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 sick with hay fever. Hay fever is a thing here in Japan every spring and it affects a significant part of the populace. 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 approach is to nurture your growth and you start with simpler lenses and then proceed to the more exotic ones. The subject of our post this time is a mysterious and unique Nikkor that many peope do not know much about.


This lens is a mystery to many people because it is the only Nikkor of its type that got into production. This lens was designed to aid photographers who use flash guns. Back in the days before TTL metering, photographers need to adjust flash output by re-calculating its power as you go nearer or further from the subject (hence GN for guide number). Nikon’s engineers had to find clever ways to solve this like the GN Auto-Nikkor 45mm f/2.8 and its unusual coupling mechanism. Another good example is the early production type Micro-Nikkor-P 55mm f/3.5 where its iris open up as you go closer to its maximum reproduction ratio of 1:2 n(ative) or 1:1 with the included M ring accessory.

This lens has a switch at the focusing ring to couple it 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 good and clever it has a catch, the focusing ring on Nikkors go the other way and 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 had to focus towards the opposite way! This is very annoying for me so I don’t use this lens as often as I would like to. It’s such a pity because I have grown to love this lens.

IMG_1774This lens is an example of precision engineering. Look at these nicely engraved numbers, this is something that you don’t regularly see in current lenses from mainstream brands. If you think this is all the engravings this lens has then just wait until you read the whole article. This is one of Nikon’s most ornate lenses and it’s one-of-a-kind.

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 don’t know what is the reason behind it. I will update this post as soon as I know what the cause is.

This lens was made specifically for flash photography and it has a nice trick in order for you to get the right exposure with your flash all the time. Please watch the video above to know how it does that because it’s easier to understand the whole thing with a video. It’s difficult to illustrate something like that with words or pictures alone.

IMG_1775The Nikon 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). Later model lenses are cheaper in the used market due to them having only 7 iris blades. This comes with a special lens hood when bought brand new. That hood 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 because the helicoids doesn’t have a linear turn rate. It had to do this because light decays in a non-linear fashion (inverse-square) and this had to follow how light decays because it was made to sync with the output of a flash gun.

GN.jpgThe silver lens to the left is the Nikkor 45mm f/2.8 Ai-P. It’s the successor to the lens in this article and both are of the same Tessar-type vaiant. I say “type” because they aren’t Zeiss-patented Tessar lenses but they derived from the Tessar design. These lenses are the only lenses of this type for the Nikon F-mount. The Tessar is a well-loved lens design like Zeiss’ Sonnar, it has a cult-status amongst classic lens users eventhough the Tessar is obsolete and superceded by many other lens formulas. The strength of the Tessar is its small size and nice sharpness (for its time) and contrast due to the relatively few lens elements that was used in the formula. It’s an old design and one of the most-copied lens formulas.

IMG_1780The lens is so thin that there’s no space for the aperture claws so Nikon decided to simply put the the aperture claes directly above the engraved numbers! In case you’re curious about what’s underneath, here it is. The numbers 4 and 8 were skipped in order to give way for these 2 screw holes.

Knowing how a lens performs is the key to successfully using it so I’ll show you a couple of pictures that I shot with this lens in real-world pictures. I’m not interested in shooting brick walls or pixel-peeping and comparing the results to a graph because that makes the whole experience of photography technical, something for engineers and not the artists.

HAW_3288.jpgI don’t shoot brick walls but I shoot accordion shutters instead. Distortion isn’t much of a problem with this lens but you will see it in the lines near the periphery of your frame. It is not a lens that you would want to use for shooting architecture or art where you need a lens that has almost no distortion.

The following sets of pictures were shot from f/2.8, f/4 and f/5.6 in that order from left-to-right. These pictures should give you an idea on how this lens performs throughout the first 3 stops where most people would want to use any lens.

(Click to enlarge)

Vignetting isn’t much of a problem with this lens as you can see here. There is a little bit of corner darkness but it’s even and not bad at all. I would say that it’s pretty good in this aspect because it’s easy to correct it by over-exposing it by about 0.7 stop.

