Repair: Nikkor 20mm f/3.5 Ai-S

Hello, everybody! I just came back from a nice dinner with the Nikon Df Japan Group last night and we had lots of great seafood! One of the things that I liked was the fried fish in batter. The fish is about the length of a finger and just as fat as thick pencil but it was so tasty! The portion was small but it was more than enough to satisfy me. Great things does not always need to be big. In fact, some great things in life are really small. In this article, I am going to show you one such thing. It’s such a great lens that it’s still popular despite being introduced around 1981! Read along to know more about this “millennial”.


Today, I’m going to introduce to you the Nikkor 20mm f/3.5 Ai-S lens! This lens is a classic, it has a cult following for many reasons. Many landscape photographers love this lens for its performance and its size. The lens is also one of the few Nikkors that were calculated to work just as well in reverse when attached to a bellows unit. In fact, if you search the net you will find some people use this lens for shooting very small things that require a magnification of 5:1 or more. The Nikkor 20mm f/3.5 Ai-S can be had for cheap because it is slower than the Nikkor 20mm f/2.8 Ai-S lens but people who know what they are doing will want this instead because of its performance when used in reverse. The minor f-stop difference doesn’t really matter when you intend to shoot with this lens at f/8 or smaller with your setup mounted on a tripod. The money you saved from buying this instead of the Nikkor 20mm f/2.8 Ai-S will be better put to use buying filters or even a tripod. Let me show you what this lens is all about in this lens repair and review article. Don’t go away!

IMG_0752The Nikkor 20mm f/3.5 Ai-S is very compact lens. It’s no bigger than usual Nikkor primes like the Nikkor 50mm f/2 Ai/K lens! It also has a very short focus throw which can be good or bad depending on your taste, I personally prefer it to be just a little longer than this so I can precisely set my focus. This is a matter of taste so don’t think of it negatively. The Ai version may be older but it has a slightly longer focus throw than this version so people who would want that should look for the older Ai version, they’re also a bit cheaper.

IMG_6273.JPGHere is Nikkor 20mm f/3.5 Ai-S together with the Nikkor-UD 20mm f/3.5 Auto lens. The old Nikkor is the first lens in the 20/3.5 series of ultra-wide Nikkors and it was one of the first  lenses to get this wide as far as SLR lenses are concerned. The Nikkor-UD 20mm f/3.5 Auto is a very good lens but it’s a huge lens compared to the Nikkor 20mm f/4K that replaced it. It also had a more complicated optical formula just to make it possible to get this wide. It had to be because retro-focus lenses haven’t been mastered yet at that time. The newer Nikkor 20mm f/4K fixed that and it had fewer lens elements and the lens ended up being a lot smaller than the Nikkor-UD 20mm f/3.5 Auto. The Nikkor 20mm f/3.5 Ai-S replaced it and is the last lens in the series, being replaced by faster lenses in the lineup. However, it is still very popular up to this day because of its great cost/performance ratio.

IMG_0726Here it is mated to a Nikon D750. This lens is very good for backpackers because it’s not a big and heavy lens so you’ll feel less pain at the end of the day. On the DX system it makes for a nice walk-around lens because it’s about 30mm due to the 1.5mm crop factor. You’ll also be able to focus really close with this lens and its minimum focus distance is short at only 0.3m! I usually will not use this lens focused on something that close but if I’m doing landscape photography, this will make for an interesting effect as I can focus on a subject that is close to my camera as a foreground element. This will give my composition depth with the incorporation of foreground, middle ground and background elements into my scene. I believe it was Ansel Adams himself who taught this technique.

(Click to enlarge)

It’s smalle even when compared to the Nikkor 28mm f/2.8 Ai-S lens, which many consider to be a small and compact lens. It also has a 52mm filter size which makes it convenient if you have invested in a set of 52mm filters. This is a good thing but the downside of this is you get some vignetting wide-open up until f/8. You would want to stop this down so it is not so much of an issue when you are shooting landscapes.

HAW_0098As expected from the Nikkor 20mm f/3.5 Ai-S, it renders foliage very well even all the way to the far corners. No smearing at all and you get a crisp picture all the way to the edge.

HAW_0100Distortion is present as you can see in the pillar but it should not be a big problem if your lines are cleverly framed. Your horizon should be as close to the center of the lens if this bothers you. I shot landscapes a lot before and this is one of my techniques to get around the distortion on my lenses. Just look at the amazing sharpness and contrast of this lens!

