105 f2.5 AI'd No Infinity Focus

Hello all:

 

I know there are several related threads, but none seem to answer this particular question. I have a Nikkor-P 105 f2.5 which has been AI'd at some point in its life. It will not focus all the way to infinity on three camera bodies (F3, F4 and D600). Because the infinity mark lines up correctly when the lens is fully "retracted", I suspect a problem with the modified mount (images follow). Other then this, the lens works perfectly, glass is very clean and produces sharp images at portrait distances. Should I remove the flange and look for an unnecessary shim, or problem? I'm not certain it's worth repairing otherwise, although there is a local shop.

 

Thanks,

Les

 

Next image

 

Last image

 

My guess is that the machining on the mount was not quite right. John White at AI Conversions has a lot of info posted, does excellent work inexpensively.

I suggest you call John White at AI Conversions. He does outstanding work, is quite knowledgeable about the subject, and is a very nice fellow.

 

Prices and Shipping

When a lens is AI-converted, the bayonet is not touched. Only the aperture ring is replaced or machined. The problem is not there.

 

The problem is likely that the lens was disassembled at some point, the focus helicoids were separated for cleaning, and then they were put back together with the wrong entry point. The correct mating point of the helicoids will have to be found by trial and error, unless the lens has another way to adjust infinity focus (as many do).

I don't think it's because of AI'd because it looks like a factory conversion.

I don't think it's because of AI'd because it looks like a factory conversion.

For a considerable time factory mounts were available for upgrades, consequently, hard to tell.

When a lens is AI-converted, the bayonet is not touched. Only the aperture ring is replaced or machined. The problem is not there.

 

The problem is likely that the lens was disassembled at some point, the focus helicoids were separated for cleaning, and then they were put back together with the wrong entry point. The correct mating point of the helicoids will have to be found by trial and error, unless the lens has another way to adjust infinity focus (as many do).

 

Would the error created this way be more than about 1-2mm of travel? That seems to be the degree of inaccuracy involved. I assume the helicoid is something like a three-ply thread?

I found this VERY helpful description of repair on what appears to be my exact lens! For once, Google actually was my BFF. There seems to be a point in the disassembly where the infinity stop can be adjusted. Since one or two of the bayonet screws appear to have been previously removed, I will assume that this was set incorrectly at that point. When I have time, I will attack this.

I agree that it's very likely the focusing helicoid was separated and reseated incorrectly at some point in the past. That's why you ALWAYS mark it when you disassemble.

 

The aperture ring is from a factory conversion kit, and is not the milled type that John White and others make. It's still an AI'd lens in my book-just a much better type of AI conversion as compared to milling the ring(or hacking it with a dremel or bench grinder, as I've seen done). These are about a 10 minute job to change and won't affect the focus at all-the mount is simply removed, the old ring lifted off, the replacement ring put on, and the mount screwed back on.

For a considerable time factory mounts were available for upgrades, consequently, hard to tell.

 

The aperture ring has a secondary aperture scale for the ADR. I don't think any after market has it.

The aperture ring has a secondary aperture scale for the ADR. I don't think any after market has it.

I believe that is correct. As I said above, I found a repair guide for a lens that looks EXACTLY like mine.

First time I've seen slotted screws in the mount - clear indication that someone "screwed" around with the lens.

First time I've seen slotted screws in the mount - clear indication that someone "screwed" around with the lens.

 

My 105 f/2.5 also has slotted screws. There were two versions of the non-AI 105 f/2.5 lens I believe wogears and I have the older version that uses the Sonnar optical formula rather than the Gauss formula used in the version made after 1972.

When a lens is AI-converted, the bayonet is not touched.

 

That is the way it is suppose to be. But only the person who did the conversion knows what they actually did.

John White at AI Conversions has a lot of info posted, does excellent work inexpensively.

Hi Sandy,

 

Great minds think alike! <BIG GRIN>

"First time I've seen slotted screws in the mount - clear indication that someone "screwed" around with the lens."

 

- Slotted screws were standard on early pre-Ai lenses.

 

Removal of the rear mount is dead easy. Not even engineering 101 standard.

 

Undo the 5 slotted screws (bolts to be accurate) and lift off the mount, together with the black baffles. The Ai aperture ring can be lifted off as well if necessary, but it shouldn't be needed.

 

As Ben said, the aperture ring is a factory Ai conversion part. AFAIK, there are no other Nikkors with a maximum aperture of f/2.5, so it's unlikely to be the incorrect ring.

 

Anyhoo. It's worth removing the mount and checking that it's been seated correctly and that no-one has mistakenly fitted a shim or trapped a pile of dirt underneath it. Nikon used to sell the conversion aperture ring directly to customers, so it's very possible that it was inexpertly fitted.

 

Dismantling the helicoid is a far more lengthy and demanding job than just lifting the mount. Better to leave the helicoid alone if possible.

 

Fingers crossed it's just a bodged mount refit!

Usually when a helicoid set is mated together incorrectly, the focus is WAY, WAY off. Most (maybe all) manual focus Nikkors can be adjusted for infinity focus without dismantling the helicoid. A lot of the small AiS Nikkors are quite easy to adjust, the older Ai and non-Ai Nikkors are a little more difficult. Sorry, no experience with this particular lens, but any professional technician should be able to fix this for you.

My 105 is the "double Gauss" version with the larger rear element.

I have that vintage of 105 as well, with serial number 465xxx and it still has the slotted screws. As for the "ten minute job" this lens might bend that rule, because unlike many it has a little spring attached to the underside of the aperture ring, which is a bit of a bear to get hooked back on. If you take one of these apart to mill it yourself (as I did) leave a little more than ten minutes to put it all back together again!

Good point Matthew!

The click-stop detent varies considerably between various Nikkor lenses. Most use a spring plate with a kink or indent in it to engage with slits in the aperture ring. However I've encountered ball-bearing detents as well. These are a complete b*st*rd if the small ball pops out under spring pressure.

 

Best to exercise caution when lifting the aperture ring from the lens.

 

Incidentally, the spring plate type of detent doesn't usually present a problem in reassembly. You just have to push the aperture ring in the direction of the plate when re-fitting. Don't force the ring into place. If it catches on the spring detent it can push it out of alignment or bend it, with the result that the click stops become rough or non-existent.

This site mentioned previously gives excellent instructions for adjustment. The error is very slight, about 1mm of turn on the focus collar, so I should not need to machine the mount or flange. (I don't have access to a machine shop any longer.) Edited May 3, 2018 by wogears

Richard Haw's site is truly a national treasure for us old-Nikkor fans. The page you linked to includes a strong hint as to how to adjust infinity focus on your lens. You will not need to separate any helicoids.

Richard Haw's site is truly a national treasure for us old-Nikkor fans. The page you linked to includes a strong hint as to how to adjust infinity focus on your lens. You will not need to separate any helicoids.

Mr. Haws agrees with you. Mismatched helicoids would cause a very large discrepancy in the necessary focus travel. I probably just have a misadjustment of the focusing collar.