Repair: GN Auto-Nikkor 45mm f/2.8

Hello, my dear readers! Despite the busy schedule I managed to pace my time in order to post this to my blog on a timely manner. Apart from being busy at work, I was down with hay fever. Hay fever is a thing here in Japan every spring and it affects a significant part of the populace. I hope that you stay healthy and keep your masks on to prevent hay fever from ruining your Spring.

As I promised a few posts before, I would like to present to you some unusual Nikkors as we have dealt with conventional ones in the previous posts. This is to nurture your growth in this craft so that you get to start with simple lenses and then proceed to the more exotic ones. The subject of our post this time is the mysterious GN Auto-Nikkor 45mm f/2.8!


This lens is a mystery to many because it was the only Nikkor lens of it’s type that got into production. This lens was designed to aid photographers with guessing flash exposure in mind. Back in the days before TTL metering, photographers need to adjust flash output by re-calculating the flash’s power as you go nearer or further from the subject (hence GN or guide number). Nikon has to engineer clever ways to work around this like like our subject for this post, the GN Auto-Nikkor 45mm f/2.8. Another good example is the early production type Micro-Nikkor-P 55mm f/3.5 where the iris opening is compensated as you go closer to it’s maximum reproduction ratio of 1:2 native and 1:1 with the included M ring accessory.

This lens has a switch on the focusing ring to couple the focusing ring to the aperture ring so that these 2 move in sync as you turn the focusing ring. For example as you focus your lens closer the iris would stop down and when you focus further the iris would open up to compensate. This allows you to shoot without even thinking about changing the power of your flash. While this is all good and clever, it has a catch. Nikon’s lenses’ focusing ring goes the opposite way, this means turning the focus ring to the right makes the lens focus to infinity and in order to make this clever iris compensation trick work this Nikkor needs to focus the opposite way so you turn your focusing ring to the opposite direction! This is very annoying for me so I do not use this lens as often as I would like to.

IMG_1774This lens is an example of precision engineering. Just look at all those beautifully engraved information and numbers, this is something that you do not regularly see in current lenses from mainstream brands.

I got this lens for a nice price, considering that the aperture ring is already factory Ai-d. I need to overhaul the lens however since the focusing was dry and there was dirt inside the lens but overall it was in a pretty OK state. I did get some artefacts on my bokeh balls when I shoot bright lights in the dark, I tried cleaning the lens but I still do not know what is the reason behind it. I will update this post as soon as I know what the cause is.

IMG_1775The D4 dwarfs this lens, making it look more like a lens cap than anything. This is a later production lens where you only get 7 aperture blades (earlier ones have 9). Late production lenses are cheaper in the used market due to it having only 7 aperture blades. This lens also comes with a special hood when bought brand new. That hood alone can fetch a high price when you buy it separately.

IMG_1776This lens is so thin it makes the Nikkor 50mm f/1.8 Ai-S look bulky. The only Nikkor that is thinner than this is the Nikkor 45mm f/2.8 Ai-P. This lens was engineered very differently from other Nikkors that you can argue that Nikon had this lens made by another company under subcontract. The Nikkor 45mm f/2.8 Ai-P was made by Cosina (Voigtlander) and to be honest, this lens feels very much like a Cosina-designed lens. One clue is that the hood for this lens looks more similar to the designs used by Voigtlander. Whatever the truth is, we will never know.

The 45mm focal length fits my taste as this is not as wide as a 35mm so I do not need to go too close to my subjects and not as tight as a 50mm or a 55mm that I am so accustomed to. The only thing that kept me from using this lens on a daily basis is the reversed direction of the focusing ring. It needs some getting used to and you will miss a couple of shots at first until you get used to it.

The lens is not so good wide open but performs just OK when stopped down. It does render nice saturated colours as you can see from my pictures above.

IMG_1780The lens is so thin that there is no space for the aperture claws so Nikon decided to simply put the the aperture horn directly above the engraved numbers! Just in case you guys are wondering what is underneath that, here it is. The numbers 4 and 8 were skipped in order to give way for these 2 screw holes.

Enough of the introduction, let us begin with the disassembly because I assume that this is the reason you are here reading my posts anyway.

Before We Begin:

If this is the first attempt at opening a lens then I suggest that you read my previous posts regarding screws & drivers, grease 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.