HAW_0111Here is a vertical shot of a tall building. You can notice a slight bowing of the lines at the left-most lines of the building. I would say that this is more than acceptable to me so long as I don’t print this really big. If you don’t go out of the way and look for it then you may not even notice the distortion in this picture. This was shot at f/8 and it’s damn sharp!

HAW_0121Going back to horizons, check out how the roofs of the buildings bow down towards the center. This is what happens when you put your horizon further away from the center of the frame. This won’t be as noticeable if I framed the horizon nearer to the center.

(Click to enlarge)

This set of images were shot from f/3.5, f5.6 and f8 respectively. You don’t really gain any sharpness (not much) by stopping this down because it’s already good wide-open. If you are a good observer then you will see the terrible vignetting of the left-most image shot at f/3.5 and is most apparent on the far corners of the frame. It mostly goes away by f/5.6 and it’s gone by f/8. Noice that the vignetting makes the picture look darker by around 2/3 of a stop. This is significant so please keep this in mind.

HAW_0124This was shot at f/5.6 and you can see that vignetting isn’t obvious at all when you stop it down by just a stop. The details at f/5.6 are also great. In fact, it’s sharp even wide-open.

HAW_0126While the DOF of this lens is deep, we still managed to get the old lady at the foreground blurred a but. I was focusing at the green building in this picture. The distortion isn’t so obvious in this picture apart from the lines at the extreme left of the frame.

HAW_0131A lens with a short focal length allows you to hand-hold your setup and shoot with a slow shutter speed. I took this at the base of the Nikon building just to check for coma. One of the highlights of this lens is its coma correction. There’s still a small amount of coma but it could have been worse. Coma is a weakness when it comes to wide lenses with a retro-focus design. This was shot at f/5.6 and as you can see, the details are all sharp!

HAW_0134This was shot wide-open, it’s very useable at this aperture and there really isn’t much of a difference when it comes to sharpness. Stopping this lens down deepens the DOF and it will also get rid of the vignetting which I think is kind of strong wide-open at the corners.

(Click to enlarge)

Finally, here are some samples shot at night on a Sony A7. I hated my Sony cameras, they are ergonomic nightmares from the design all the way to the menus. I’m only using them because they’re the only company making full-frame mirrorless cameras at the moment besides Leica. You can see that the vignetting isn’t really a problem when shot at night. I barely noticed it in my shots but it’s there when you know how to look for it.

That’s it for the introduction. I hope that I gave you a good idea on how this lens works in real life. My sample pictures weren’t really great and I may add some more in the future. I still haven’t finished my roll of Provia and I think I shot a few pictures there using this lens. Come back again for more updates. Let’s now move on to the lens repair portion!

Before We Begin:

If this is the first attempt at opening a lens then I suggest that you read my previous posts regarding screws & driversgrease and other things. Also read regarding the tools that you will need in order to fix your Nikkors.

I highly suggest that you read these primers before you begin (for beginners):

Reading these primers should lessen the chance of ruining your lens if you are a beginner. Also before opening up any lens, always look for other people who have done so in Youtube and the internet. Information is scarce, vague and scattered (that is why I started this) but you can still find some information if you search carefully.

I highly recommend that you also read my working with helicoids post because this is very important and getting it wrong can ruin your day. If I can force you to read this, I would. It is that important!

For more advanced topics, you can read my fungus removal post as a start. This post has a lot of useful information here and there and it will be beneficial for you to read this.

About Me:

I am a photographer based in Tokyo who scours the junk section of the camera shops here on a regular basis. I started this hobby of fixing old lenses and cameras because I kept on getting scammed before when I purchase online. This turned out to be a blessing because I now buy junk lenses for a fraction of the price and then fix them to add to my now-growing collection. This allowed me to buy gear that were otherwise off-limits to me if I were to get them in better condition.

I was a scale modeller before who built models for other collectors for a very longtime, got an education as a dental prosthodontist (but didn’t pursue dentistry) and growing up in a watch repair shop gave me the useful skills that would help me in this craft. Fixing my car also contributed some know-how.

Having mentioned the above, I will tell you now that I am not a professional repairman! So please take what I do with a grain of salt and I will never be held responsible for anything wrong that will happen to you and your project. I hope that the pros would guide and teach us but nobody is writing anything.

