Repair: Nikkor 20mm f/3.5 Ai-S

Hello, everybody! I just came back from a nice dinner with the members of 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. It is about the length of a finger and just as fat as a thick pencil but it was very tasty. The portion was small but it was more than enough to satisfy me. Great things do not always need to be big. In fact, some amazing things are really small. In this article, I am going to show you one such thing, it’s so good 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. This lens is a classic, it has a cult following for many reasons. Landscape photographers love this for its great performance and compactness. It’s 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 for shooting small subjects that require magnifications of 5:1 or greater. It can be had for cheap since it’s slower than the Nikkor 20mm f/2.8 Ai-S but people who knew what they’re doing will want this one instead because of its performance when used in-reverse. The minor f-stop difference does not really matter when you intend to shoot this at f/8 or smaller with your setup mounted on a tripod. The money you saved will be better put to use buying something else.


This is compact, it is no bigger than most Nikkor primes like the little Nikkor 50mm f/2 Ai/K. This has a really short focus-throw which can be good or bad depending on your taste, I prefer it to be just a little longer so I can precisely set my focus. The Ai version is older but it has a longer focus-throw so those who need it should look for the older Ai version, they’re also a bit cheaper.

It has an 11-elements-in-8-groups design which it inherited from the Nikkor 20mm f/3.5 Ai that replaced the venerable New-Nikkor 20mm f/4. It was sold from 1981 to 1984, quite short in my opinion. This was then replaced by the Nikkor 20mm f/2.8 Ai-S which is faster, bigger and more expensive. Despite all that, this still remained as a favorite amongst landscape photographer. I never really liked any of Nikon’s 20/2.8 lenses for one reason or another so I could understand why people would choose this over the newer Nikkor.


Here it is together with the Nikkor-UD 20mm f/3.5 Auto. The older 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 amongst SLR lenses. The Nikkor-UD 20mm f/3.5 Auto is a very good lens but it’s bigger compared to the New-Nikkor 20mm f/4 that replaced it. It also has a more complicated optical design to make it go that wide since retro-focus lenses haven’t been mastered yet at the time. The New-Nikkor 20mm f/4 fixed that, it has fewer lens elements and the lens ended up being a lot smaller. The Nikkor 20mm f/3.5 Ai-S replaced that, it is the last lens in this family. It was replaced by faster lenses but it’s still very popular to this day because of its great cost-to-performance ratio.


Here it is mounted to a Nikon D750. This is great for backpackers since it is not as big and heavy so you’ll feel less pain at the end of the day. On the DX system it makes for a great walk-around lens. It will give you a 30mm field-of-view due to the 1.5mm crop factor. This focuses really close, its minimum focusing distance is short at only 0.3m. I usually won’t use it at this distance but if I’m shooting landscape photography, this will make for an interesting effect as I could focus on a subject that is closer to my setup as a foreground element. This will give my composition some depth with the incorporation of foreground, mid and background elements into my scene. I believe it was Ansel Adams himself who taught this technique.

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It’s even smaller compared to the Nikkor 28mm f/2.8 Ai-S, which many folks consider to be a small and compact lens. It also has a 52mm filter size which makes it convenient if you have invested in that filter size. That’s really nice but the downside to this is you’ll get some vignetting wide-open if the filter’s ring is tall enough to cause mechanical vignetting.

Knowing how your lens performs is key to maximizing it. You’ll learn how to utilize its strengths and avoid its weaknesses, this knowledge will help in determining which lens to bring on a shoot and which one to leave in your cabinet. I don’t shoot charts or do any tests in a controlled environment, it’s more important for me to take photos that simulate real-world use. What I do is observe my photos and share my opinions. I shot the following photos from f/3.5, f/5.6 and f/8 because we’ll see the most changes happen in these apertures.

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Distortion is quite high and the profile looks a bit complex which is a shame in my opinion. You could see it prominently if your scene has straight lines. There’s no way to avoid this, really. Vignetting is quite high wide-open, stop the iris down to f/5.6 alleviates it but you’ll want to shoot this at f/8 at least. I could still see traces of it even at f/8 but at least it’s not that bad.

