AI conversion kits - are they really just the aperture ring?

[whatdoido]

Are AI conversions for K lenses are as simply as an aperture ring replcement?  And if so, would the K-AI conversion kit (aperture ring) be the same as the AI ring?

Example:  would the 20mm f/4 K conversion kit have the same aperture ring as the to 20mm f/4 AI?  This kit was listed in the cheaper tier of conversion options based on Roland's info http://www.photosynthesis.co.nz/nikon/aimod.html which suggests labour and parts (and shippping) were reasonable simple.


The background to this is I have a Nikkor 20mm f/4 AI but it's got some problems but have an option on a 20mm f/4 K that is in good condition - I wonder if i can simply swap the aperture rings (from AI onto the K) and have an AI'd K lens.  Milling/filling the K ring is something I dont want to do since I have an AI aperture ring.

[David H. Hartman]

Take care to soften the white locking "paint" on the screws. Do NOT strip the heads or torque them off! I did a DIY conversion of a NIKKOR-P·C Auto 1:2.5 f=105mm. I torqued off the head then with the aid of a vacuum cleaner and a pin vice I somehow drilled a tiny hole in the torqued off screw. I could not buy an extractor small enough so I used a larger drill to bind in the hole and successfully extracted the broken screw.

Stupid! Damned lucky! Learn from my mistake!

Later I used acetone to soften the Loctite thread locker (white, permanent?). A camera repairman friend use a small. butane soldering iron to heat the screws.

Dave

It's been so long but to my knowledge an official Nikon AI conversion kit contained only an aperture ring, AI style rabbit ears and a few screws. John White may still be doing AI conversions as of March 2026.

---

I can't answer the original question. Someone with more experience will surely answer. If you are already experienced with the procedure for removing the rear bayonet screws please accept my apology.

My advice is pickup a clean, used NIKKOR 20mm 1:3.5 AIS as they are highly resistant to flare and ghost and can be turned directly into the sun.

[David H. Hartman]

here is my go to short lens review site...

http://www.naturfotograf.com/lens_wide.html

[pluton]

Are AI conversions for K lenses are as simply as an aperture ring replcement?  And if so, would the K-AI conversion kit (aperture ring) be the same as the AI ring?

Example:  20mm f/4 K conversion kit have the same aperture ring as the to 20mm f/4 AI.  This was listed as the cheapest conversion option based on Roland's info http://www.photosynthesis.co.nz/nikon/aimod.html which suggests labour and parts (and shippping) were reasonable simple.


The background to this is I have a Nikkor 20mm f/4 AI but it's got some problems but have an option on a 20mm f/4 K that is in good condition - I wonder if i can simply swap the aperture rings (from AI onto the K) and have an AI'd K lens.  Milling/filling the K ring is something I'd like to avoid since I have an AI aperture ring.

My limited experience swapping K and Ai aperture rings is that the Ai kit ring can (obviously) swap onto a K lens, and the K ring should go onto the Ai lens without issue.  My experience: I have switched K to Ai, and Ai to K, on a 28/2.8 K/Ai lens a few times between the two copies of the lens. 

[whatdoido]

Dave - thats a name I havent seen since the strobist forum days!  Yes i'm ok with dissaembly/tear down of these lenses, I have a number of the 20mm f/3.5 AI already (still have one) but all of them aren't that sharp until you get f/8 and beyond and even then its just "ok" whereas direct comparison with the f/4 is sharper, even at f/4 and f/5.6 for both near and far.

https://whatdoineed2do.blogspot.com/2026/01/widely-regarded-nikkor-20mm-f4-ai.html

Pluton - thanks.  Roland's site also notes the 28mm ff/3.5 kit was 18$ but thats not a defnitie indicator.  if the K seller can do a good price i may just chance it.  The only reference I have is a factory AI ring on a 55mm f/3.5 Auto (I dont have the original aperture ring though) where the installed aperture ring just looks like the one of the 55mm f/3.5 AI although I know there's K variant and so not direclty comparable.

[Erik Lund]

The internal design are different on Ai compared to Ais lenses.
Therefore the aperture rings are not interchangeable between the two types of lenses.

[Roland Vink]

Are AI conversions for K lenses are as simply as an aperture ring replcement?  And if so, would the K-AI conversion kit (aperture ring) be the same as the AI ring?

Example:  would the 20mm f/4 K conversion kit have the same aperture ring as the to 20mm f/4 AI?  This kit was listed in the cheaper tier of conversion options based on Roland's info http://www.photosynthesis.co.nz/nikon/aimod.html which suggests labour and parts (and shippping) were reasonable simple.