(Click to enlarge)

This lens doesn’t focus close at all and its minimum focusing distance is a mere 3ft away from your subject from your film plane. This is important because nobody uses this lens with flash at that distance anyway when the aperture ring and focusing ring are coupled. The good news is the performance is nice throughout the focusing ranges that matter. It’s a great lens as you can see from the picture above, the contrast is great at f/2.8 but a little bit of flaring and aberration is masking the sharpness of this lens. Look at the bell on the bicycle and you will see what I mean. The smily face in the first set also illustrates that. It was shot intentionally like this so that the white background will “bleed” into the smiley face’s black border. To be frank, it’s not as bad as I was expecting. Stop it down to f/4 and the sharpness improves a bit because the flaring is now gone. Contrast improves a little bit because the contrast is already looking great wide-open. The frame is clean and this is a very good aperture for using this lens. By f/5.6, the resolution is beginning to peak and you can see that there’s nothing much to improve at this point. The lens still looks great by f/11 and I haven’t seen the usual ugly effects of diffracion. This was probably intended by the lens designers because shooting with a flash means that you will probably stop it down most of the time. It’s also worth noting that the bokeh looks decently smooth. This goes against the popular myth that Tessars have terrible or harsh bokeh.

(Click to enlarge)

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’m so used 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 after using it for a couple of days.

(Click to enlarge)

These sets of pictures were shot in order to see how this lens performs when focused on the middle distances from 2-20m. I assume that this range is what many people will want to use this lens with. So far the results mirror that of the previous sets wherein we check how it looks in closer distances with the difference that the aberration or magenta cast is absent wide-open. This is a good lens if you’re going to ask me. The flare resistance is not perfect but it’s not terrible at all and it adds to the image more than it takes away from it. Again, bokeh isn’t the best but it’s not terrible at all.

HAW_3198It’s now time for some real-world pictures! I took this at f/8, the colors tend to be on the cooler side but the rendering is very natural. I think this was intended because the big flash bulbs of years-past were warm and older lenses tend to be calculated for the black and white films which benefit from a slightly-blue cast. While there were bulbs that were made with different colors for different lighting conditions, I will just assume that Nikon did this for the sake of the big-blue bulbs that many photographers used. This bluish-cast isn’t an issue because it’s easy to fix your white balance these days in post. It also gives a nice and unique look to your pictures and adds to that “vintage look” that many people are into these days.

(Click to enlarge)

These were all shot at f/8 and you can see just how nice everything is and the resolution is great. It’s apparent in the details of the skin, hair and face of the subjects. You can also notice the cooler cast in the pictures as evident in the 3rd one with the 2 girls. The shot with the train was taken near the “Golden Hour” and the bluish cast helped make it look more natural instead of 24K solid gold. I like the look a lot, do you agree with me?

(Click to enlarge)

Here are some more shots that I took at Korean town. I don’t know what’s going on with the pretty boys and the make up, I thought it went away with The Human League. Again, the cooler cast of the pictures are obvious despite me shooting in AWB mode. This is such a fun lens to use if only I can get used to focusing the opoosite way!

Let’s now see how this lens performs with film. It’s important for us to study how a lens performs with pictures that were shot using film. Film has a distinct look, something that is almost impossible to simulate in digital because film grain’s pattern looks organic and not serialized like what we have in a sensor. I shot the following pictures with flash and I coupled the focusing ring and the aperture ring so the lens will change its iris as I focus. I used a manual flash for this and I set the ideal aperture and focusing distance according to the scale at the back of the flash. I then coupled the coupled the focusing ring and the aperture ring so they’re locked together. I recall that I was shooting with f/16 for these or maybe f/11, I wanted maximum depth-of-field so I won’t have the need to focus using the viewfinder. Focusing using the lens’ focusing scale is convenient and I am used to it since I shoot with manual-focus lenses regularly.

FH000008Since I am shooting with a flash I can use smaller apertures for more depth-of-field. This is convenient when you need to take a snap-shot quickly.

FH000013Since I was focusing using the scale I sometimes mis-judged my subject’s distance, giving me a blown-out result. You will have to remember this when using this lens, always see if your subject is at the close or far-end of the scale because the flash won’t compensate for it since it’s all-manual. The GN Auto-Nikkor 45mm f/2.8 and your flash will give you good exposures at the correct distance and not what’s before or after that.

FH000023I am impressed by this clever gimmick. Flash exposure wasn’t perfect but it is still better than guessing the right exposure using your head or a flash meter. I felt like a newspaper photographer from the 1960s when I was taking these pictures.

(Click to enlarge)

Here’s the rest of the set. Notice that the exposure of some of the photos look a bit off. If the lens is focused at a farther distance then your subject will over-exposed if he/she is at the near-end of the distance scale. This is what happened to the picture of the lady with green hair. The reverse is true for the picture of the couple. They’re both at the far-end of the distance scale and so they were a bit under-exposed. They’re also not focused, I think I mis-judged the distance in this picture. The photo with the lady walking is exposed just-right since I got her closer to what’s in the center of the distance scale. I hope that I didn’t confuse you with all this babble, the best way to see this work is to shoot using a similar setup as mine and see the results in your LCD or negative.

