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.



To overcome the unremovable focus ring collar, which overhangs and obstructs the focus ring screws, we can file an angled groove that is wide and high enough to allow a driver onto the screw head and sufficient clearance for the screw to clear its slot. Filing the groove at an angle will allow for an unmarred collar when assembled. Once removed we expose the focus limiter ring screws



The focus limiter ring on this copy was stuck even after removing the delicate slotted screws - there is a slot at the side of the ring that I applied accetone and IPA that helped free it.



To strip down to access the helicoid key, the rest of the action is at the rear. The usual rear mount and aperture ring come off first and then we need to remove the aperture ring coupling screw along with the ident spring. This will give us the remaining chrome grip that is glued with the distance scale.

The helicoid key has a lip that faces towards the front of the lens, which is unlike similar contemporary construction lenses such as 20mm f/3.5 AI, 35mm f/2 AI and 50mm f/2 AI which have a near identical key where the lip faces the rear of the lens.



Separating the outer helicoid is standard fare but care must be taken with separating inner and central helicoid. The limiting block on the inner helicoid has its screws entirely obstructed by the focus limiting ring. This is unlikely other latter AI lenses where the top of limiting block can be exposed. To separate the inner helicoid I found that with the limiting ring freely moving, we can carefullly turn both helicoid and the limiting ring as to remove from the front. There isn't a lot of turns before it separates so be careful - I did find that there are only 2 mating positions so if, as I did, the inner helicoid separates inadvertantly you won't suffer too badly: the remated helicoid will must be able to sit the holes aligned to secure the helicoid key.



One note - the unserviced lens had a heavy focus and this remained even after new grease applied - with not lens objective in the barrel, the lens' focus is smooth and lightly damped. Inserting the lens objective is a very snug tight into the barrel and once inserted the focus becomes heavy. There is nothing catching (the aperture fork and activiation key are correctly positioned) and its not apparent what causes this heavy feeling unless its the weight of the lens elements pressing into the inner helicoid.

On the the lens objective, we can get to the rear 4 elements by unscrewing the rear collar that also houses lens elements. However because of the impossible adhesive at the front, other access is impossible.

So a warning, servicing the optics nor the helicoids is impossible if the lens was assembled in a similar fashion to this copy - no heat, acetone, IPA nor tapping would break the adhesive 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 become available will almost certainly require servicing given it will nearly be 50 years old. This is huge shame as the 20mm f/4 AI is a nice little compact lens with nice optical output but an alternative can be found in the Nikkor 20mm f/3.5 AI/AI-S is quite similar in size and optical performance.