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This is how it looks like on a real-world scenario. You won’t see much of the dark corners by f/8 which is good. People who shoot landscapes know that it is important to stop the iris down to at least f/11 in order to get more depth-of-field I don’t think this will be an issue at all.

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It’s quite resistant to flare and ghosts, really impressive because I recall that many of my ultra-wide lenses have poor performance in this department. It was something I had to live with during my landscape photography days.

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Well, you don’ shoot with this lens for its bokeh characteristics but it’s quite acceptable to above-average for an ultra-wide lens. These photos should be useful if you’re wondering about this topic.

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Chromatic aberration can be quite severe in extreme cases. The bicycle has many overblown details that could exhibit this. Look at the 2nd set and you could see what I mean. It’s kind of high wide-open but it cleans-up when the iris is stopped-down to f/5.6 but you’ll still see traces of it even by f/8. This is only true for terrible cases like what we have here. You normally won’t see this beyond f/5.6 and it’s still at acceptable levels wide-open. The last set is a good example of a real-world scenario. You could see some of it wide-open but it improves significantly by f/5.6. Spherical aberration is quite high, too. It’s not as distracting as the former but some people don’t like it.

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Sharpness in the center is excellent wide-open, it improves a bit by f/5.6 and marginally gets even better by f/8. This is because it is already quite good at f/3.5. Resolution is more than adequate wide-open and gets better when the iris is stopped-down. The corners are a different story, it only starts to look acceptable to me by f/8, it’s not bad at f/5.6 but I expected more from this an ultra-wide lens. Saturation and contrast looks quite-even across the range. I love how it renders since the photos have this natural-look to them. I would say that it performs better at moderate ranges but it really is nice across its focusing range.

It renders with a nice “vintage-look” that some people either love or hate. It could take photos with nice, natural-looking tones.

Position your straight lines diagonally to prevent them from bowing. This is how I would use lenses with high-levels of distortion. I wouldn’t never use it for shooting architecture at all, there are better lenses for that.


As expected, it renders foliage very well even all-the-way to the far-corners. No smearing at all.


Distortion is present as you could see in the pillar but it should not be a big problem if your lines were cleverly framed. Your horizon should be as close to the center to avoid it curving a bit. I shot landscapes a lot before and this is one of my techniques to get around this issue. It’s sharp and the contrast is quite nice, too.


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

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Well, I would have positioned the horizon closer to the center of the frame but I don’t think I had much choice since it would’ve made the composition look off. It’s still acceptable to me in this case.


Here’s a vertical shot of a tall building. You can notice a slight bowing of the lines at the left-most edge 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 it in this picture. This was shot at f/8, it’s really sharp.


This was shot at f/5.6, you can see that vignetting isn’t obvious at all when it is stopped-down to f/5.6. The details at this aperture looks great. In fact, it’s sharp even wide-open.


Despite ultra-wide lenses having deeper depth-of-field, we still managed to get the old lady at the foreground blurred a bit. I was focusing at the green building in this picture. The distortion isn’t so obvious here apart from the lines at the extreme left of the frame.


A lens with a short focal length allows you to hand-hold your setup and take photos with a slower shutter speed. I took this outside of the Nikon building just to check for coma. It’s well-corrected for coma but there is still a small amount 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 by the way.


This was shot wide-open, I would normally shoot with this on a tripod but I don’t have one with me at that time. I mentioned that because I would have shot this at the smallest-practical aperture available to me just so I could get nice and sharp corners.

Let’s now see some pictures that were shot with film. It’s good to see photos from both film and digital so you could see how a lens renders with both. It (film) has this unique look that’s difficult to replicate using a digital camera because of grain and other factors. It also has certain properties that makes it react differently to light such as being less-reflective, making it resistant to internal-reflections. That is why many manufacturers make new, digital-versions of their existing lenses back-in-the day when DSLRs were starting to become popular. I took these pictures using a Nikon F4 and loaded it with Fujifilm Industrial 100.


This was probably taken at f/5.6, the focus is at the building in the center of the frame where the lights are. Its exposure is about 4 seconds so the trains look like streaks of light. It’s sharp at the center but focus is a bit lacking in the foreground. Maybe I should stop the lens down more to get more things focused.