The background to this is I have a Nikkor 20mm f/4 AI but it's got some problems but have an option on a 20mm f/4 K that is in good condition - I wonder if i can simply swap the aperture rings (from AI onto the K) and have an AI'd K lens.  Milling/filling the K ring is something I dont want to do since I have an AI aperture ring.

The short answer is yes - the AI conversion ring is simply an aperture ring replacement. For lenses like the 20mm 1:4 where the K and AI lens are basically the same, it is reasonable to assume that the AI conversion kit for K lenses was simply the aperture ring from a standard production AI lens.

There is at least one case where the conversion ring is not the same as a standard AI aperture ring. The K 105mm 1:2.5 stops down to f32 and the AI conversion kit retains this aperture range. However, the standard AI 105mm 1:2.5 only stops down to f22 even though it is mostly identical to the K version. There may be others where the AI conversion ring is different from the standard AI ring, but they should still be interchangeable since they need to fit the same lens.

Note that the screws on these lenses are notoriously difficult to budge. You need to use the right tools to avoid damaging your lens.

I think I have an unused AI conversion ring for the 20mm 1:4 lens - let me know if you are interested.

[whatdoido]

Roland, thanks.  Let me secure the 20mm f/4 K and will report back but the one I was looking out might have gone.  Knowning the 20mm f/4 K and AI rings are interchangable opens the search options.  Theres not that many locally in here but maybe places like Tokyo / Hong Kong will have more.

FWIW, I serviced the 20mm f/4 AI I have in my possession and it was the most glued lens I've worked on but the rear mount screws were surprisingly easy.  The pain was the focus ring collar wouldnt unscrew (i filed a notch around it to remove the focus ring) and niether could I unscrew the front elements from the objective and I tried everything.

https://whatdoineed2do.blogspot.com/2026/01/widely-regarded-nikkor-20mm-f4-ai.html#servicing

Horrible lens to work on if that one sample is represenation - on a FB lens repair group one service tech said all 20mm f/4 AI he worked on were the same pain.

[Roland Vink]

I read your post on servicing this lens. Near the end you write: "One note - the unserviced lens had a heavy focus and this remained even after new grease applied"
I wonder of the focus ring is slightly out of round? That would put more pressure on the focus helicoids. I have an AI-S 85/2 which had uneven focus action when I bought it, it was a bit tight at certain distances. I had it serviced many years ago and whatever they did, it has been one of the smoothest focusing lenses I own. Recently a grain of sand has worked its way in so it's slightly rough, might need to get it serviced again...

[pluton]

Uneven resistance in helicoid focus mechanisms can be caused by uneven or excessive grease in the threads.

[whatdoido]

I read your post on servicing this lens. Near the end you write: "One note - the unserviced lens had a heavy focus and this remained even after new grease applied"
I wonder of the focus ring is slightly out of round? That would put more pressure on the focus helicoids [...]

I dont believe anyone else has been inside (all seals, screw heads etc were clean) this lens other than me so any issues might be from the factory.

The focus action with the lens objective removed (all the glass/aperture-iris mech) is smooth and light after my service but once the objective is inserted - its a tight fit  which i've not seen before - the focus action is heavy.  But yes, I'm starting to think the lens objective is out of shape although visiually i can't tell and its somehow putting pressure on the helicoids but this is just one of the reasons i'm likely to use this current f/4 AI for parts, including its aperture ring on any possible f/4 K.  The grease I used was the modern Nikon NPC FC4 alternative, Nippeco NPC BC-13A, which is for lens helicoids so not expecting any special grease is required for this 20mm.

Uneven resistance in helicoid focus mechanisms can be caused by uneven or excessive grease in the threads.

I  tthought so, but it had same heavy feel pre/post my clean-regrease.

[paul hofseth]

For the other kind of AI, Artificial Incompetence just regreasing will not help-

p

[Matthew Currie]

There is one difference that modifying won't make, and that is the aperture indexing post in the lens.  I'm not sure how many cameras use this, but the F4 does, and it determines whether the lens can use matrix metering. I saw it identified as having to do with certain flashes as well, so it might influence how a dedicated flash operates on an F4.  Anyway, the F4 will not matrix meter a lens without that post. It also senses the AIS slot, which I think is also ignored by newer digital SLR's, as well as at least some  film ones (the F100 ignores both). 

e.t.a. The FG-20 appears to have the internal lever for the AI post (it's down along the bottom), but nothing in the instructions suggests that it makes any difference to the camera. The FG-20 loses both the program mode and the TTL flash mode of the FG,  so I wonder what, if anything, that lever does.