Need nerd help Re: Nikon NAI GN 45mm

[Gene Jackson]

  I recently bought this lens, and aside from being misrepresented as a 9 bladed aperture (its not  ) the lens has another problem.  The focus ring and the aperture ring both turn freely, and do what they are supposed to do, if operated without the GN tab engaged.  However, when I engage the GN tab, the lens locks up, for lack of a better term.  I understand that when the aperture ring and focus ring are locked together, the action will be stiff, but I get no movement on the rings if I apply what I consider to be reasonable pressure, and I would rather consult the Nerd Oracles about their opinions rather than forcing the issue, so to speak. 

You might well think to your self, or ask, why I would like this aspect of this sorta marginal lens to be functional.  I can only answer that I want to use it as designed and intended for on nikkormats with an appropriate flash and period lenses. 

Thanks in advance for any advice.  I'm not (very) afraid of tearing into a lens, but I would appreciate pointers as to what to look for in there regarding this particular problem.

TIA,

gene

[Bent Hjarbo]

Richard Haw has a blog with advice on this:
https://richardhaw.com/2016/03/13/repair-gn-auto-nikkor-45mm-f2-8/#more-4427

[Bjørn Rørslett]

The cam coupling focus and aperture might be worn down. A common issue with this construction. Even the brand new lens had a quite "stiff" focusing feel to it with the GN tab engaged and the wear must have been formidable over time.

I removed the internal coupling on mine and added a CPU. Now, the lens is fully functional once I got the hang of its "wrong" direction of focus travel I in fact prefer the older GN over the newer Ai-P lens because the former is easier to operate in winter with gloves on. Much easier.

[Roland Vink]

The 9 blade aperture was only found on early production lenses, serial nos from 710101 to at least 725535. By no.725743 the lens is made with 7 aperture blades.  I believe that the aperture is circular at f/8, perhaps you can confirm this for me? If so this is the earliest example of rounded aperture blades on a Nikon lens.

I'm not sure why Nikon made a special effort to create a rounded aperture opening (either 9 blades or 7 rounded) on a lens with a relatively short focal length and slow aperture. It can't have been for bokeh. My best guess is that due to the GN feature, the lens could go to a very small aperture when focused at close range, and the rounded aperture helps to minimise diffraction. Nikon didn't start to use rounded aperture blades for bokeh until the 135DC and 105DC lenses from the early 1990s.

[Bjørn Rørslett]

My GN is 729075, has 7 blade aperture and the blade edges are rounded. At f/8, they form an almost perfect circle.

[Bill De Jager]

I have two copies: 713297 and 717552.  Both have 9 aperture blades.  The aperture has a circular shape at f/8 but not above or below that stop.

[Roland Vink]

Thanks for confirming that, I didn't know the 9-blade version also has a rounded opening at f/8 (the 45GN is one of the few lenses from this era that I don't own, so never had the chance to inspect one closely). It seems strange the opening is only circular at this setting and not others, I wonder why it was designed this way. So the 9 and 7 blade apertures are essentially the same, the reduction to 7 was probably to simplify production, 7 blades on this lens is perfectly sufficient.

[Akira]

Sorry to stray, but I had an old Sonnar type Nikkor 105/2.5 with 9-blade aperrture version.  The aperture blades made nicely rounded openings at any settings.  They never formed the gear-like shapes even at f2.8 or f4.0.

[Bill De Jager]

Thanks for confirming that, I didn't know the 9-blade version also has a rounded opening at f/8 (the 45GN is one of the few lenses from this era that I don't own, so never had the chance to inspect one closely). It seems strange the opening is only circular at this setting and not others, I wonder why it was designed this way. So the 9 and 7 blade apertures are essentially the same, the reduction to 7 was probably to simplify production, 7 blades on this lens is perfectly sufficient.

Sorry to stray, but I had an old Sonnar type Nikkor 105/2.5 with 9-blade aperrture version.  The aperture blades made nicely rounded openings at any settings.  They never formed the gear-like shapes even at f2.8 or f4.0.

I've never seen this kind of behavior on any other lens.  I suspect that each blade, on its inner edge, has two straight sections joined by a curved section.  I think the way it works is that at f/8 ~the entire border of the pupil consists of the curved sections, and at other apertures the straight edges make up the border.

[David H. Hartman]

I had one in my hand, mint; it had multi-coating but was not designated as such,  no "Auto NIKKOR-C". It was in a Dutch auction. I didn't bid due to the reverse focus. Leica focus direction? I remember fairly stiff focus with the GN feature engaged.

Dave Hartman

[Gene Jackson]

Thanks to all for the additional information about this lens.  Since the focus and aperture rings both operate smoothly, I probably won't risk trying to fix the GN coupling prong. 

Roland Vink: my lens is #725705, and has seven aperture blades.  The serial number is in between the ones for the nine bladed and seven bladed versions from your invaluable compendium of Nikon lens information, so may represent a new data point.

Thanks again to the group for an interesting discussion.

best

gene

[Roland Vink]

Hi Gene, yes your lens is a new data point. if it's not too much trouble, can you send me a small picture showing the serial number and aperture blades so I can confirm the details, I'll add it to my database