It has average-looking sun-stars. I was hoping that the sun-stars would look better than this but this is all that it could offer.


Great dynamic range considering that this is a cheap film stock. I wanted to have the sky look clear so I stopped the lens down to f/8 to prevent any dark corners. The details are crisp, clear and sharp at this aperture.


A wide lens has to focus closer than normal lenses or else its use is limited.

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These were taken at the minimum focusing distance. I love the details, I can almost feel the textures in the scene. Film grain looks organic compared to noise, it can help give rough details in your scene a gritty feel.

I used to shoot a lot of landscape photos so I’m embarrassed that my photos look mediocre but these will do for now just so I can have pictures to show. For those who could see its potential and would like to buy a used sample I will advise that you check the iris carefully. It should be dry, snappy when actuated. It can be prone to the oily iris problem because of its construction as you will see later. There isn’t much that can go wrong with this lens since it’s a simple design. It’s compact, light, tough and reliable so it’s perfect lens for backpacking. Videographers will like it as well but the short focus throw is going to be frustrating, for that you may want to consider the older New-Nikkor 20mm f/4 Ai instead. With that said, you couldn’t go wrong with it. Its popularity and reputation is well-deserved and it will still be a relevant lens for decades to come.

Before We Begin:

If this is your first attempt at repairing a lens then I suggest that you check my previous posts regarding screws & driversgrease and other things. Also read what I wrote about the tools that you’ll need to fix your Nikkors.

I 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 novice. Before opening up any lens, always look for other people who have done so in Youtube or 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 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 and it will be beneficial for you to read this.


It doesn’t have a lot of parts so I’ll condense everything into one section. The construction is typical of many Nikkor primes from the Ai-S era wherein its optics were incorporated into the barrel’s design. When I worked on it, I did not even have to look for a reference since it is of a conventional design and I have worked on many similar lenses before. A beginner is cautioned from working with this since it can be tricky to get the helicoids to mesh properly at the correct height. Do this only after you’ve worked on a couple of lenses, you’ll need a keen eye and some skills of an experienced hobbyist. Send this to a qualified repairman if you are not comfortable with working with it.


You could remove the rubber grip later if you wish but I opted to remove it now. Carefully run a toothpick under the circumference of the rubber grip to lift the glue and carefully pick it off with your fingers. Make sure that you don’t tear it.


It’s similar in construction to the Nikkor 28mm f/3.5 Ai-S, you’ll need to start from the rear if you want to totally disassemble it. Extract these to remove the bayonet mount. Many people get stuck here because they have stripped the heads of the screws by using the wrong type of drivers or not having the right skills to do it. They get stuck and sell the lens after getting frustrated, I recommend that you read my article on how to remove bayonet screws, the article will show you how to do it properly and the tools that you will need to get the job done properly.


Remove the bayonet mount. The stop-down lever mechanism is part of the assembly, that tiny notch at the lever should be coupled to the post located in the iris mechanism, you could see that to the left of the photo.


This is the iris regulator. It is a ring with a tab that rotates and is coupled to the aperture ring via the tab. Remove this by picking it out from the barrel.


The aperture ring can now be safely removed.


I always work with the barrel focused at infinity as much as possible so I’ll have a point-of-reference when I take my notes.


With the barrel still set to infinity, take plenty of notes so you will see how things are aligned before you remove anything. Extract these to remove the helicoid keys. The helicoid keys are responsible for keeping all helicoids in-sync as you turn the central one, this enables you to extend-and-retract the barrel. Be sure to note which key goes where since you’ll want to put them back to their respective slots again later.


Once the helicoid keys are off, you can remove the inner helicoid. The inner helicoid is also part of the objective’s housing so be careful. It’s a clever trick that makes the barrel small but it also has a few drawbacks. One of them is the iris is more prone to get contaminated with oil because it’s closer to the helicoids. Another is it makes servicing the objective or iris more difficult, it requires that you take the lens apart just to service the optics or iris.