That’s it for the introduction. I hope that you got a good idea on how this lens performs. I like this lens a lot mainly because it renders in a unique way and it’s sharp. If you want a lens that renders this way then this is the lens for you! The only alternative to this is the Nikkor 45mm f/2.8 Ai-P but it doesn’t render pictures like this since it’s a modern lens in every way. It’s one-of-a-kind and the prices for these aren’t getting any cheaper because collectors want several of these. I find this funny because this lens was once the cheapest Nikkor you can buy but they sell for way-more than the exotics that were sold in its time. If you want a small lens then by all means, buy this lens and enjoy the pictures. Hunting for one in the used market isn’t difficult but you should find one with an Ai ring like the one I have here or else you won’t be able to use it with Nikons that have the Ai interface. This is not a problem for people who shoot mirrorless cameras and they’re sold cheaper so this is a good chance to buy them for a good price. Prices vary from $150-250 and that will depend on the condition, the inclusion of an Ai ring or the rare hood and the 9-blade iris which is the most desirable model. Let’s now begin our repair article.

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. Please also read what I wrote about 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 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 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.

Disassembly (Front):

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.

I started from the rear as I didn’t 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 just 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’ll explain this later when we come to the focusing ring and why I did this.

That’s it for the front part of the lens. If you intend to use this lens exclusively with flash then I will recommend that you remove the detent spring but otherwise just leave it so it can click into the proper aperture numbers properly. Just make sure that you don’t lose it because this part is unique to this lens and fabricating one is a waste of time.

Disassembly (Focusing Ring):

This part is easy but you’ll have to be 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 part, 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 don’t want what is under this thing to fly across the room or get lost in the floor! You don’t 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’s 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/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. Sorry for the blurry picture.

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 because it wasn’t glued.

I hope that you kept everything and not lose the small parts. Follow my guide and you’ll be fine because you know what you’re going to expect. Using a tray will help prevent this from happening because the tray will catch anything that fell from the lens as you work on it. I hate working with spring-loaded things because I hate surprises!

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 cemented in place so a drop of alcohol helps to soften it.

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!

This section is probably the easiest because you don’t need any special tools. If something is stuck, just used some alcohol or acetone to soften the cement and wait for it to work. It will probably take a few applications depending on what was used so you’ll have to be a little patient. It’s not unusual for me to wait over-night and attempt another try the next day. It’s better to wait than make something that you’ll regret later.

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. Take all the notes and pictures you can because that will save you a lot of time and guessing later on.

IMG_1795I started by removing the rear lens assembly. Simply use your hands to unscrew the ring from the objective’s casing. 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 don’t want to accidentally damage the rear element so be careful! Any harm to this part will affect your pictures no matter how small the damage is.

IMG_1797Now, on to the focusing unit. The iris (aperture blades) is vulnerable so be careful not to damage the blades. 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 will 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 was made differently from what we are used to and instead of having helicoid keys we have these posts instead. Remove these so that you can freely turn and separate the focusing mechanism.

IMG_1801These are fragile and I wonder why Nikon didn’t use more robust parts for this. Now that you are aware of this, be careful when turning your lens because these can snap off from its neck or base 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. While these threads have only 1 insertion point, you’ll still want to take pictures as to where they separate to give yourself some peace-of-mind. If you are new to working with lenses, please read my article on working with helicoids. I wrote that article because there are many people who get stuck in this part. You do not want to end up being one of them so please read my article on helicoids.

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 slides-off. Notice how these slots have etched the inner brass 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 part in place. You can now remove the objective and iris assembly.

That’s all there is for this lens. It’s not a complicated lens at all, it’s just unlike most lenses we are used to work with and that makes it a challenge for many people including those who are experienced with repairing Nikkors. If you are mechanically-inclined, this lens is going to be a fun project and you will learn a lot of new things just by working on one of these. I had a great time with this lens, it was all worth it.


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 was accustomed to just to get this to focus smoothly with a nice damped feel. I made a mistake in my first choice of grease and I should have used a heavier variety for this so keep this in mind. I initially wanted it to focus with minimal resistance so I used my usual grease but it turned out that a grease with more resistance is best for this. It now feels great now that I applied a thicker grease variety and it feels so good to focus. Be sure not to apply too much or it will migrate to its iris and this will end up with an oily iris problem.

IMG_1809These are the bigger parts of the lens. I cleaned them very well before applying grease to 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 why. This was necessary so focusing will always be synced with aperture ring when they’re both 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 sample 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 its engineering as well as design decisions that were made 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.


13 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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