As my collection of repair notes grew, I get requests from people with similar interests and from professionals who just needed some notes just in case. Now, I am sharing my library with you for your entertainment and reference. I hope that you find my work useful.


The lens doesn’t have a lot of parts so I will condense everything into one section. We will be joining the lens barrel and objective’s disassembly sections into one long section. This lens is very typical of Nikkors from the Ai-S era. When I worked on this lens, I didn’t even had to look for a reference or wonder how I should go about working on this because it’s of a conventional design and I have worked on many similar lenses before. A beginner is cautioned from working with this lens because it can be tricky to get the helicoids to seat properly on the correct height. Attempt this after you have worked on a couple of lenses, you will need a keen eye and note-taking skills of a more experienced hobbyist. Send this to a qualified repairman if your not comfortable with working with this.

IMG_0753You can remove the rubber grip later if you wish but I opted to get rid of it now. Carefully run a toothpick under the circumference of the rubber grip to loosen any glue and then carefully pick it off with your fingers. Make sure that you don’t tear the rubber!

IMG_0754This lens is similar in construction to the Nikkor 28mm f/3.5 Ai-S lens and this means that you’re going to start from the rear of the lens if you want to totally disassemble it. There are 3 screws that secure the bayonet and you should remove these to continue.

IMG_0755The bayonet can then be pulled away once the screws are gone. Note that the stop-down mechanism is part of the bayonet mount assembly. The notch on the swing arm should be coupled to the post on the iris mechanism that you can see on the left.

IMG_0756This is the iris regulator. It is a ring with a tab that rotates and is coupled to the aperture ring via the tab. You can remove it by picking it out from the lens with your fingers.

IMG_0757The aperture ring can now be safely removed just like this.

IMG_0759I always work with my lens focused to infinity as much as possible so I’ll have a point of reference when I take notes or when I reassemble my lens.

IMG_0758With the lens still set to infinity, take plenty of notes regarding how things are when your lens is set to infinity before you remove anything. Unscrew these screws to remove the 2 helicoid keys. The helicoid keys are responsible for keeping all 3 helicoids in-sync as you focus your lens in or out.

IMG_0760Once the helicoid keys are gone, you can now safely remove the inner helicoid from the rest of the lens. The inner helicoid is also part of the objective’s housing so be careful. It’s a clever trick that makes the lens small and light but it also has a few drawbacks. One of the drawbacks is that the iris is more prone to get contaminated with oil because it’s very close to the helicoid. Another is it makes servicing the objective or iris more complicated because you will have to pull this out instead of just removing the objective on its own.

Remember to mark or take not where the helicoids separate because this is also where it is going to mate. If you forgot to do this then you’ll have a hard time guessing where you should mate the helicoids. If you are new to this, read my article on helicoids.

IMG_0761Before you go any further, remove the focusing ring by unscrewing these 3 screws.

IMG_0762The focusing ring is held in-place by the thin brass ring and the 3 screws secure it. Pick it off from the lens while still maintaining the your focus at infinity.

IMG_0774The front bezel of the focusing ring can be unscrewed and I usually remove it to remove gunk that has accumulated under it. It’s not necessary for you to do this but I just wanted to be sure that my lens is clear from contaminants like old grease and other nasty things.

IMG_0763Going back to the main barrel, separate the central and outer helicoids and mark where they separate.

IMG_0764To remove the sleeve and the grip, simply unscrews these 3 screws. Use the proper screw driver so you won’t damage the fine knurling of the grip.

IMG_0765The sleeve should come off effortlessly just like this. Old grease and dirt usually settle in the spaces under the sleeve so it’s a good idea to clean this part thoroughly. Think of it as a lens’ foreskin. Sorry for the crude gym locker humor if that offended you.

IMG_0766To open up the objective, begin by removing the front barrel. First, remove this little grub screw that secures it. Make sure that your driver fits the slot of the screw perfectly. Yuck, the old grease is starting to deteriorate and it’s a good thing that I opened this! If I got to this any later, the oil may end up fouling the iris and that’s going to be a bigger problem!

IMG_0767Once the screw is gone or loosened you can now unscrew the front ring off.

IMG_0768The front elements group can be unscrewed from the objective just like this.

IMG_0769The inner elements group can be removed by using a lens spanner.