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 growing now 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.


This lens is not your usual Nikkor so the conventional way of removing the objective first and then taking apart the focusing unit will not apply. You will find that I go about doing things in no particular order simply because I do not know what I am going to get when I remove this or that part. I hope that this guide will be useful for experienced technicians and professionals so that they can plan their steps better before fixing this lens.

Disassembly (Rear Part):

I started from the rear as I did not know what this lens has in store for me. Opening a lens from the rear will give you an idea of how a lens was engineered since you can see a lot of things when the bayonet mount is off.

IMG_1781Start by removing these screws on the bayonet mount. Mine came off easily but if your are stuck, just drop some acetone into it and wait for the acetone to work on whatever Nikon used to lock these things.

IMG_1782The lens can now be easily taken apart now that the bayonet mount is gone. See how tight and packed it is inside? Despite this, Nikon still made the objective separate.

IMG_1783This is the mechanism links the aperture ring to the iris assembly. Be careful not to bend or damage this part! You can unscrew it if you want to.

IMG_1784The aperture ring easily comes off just like that since the bayonet mount is the only thing that is holding it in place. Notice the pretty engraved information on the aperture ring!.

IMG_1785This is the aperture ring detent spring. Videographers remove these to de-click their lens’ aperture rings. I left mine intact except that I made the springs  shallower so that the ring does not need a lot of effort to click. I will explain this later when we come to the focusing ring and why I did this.

Disassembly (Focusing Ring):

This part is easy but you have to be a bit careful because of the small mechanisms that are involved with the focusing ring so go through each step slowly. Also be sure not to work on a carpeted floor or rug or you might lose some of the fine and tiny parts that can be found in this assembly!

IMG_1787Remove the focusing ring by unscrewing a couple of screws. Check the picture above, the encircled parts are the places where the screws used to be. While removing this, be careful with the aperture coupling switch. See the next 3 picture to understand why.

IMG_1788Apply pressure to this switch while you remove the focusing ring. You do not want what is under this thing to fly across the room or get lost in the floor! You do not want to remove these screws in this step as well. These 2 little screws simply hold the grip and the switch together and has nothing to do with it’s mechanical function.

IMG_1789Carefully remove the aperture coupling switch and be mindful of what is underneath that thing (encircled). Check out that pin under the switch, that pin is inserted into the slots underneath the aperture ring when you slide the switch down. When the pin is inserted into any of the slots, the aperture ring and focusing ring are now synced so that they will both turn when either one is rotated.

This pin is prone to wear and tear so I modified my aperture detent spring to be shallower. This will save the pin from excess wear each time the focusing ring is turned.

IMG_1790This little bead is spring loaded. Underneath this bead is a little brass cup and a spring. See how small and delicate they are? This is the reason why I told you to be careful with this.

IMG_1791Time to remove the narrow grip. Remove the screws that secure this part to the lens.

IMG_1792You can now safely remove this ring. Mine came off easily as there were no adhesives that were used in this part.

Disassembly (Front Part):

Now, back to the front! You can actually start from the front instead of the rear like what I did, it does not really matter as far as this lens is concerned.

IMG_1793Start by removing this ribbed cone with a rubber stopper. These are usually glued in place so a drop of acetone helps.

IMG_1786Next, the front barrel should be removed by using a wide rubber stopper. This part is too narrow for my bare hands so a rubber stopper or a rubberised cloth helps.

IMG_1794And off it goes…

Disassembly (Internals):

This part can get confusing (or frustrating) because the lens’ engineering is unorthodox. You should carefully document each step and do not rely on my notes alone.

IMG_1795I started by removing the rear lens assembly. Simply use your bare hands to unscrew this metal ring from the objective (optical block). If yours is being held by glue, drop acetone into the holes and wait for it to soften the glue before proceeding. Be careful not to use too much acetone because it might ruin the cement (Canada balsam) used to glue the doublet together. The rear element assembly is called a doublet because it is basically 2 separate lenses glued together to form a single unit (check the picture).

You do not want to accidentally damage the rear element so be careful! Any damage on this part will affect your picture no matter how small the damage is.

IMG_1797Now, on to the focusing unit. At this point the iris (aperture blades) is vulnerable so be careful not to damage it. The 3 screws you see here holds the objective down so remove these to remove the objective.

