Widely Regarded: Nikkor 20mm f/4 AI

The Nikkor 20mm f/4 AI (and its near identical predecessor, the K) was the optical successor to the very well regarded Nikkor-UD 20mm f/3.5 Auto. The K was produced from 1974 to 1977 and the AI released for a short period til the start of 1978, at which point the Nikkor 20mm f/3.5 AI had been released. Both the f/4 AI and f/3.5 AI had good reputations but how are they really?



Physically, the f/4 is small - the f/3.5 itself is already small but this is even shorter, with it just a little larger then the Nikon 50mm pancakes. Focus throw is longer at 140 degrees compared to 100 degrees for more minor focus corrections and about 20g ligther.


<sup>Nikkor 20mm f/4 AI height vs Nikkor 20mm f/3.5 AI


Nikkor 20mm f/4 AI height vs Nikon 50mm f/1.8 pancake</sup>

Optically, the f/4 has a 10 element, 8 group design and its housed as most smaller AI lens in a single objective, whilst the f/3.5 AI was mored advanced with a 11 element, 8 group design.


^{(C) Nikon - Nikkor 20mm f/4 AI optical cross section}

Both lenses are comfortable shooting into the sun, with very few ghosts and very glare resistant.




The 20mm f/3.5 AI is similar in the same conditions.

There are some minor ghosts but nothing concerning.


<sup>Nikkor 20mm f/4 AI @ f/4


Nikkor 20mm f/4 AI @ f/8</sup>

Sharpness is pleasant and sufficient - not biting sharp but good enough; between the two 20mms, I'd say they're pretty close and difficult to see one over the other. Both are centre crops wide open.


^{f/4 at MFD}



Distortion is there and seems better than the f/3.5.

Servicing

This is where this lens hurts - as with a number of early AI lenses, there was a tendancy to apply a lot of adhesive seals. The front name plate has a set screw that reveals a retaining ring sitting on the focus ring - this was impossible to remove even with heat. The lens objective itself has the bulk of its lens element (6) housed in a unit that has to be unscrewed from the front - there are lens spanner slots but I don't know why they bothered - the technician who assembled this at the factor has thread locked this so good that I bent one of my thinner spanner tools trying to unseat it.

So a warning, servicing the optics nor the helicoids is impossible if the lens was assembled in a similar fashion - no heat, accetone nor IPA would break the bonds. This is incredible frustrating as this is lens does not appear so often on the 2nd hand market, with about 10,000 lens made based on Roland Vink's data, and any that do appear will almost certainly require servicing given it will nearly be 50 years old.