Nikkor 5cm f2 LTM – different shim sizes

[lukx]

Hi all, I have two black rim Nikkors that I am trying to make one nice and one bargain lens out of.

One lens (#729...) has nice mechanics, a good clean barrel, and is modded for 0.7m coupled focusing. But it has some internal coating issues and some semi bad wipe marks front and back. This lens is a good performer stopped down but is quite soft and glowy wide open. I suspect the defects in the glass contribute to that to some extent. The other (#760...) has okay focus mechanics that need a CLA, is not focus modded, has a less nice cosmetic appearance, but much nicer glass.

I would like to enjoy the best of both worlds if you will and merge them into my perfect little Japan-Sonnar.

SO: I found that, while looking identical when comparing the lens blocks and mounts, the lenses have different shims installed.

The first lens (#729... with good mechanics) has two shims stacked, one 1.24mm, one 0.31mm, for a combined 1.55mm. I found that this combination achieves good focus accuracy on my Leica at f2.8 with the originally installed, faulty glass. The other lens (with the nice optics) came with just one shim of 1.35mm. So a total of 0.2mm difference. So far this lens is not film tested on my Leica.

If my studies of Brian Sweeney's work on Sonnars have taught me anything, such a difference would result in a substantial amount of focus discrepancy when applied falsely.

MY QUESTIONS to the experts:
(Obviously whatever I do I will ultimately need to try it on my Leica anyway, but I am looking for a good educated guess to start with.)

Would you say I should swap shims when swapping optics blocks, or should the shims stay with the mount?
Does anyone have any experience with the shims in these lenses? How much variation was there?
Are my thoughts moot, because shims are chosen for a distinct combination of optics and focus mount, with too many possible combinations?

I have a digital Fuji I can test with, or film test with my Leica. No digital Leica with live view available. I would prefer to not tear the optics units apart and swap all elements from one housing to another. So trying that would be my last resort.

Thanks in advance
Lukas

 

[Sonnar Brian]

The shims are set individually in each lens to account for slight variations in mechanical and optical tolerances. Once you make the best-of-two lenses, you will need to shim the new lens for best performance.

I've done what you are doing now with the Nikkor 5cm F2: transferred the good optics from an S-Mount lens to the barrel of an LTM lens with damaged glass. Once that was done, set the shim and had a lens that focused across the full range.

As you have a mirrorless camera, before tearing the lenses apart- test them at infinity and at 1Meter. Make a test target of a tape measure, carefully mark where you are using the mirrorless camera. Once the optics are transferred, set the shim using the mirrorless camera set up as it was before. Start at 1Meter and wide-open, then test for infinity. If the focus across range is good, the shim is set correctly and the focal length is good. Focal length depends on the front and rear section of the lens being the proper distance apart.

 

[lukx]

Thanks for the input, Brian.

I have tried both lenses in different combinations of shims and mounts on my Fuji. It seems like the lenses focus more accurately when the shims stay with the respective lens block, but the results are inconclusive as my adapter seems to be too short for proper focusing. All my lenses focus past infinity (the moon, in my case) with this combination, making it useless for my testing purposes. A common issue with cheap-ish mirrorless adapters.

So it seems I have no way of proper testing apart from film testing with the Leica. Hence I have resorted to cleaning up the new lens and see how good (or bad) the mechanics really are. Maybe it'll clean up nicely, they are very well made after all...

 

[Sonnar Brian]

If the lens is focusing past infinity with your adapter: you could make a shim to fit between the lens and the adapter to correct the distance scale. I would start with paper rings, cut so they fit around the base of the Nikkor. It's thick enough when the Infinity scale and actual focus agree, with the original lens with its original shims.