f/16, because of diffraction.

All tests were done using the D610, with 24 MP (5.95 micron pixels). I only shoot un-sharpened RAW for the resolution tests.

To convert the MTF50 lp/mm measurements into LP/PH, simply multiply readings by 24.0. To convert into LW/PH, take the LP/PH values and multiply by 2.

Sample Photos

Sun just out of frame. Palm fronds near frame edge are sharp, D610.

The “wavy” distortion is quite minimal, D610.

Infrared, Hoya R72, D7000


If you're willing to stop this lens down to f/8 or f/11, the results are about as good as any ultra-wide lens made today. You won't find a more compact lens anywhere. I love that it uses 52mm filters, too.

This has become my go-to lens for infrared photography. My D610 and D7100, by the way, are basically useless for IR unless I put the DK-5 eyepiece cap over the viewfinder. My other cameras are fine without the cap, unless I switch to my 850nm IR filter, which requires exposures of 2 or 3 minutes; all cameras need an eyepiece cap when exposures get that long.

I understand that this lens is a bit rare these days; I have no intentions of ever selling mine.

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