IMG_0770You can extract it with your fingertips or with the aid of a lens sucker.

IMG_0771The rear elements group can be unscrewed by using your barehands.

IMG_0772It should come off just like this. Be careful not to drop this!

IMG_0773If you want to service your iris mechanism, begin by removing these screws that secure the iris mechanism to the housing of the objective. I didn’t have to do this because mine is clean and it’s pointless to remove it just for the fun of it. It’s a precise adjustment and if you need to remove yours make sure that you take notes or mark it so you’ll know how it should be oriented. he iris mechanism shares a lot in common with the one found on the Nikkor 85mm f/2 Ai-S lens so if you can use the link to that article as additional reference.

That’s all for the objective. I’m sorry that this article didn’t have the complete information for working with the iris but like I said, there was no reason for me to do so on this lens. I may update this in the future if I ever got a chance to work on another and if that has an oily iris problem. Like I said, lenses of this design wherein the iris mechanism is too close to the helicoid because the housing of the objective has the helicoid’s thread milled into it has greater tendency to develop that problem. I hope that mine will be fine in the future!


This lens is simple as far as I’m concerned. It’s pretty conventional and is in-line with the Nikkor primes of the same vintage. It took me a whole evening to do all this (3 hours?) so it was a nice exercise for me. Since this lens has a very short focus throw, I used a thicker grease to give it the proper feel. If I used a lighter grease then the focusing ring won’t feel as damped as I would want. It’s also feel cheap and gritty. With a damped focusing ring, I can also do smaller turns and that will also make my focusing more precise. A loose ring will just make me over-turn the ring and potentially lose my focus as I focus pass it. The resistance of a focusing ring is essential in how a lens operates.

IMG_0775Calibrating the focus of this lens is a must even though it’s DOF is very deep. To calibrate it, simply reassemble your lens to this state where everything is back in its original place but leave out the front parts. At this point, the objective may drop to the floor because the front barrel that secures it is absent. Loosen these 3 screws and then calibrate the focus of your lens. If you don’t know how then read my article on how to calibrate the focus of your lens. If your lens cannot achieve infinity focus then it’s useless for landscapes!

IMG_0776Here’s mine! It has been properly calibrated and the infinity symbol is dead-center on its centerline (dot). Tighten-up the 3 screws and do more tests and see if everything is OK. If it’s still off by even a bit then you will have to re-do the steps until you are satisfied!

IMG_0777Finally, reinstall the front ring and reassemble your lens completely. Good job, get a beer!

And that’s it for this article! Did you guys enjoy this? I hope that this article will help you in the future when your lens starts to show signs of wear or dry helicoids. There’s no one talking abut the repair of this lens online so I decided to make one just in case somebody will need one in the future. This is not a difficult lens to service for an experienced and resourceful repairman. I enjoyed working on this and it was a good break from what I’m used to doing which is fixing normal to tele primes. See you guys again next time and if you liked this, please share this with your friends at social media! Until next time, Ric.


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Maintaining this blog requires money to operate. If you think that this site has helped you or you want to show your support by helping with the upkeep of this site, you can simple make a small donation to my account ( Money is not my prime motivation for this blog and I believe that I have enough to run this but you can help me make this site (and the companion facebook page) grow.

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Thank you very much for your continued support!


Helping support this site will ensure that this will be kept going as long as I have the time and energy for this. I would appreciate it if you just leave out your name or details like your country and other information so that the donations will totally be anonymous it is at all possible. This is a labor of love and I intend to keep it that way for as long as I can. Ric.


4 Comments (+add yours?)

  1. Trackback: World of F-mount Nikkors (2/3) | Richard Haw's Nikon Maintenance Site
  2. Petr
    Oct 08, 2018 @ 12:52:44

    I recently purchased 20mm f3,5 UD. Non AI.
    Any chance you have disassembled and assembled this lens?
    My front of the lens wiggles a bit… and I don’t know what to do.
    Thank you


    • richardhaw
      Oct 09, 2018 @ 00:59:08

      Hello. I haven’t worked on one yet but I will have to have it here in the future. Ric,


      • Petr
        Oct 11, 2018 @ 00:28:43

        Do you have any idea how to take the front off?
        There is the small lock screw , but the ring does not come of , seems like there is another ring holding it possibly the name around the front glass f
        Ring that holds it all in…?

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