Always mark or take some notes as to how the helicoid separated, people do not do this sometimes and they end up getting frustrated because they have to guess how the helicoids should mesh. In order to prevent this, read what I wrote about working with helicoids and follow it so it won’t happen to you.


Remove the focusing ring by extracting these.


It’s secured by this brass ring.


The 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 but I just wanted to clean the barrel thoroughly.


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


To 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.


The sleeve should come off easily. Grease or dirt usually settle in the spaces under the sleeve so it’s a good idea to clean this part thoroughly.


Remove the front barrel to open the objective’s housing. Extract this, be sure that your driver fits the slot perfectly. This is usually sealed with lacquer or paint, place a drop of solvent to soften it.


Once the set screw is gone you can unscrew the front ring off.


The front elements assembly can be unscrewed with your bare-hands.


The inner elements assembly can be removed by using a lens spanner with long bits installed. Be sure not to damage anything in the process.


You can pick it up with your fingertips or with the aid of a lens sucker.


The rear elements assembly can be unscrewed easily.


Be careful while removing this, you may bend something while you grip the whole thing.


If you want to service the iris, begin by removing these but do not forget to mark the position of the iris mechanism’s housing in relation to the casing of the objective. It’s an adjustable

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 you’ll need to put it back properly or else your exposure won’t be accurate. Its construction shares a lot in common with the one found on the Nikkor 85mm f/2 Ai-S so you could use that article to help guide you through the disassembly.

Clean the helicoids thoroughly and don’t leave any residue. Since this has a rather short focus-throw it’s best to use grease with more resistance. It will make it turn a lot better and you could focus more precisely because of it. I would like to warn you against applying too-much grease, that excessive oil will end up in the iris at some point, only apply a thin film, that’s more than enough to make things feel smooth.


It took me a whole evening to do all this, it was a great exercise. It was a bit unnerving when I was reassembling the helicoids since it was difficult and a bit confusing due to the fine-nature of the threads. If I turned it more than I should I would’ve screwed-up the reassembly and start all-over-again. It’s best to use an electronic caliper before you dismantle anything to measure the exact height of the barrel. Use that measurement as a reference for you to recreate. Having said that, this is not a lens for a beginner to play with, it is best left for the experienced repairman,


Calibrating the focus is a must despite its deep depth-of-field. To calibrate it, simply reassemble it to this state where everything is back but leave-out the front parts. At this point, the objective may drop to the floor, the front barrel that secures it is absent. Loosen these and calibrate the focus of your lens. If you don’t know how, read my article on how to adjust the focus of your lens. If your lens couldn’t focus to infinity properly then it’s useless.


Mine has been properly calibrated and the infinity symbol is dead-center on its centerline. Tighten the screws and do more tests and see if everything is fine. If it’s still off by even a bit then you’ll have to re-do the steps until you are satisfied with your results.


Reinstall the front barrel and reassemble it completely.

At the time of writing this, I haven’t found any articles online talking about the repair of this lens. I hope that this will become helpful to somebody out there looking for a guide. If you enjoyed this article, please share this with your friends. Every view counts, this site only gets $0.30 day from it alone. I would like to thank the people who support this site, it helps me maintain it and pay for its hosting. You’re also helping me offset the cost of purchasing, processing and scanning film. Thank you very much and see you again next time, Ric.

Help Support this Blog:

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 site’s upkeep, you can make a small donation to my ( 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.

Buy me a roll of film or a burger?

Thank you very much for your continued support!

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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’s name or other information so that the donations will totally be anonymous. This is a labor of love and I intend to keep it that way for as long as I can. Ric.

8 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…?

      • Vince
        Jul 15, 2019 @ 21:36:12

        I have the same lens as Petr, with the same problem. Hoping you can provide some insight at some point. Thanks for the great site by the way!

  3. Trackback: Repair: Nikkor-UD 20mm f/3.5 Auto | Richard Haw's Classic Nikon Repair and Review
  4. Trackback: Repair: New-Nikkor 20mm f/4 | Richard Haw's Classic Nikon Repair and Review
  5. Trackback: Repair: Nikkor 20mm f/2.8 Ai-S | Richard Haw's Classic Nikon Repair and Review

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