IMG_1798Before removing the screws I made a mark a mark so I know how these should be aligned when I reassemble these parts together later. If you got this wrong, your iris will either not open or close correctly and you will end up with the wrong f-stop all the time.

IMG_1800This lens is engineered differently from what we are used to so instead of having helicoid keys, we have these posts instead. Remove these so that you can freely rotate and separate the focusing mechanism.

IMG_1801These are fragile and I wonder why Nikon did not use more robust parts for this. Now that you are aware of this, be careful when you turn your lens because these snap off easily.

IMG_1802And here is the other on on the other side.

IMG_1803Before I forget this, be sure to mark where these posts should be when the lens is focused all the way to infinity so that you know how far you need to screw the focusing unit back when you reassemble your lens.

There was also no need to mark where these separate as this is just a one-way thread. Your usual focusing unit (for most lenses) uses a helicoid where there are several positions the helicoids can mate. This one is simply a screw thread and these parts can only mate in one position so I did not even bother but just for my sanity’s sake I took a picture of where they separated.

IMG_1804And off it goes. Notice that the groove and how it is not linear (exponential). It had to be this way because of the inverse square law and how it affects light. You see, this is a lens that automatically adjusts your lens’ iris by coupling the aperture ring to your focus ring when you have the pin engaged. If this groove was linear then the change will not match the light that the flash is putting out. This is the reason why this lens has a cam instead of proper helicoids. Now you know!

IMG_1805The inner part simply slides off. Notice how these slots have etched the brass inner barrel, these should give you a clue on where these should be positioned.

IMG_1806Remember these screws that we removed a few steps before?

IMG_1808Those screws hold this thing in place. You can now remove the objective and iris assembly.

Post Mortem:

Putting the lens back together was not difficult at all but I need to be aware of the order in which these things should go together as this is a very unusual lens as far as Nikkors are concerned.

I also found myself using more lubricants than what I am accustomed to just to get this to focus smoothly with a nice damped feel. I made a mistake in my choice of lubricant and I should have used a heavier grease variety for this so keep this in mind. I initially want this to focus with minimal torque (I like feather smooth focusing) so I used my usual grease but it turned out that a heavier grease is best for this. I might re-grease this lens in the future but I do not use this lens often enough so this is not urgent.

IMG_1809These are the bigger parts of the lens. I cleaned them properly before lubricating the parts that are involved in the focusing of this lens.

IMG_1810The focusing has a tendency to ramp or “bunch up” from the minimum focusing distance to just about normal distances and this slot explains it. This was necessary so that focusing will always be in sync with exposure when the aperture ring and focusing ring are coupled.

IMG_1813And this is the view from the other side.

IMG_1811These thin brass shims are used to adjust the lens’ infinity focus so please do not damage or lose these rings! Each lens is uniquely calibrated so you may have more or less of these.

I also cleaned the lens elements because they are dirty as you can see from my pictures.


This lens was a lot of fun to service in the sense that I learned a lot of things regarding it’s engineering as well as design decisions that were considered to make this lens as compact as possible. I was also itching to fix this thing for a long time because I want to know what is inside this thing so that certainly satisfied my curiosity!

I hope that you have enjoyed this post and if you would like to support me, please share and repost this to your favourite Facebook page and websites so that I get more hits and other people also get entertained reading my repair notes and commentaries.

I also intend to extend this site with e-commerce because I get questions regarding tools and parts from time to time. This will also enable me to pay for this blog’s overhead and it enables me to buy more pricey junks to dismember.

Until next time, please stay safe and enjoy your craft. With love, 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 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.

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.


10 Comments (+add yours?)

  1. Ron V.
    Mar 13, 2016 @ 22:08:24

    Always wondered what these look like on the inside.
    Thanks Rick.


  2. richardhaw
    Mar 14, 2016 @ 00:38:17

    Thanks, Ron. I am going to look for the 45P so I can see what’s inside of that too


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  4. Page Admin
    Mar 03, 2017 @ 03:08:32

    Thank you. This was very helpful. I just took out everything and did a cleaning for my 7 bladed 45mm. Everything works great now.


  5. mmarquar
    Apr 09, 2017 @ 15:47:02

    Amazing! Very, very nice. Thanks.


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