Nikon have failed to produce a (effective) wide and lightweight prime for their DX systems and the 20mm f/3.5 AI can potentially fill this gap (giving a 35mm equivalent FOV of 30mm) with the added benefit that the lens would not overlap with any 24-70mm focal lengths.
The Nikkor 20mm f/3.5 appears to be reasonably popular lens with ebay prices around 180-200 GBP; the f/2.8 AI/AI-S variant commanding ~250 GBP. The equivalent modern Nikkors are significantly more expensive: 20mm 1.8G (~650 GBP) or the older 20mm 2.8D (~500 GBP). Other options would have been the (cheaper) 24mm or (more expensive) 28mm AI/AI-S but that would have pushed the FOV away from the wide end on DX.
The AI versions are reported to have a longer focus throw when compared to the AI-S version (100o vs 70o) and it appears there were two separate runs of the AI lens - on mine, the rear mount has 3 screws for the mount and what confirms this as a AI lens is the lack of the notch cut into the mount found on AI-S lenses.
Flare and Ghosts
The 20mm f/3.5 AI/AI-S has legendary flare resistance and it does lives up to its billing and also with very little ghosting with no reduction of contrast across the image when stopped down a little.
DX | f/5.6 @ 1/8000th
Even with raking the sun across the image, we only get minimal ghosting which I find acceptable. The ghosting becomes a little more pronounced when stopping down the lens but it is still very marginal.
DX | f/11 @ 1/640th
DX | f/3.5
Wide open there is some flare but nothing overly troublesome.
Adding a protective or UV filter may affect performance though.
Sharpness
Bjørn Rørslett notes that the 20mm f/3.5 AI-S yields excellent sharpness used up close which mirrors a number of opinions online. Testing my sample, I wasn't so impressed - the lens is ok, perhaps a little above average but centre sharpness doesn't excite compared with other AI-S lenses such as the 50mm f/1.4 AI-S or 105mm f/2.5 AI-S. The 20mm is definitely sharper at f/4 but from then it doesn't really get better on my sample.
close focus - 100% centre crops: f/3.5, f/4, f/5.6
medium focus - 100% centre crops: f/3.5, f/4, f/5.6
Samples above were taken on a tripod, focused using liveview zoomed and tripped via release cord. Also notice the significant barrel distortion observed in the middle bookshelf.
f/3.5, close focus
f/3.5, 100% view of focus area (edge of red leaf above green stem)
Adding meta
Whilst a number of Nikon bodies allow you to add non-CPU lens info (focal length and max aperture), we can also easily add the lens name to the exif directly to the raw files using exiftool exiftool -overwrite_original -FocalLength=20 -LensMake=Nikkor -LensModel="Nikkor 20mm f/3.5 AI" _DSC8470.NEFLens hood
The official lens hood for the 20mm f/3.5 is the HK-6, which is an all metal slip-on hood with rubber grips. However because it is a slip-on hood which needs to 'grab' onto the front of the lens, it means that you can't easily use profile circular polarisers since the hood will grab onto both the rotating and non-rotating parts of the filter. With the hood installed the original clip on 52mm lens cap will also not attach but the diameter of the lens hood is wide enough (external 80mm/internal 78mm) to accommodate a standard 77mm lens cap.
The lens hood is probably not a must-have item since the lens is pretty resilient to both flare and ghosts but it's useful as an additional barrier against knocks on the front element even if it is reasonable shallow, standing at 13mm tall, although it's relatively wide (80mm) as you'd expect for a wide angle lens hood.
Cleaning the optics
The optics are complex with 11 elements in 8 groups. There do not seem to be many resources for this lens althought Richard Haw has a write up for the AI-S version (which is the same optically as the AI version) but mechanically its not the same as the AI lens
(c) mir.com.my - 20mm f/3.5 AI optical cross section
The front optics appear to be accessible via a front collar that is secured via a set screw, but this screw is obscurred by the focus ring/main barrel. The rear element is removable using lense spanner.
General Use
As with other Nikkor lens from this time, it's nice to use - it's solid and the focus ring is smooth to operate but using hand held as a walk-around lens it can be challenging for focus at wider apertures. The DX field of view makes it a very easy choice as a walkabout lens.
DX | f/3.5 - giving closing enough to 28mm-equiv FOV on DX
DX | f/5.6
DX | f/5.6
DX | f/5.6
However, it is the dual purpose (DX and FX) use case that makes this lens interesting combined with the resistance to flare (and the reason I went for the f/3.5 instead of the older f/4) - I can essentially point this lens at any subject, as you would with a modern lens with its various advanced coatings, without having to remind yourself to avoid backlighting that makes this a fun lens to use.
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