Zorki/FED lenses - RF coupling question.

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Grytpype

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How reliable is the positioning of the range-finder coupling ring on the back of FSU lenses?

I'll be doing a CLA on a Zorki-4K with Jupiter-8 sometime soon. The lens-mount needs shimming, and I can check the infinity focus of the lens when I've done this, but before I tweak the range-finder I'd like to be sure the lens coupling is correct, or I might be doing more harm than good.

Does anyone know what the exact position of the coupling ring relative to the flange should be at infinity focus? This must be part of the LTM specification. Is it adjustable, or does it rely on accurate machining at the factory?
 
Thanks for the replies.

I know that the lens-mount needs shimming from measuring the depth from the flange to the film-plane with a depth micrometer (it's about .005" short) but I only mentioned this, and checking the infinity focus of the lens, as being the things I need to do before checking the range-finder.

Really the reason for my question is that there is no point in adjusting the range-finder for the lens on the camera if it is then going to be wrong for any other lenses I might fit, so I want to be able to check that the position of the RF coupling ring on the lens is correct. Obviously, for the range-finder to work accurately, the coupling ring at the back of every lens must be in the correct position relative to the flange at infinity and all other focus settings.
 
Sorry Pitxu, I didn't make myself clear. It is the lens-mount on the camera I am referring to when I mention shimming, and 5 thousandths of an inch (.13 mm) is well out of tolerance here!

I haven't been able to find any details online on the LTM specification, but it must include the position of the coupling ring on the lens relative to the lens-flange, at infinity focus, which is really what I am looking for. If the FSU lenses are all made to correct tolerances then I don't need to worry about it, but are they?
 
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Pitxu said:
The "coupling ring on the lens relative to the lens-flange, at infinity focus" is standard for all FSU made cameras and lenses in 39mm LTM.
View attachment 64055
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That is really the question - can I rely on it being correct? As I mentioned before, I don't want to set the RF for the lens that happens to be on the camera, and then find every other lens I might get is different. If I know what the correct position should be, I can check my lens and make allowance for any error.

The shimming of the lens-mount on the body is not a problem, and really nothing to do with my question. I just mentioned it to show that I'm not going off half-cocked and adjusting the range-finder without checking this first!!

I must admit I had wondered what that paragraph in the manual meant, Pitxu. It does seem to be saying that the lenses are not really interchangeable at all, and you would have to set up each lens to a particular camera, and only use them on that camera, which does not seem right at all.
 
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I'll chip in here for what it's worth...

First, the correct film-to-mount distance is 28.80mm +/- 0.02mm, that's what you need to set on the camera body. Next, it's near-impossible to adjust the coupling cam on any FSU lens. However, it *is* possible to adjust the optical module so that the lens focus agrees with the cam (which then agrees with the RF too). On the J-8, the whole optical module simply unscrews and behind it there is one (or more) shims. With the lens at infinity, the body correctly agreeing with that and the lens module shimmed correctly, the lens will indeed be at infinity and the RF will say so too.

Speaking from my own experience, I've only found one FSU lens (a J-8, as it happens) to be far enough out to need adjustment. Kim Coxon altered it for me but I was told that it was a case of altering the focal length by altering the element spacings (not easy) rather than correcting the shimming. There was also a comment that it was unusual for it not simply to be a case of shim-adjustment needed.

There were some non-standard early model FEDs and Zorkis, where the body was shimmed to match the lens so they were, essentially, a matched pairand not strictly "interchangeable". For later bodies and lenses, however they were (in theory) fully interchangeable.

As Pitxu suggests, try it all out first and see what the results are like. Make sure you've calibrated the RF at 1m and also at infinity (I'd suggest a target at least 500m away). Then go from there.

One last point: if the lens-to film distance is not correct, then a lot depends on the depth-of-focus of the lens (and that is NOT the same as depth of field). Depth of focus is a feature of the lens and will vary according to design. Therefore, some lenses may work well whilst others do not on a body with incorrect lens-to-film distance.
 
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Grytpype said:
I haven't been able to find any details online on the LTM specification, but it must include the position of the coupling ring on the lens relative to the lens-flange, at infinity focus, which is really what I am looking for. If the FSU lenses are all made to correct tolerances then I don't need to worry about it, but are they?
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I think I have seen this spec somewhere but I can't recall where - try the optics or Leica sub-forums. Measuring a random selection of FSU lenses gave me 7.45mm and they were very consistent.
 
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Many thanks for all your help, Pitxu and wolves3012. That looks like an extremely useful link, Pixtu. Somehow in all my searches I have never come across it before. I will spend some time browsing through it.

It is good to know that the coupling ring position appears to be reasonably consistent. I've actually got two J-8s, so once I have checked their infinity focus I will measure the position of their coupling rings for comparison.

Probably, as you say, the lenses will turn out to be accurate enough. It's just that on a rangefinder camera you have to rely on quite a lot of things being right for good focus, and several small errors can add up to one big one! On an SLR, as long as focus indication in the viewfinder matches true focus at the film plane, any other errors don't matter much.

You may well have read it already, but I found an interesting article at http://www.dantestella.com/technical/compat.html . It is quite long, and there are one or two "typos", but he goes into detail about how the rangefinder coupling works on lenses other than the standard "50 mm" lens and FSU/Leica incompatibility.
 
A brief follow-up: I still haven't found any authenticated spec. for the LTM flange/coupling ring distance, but according to this thread, http://photo.net/leica-rangefinders-forum/00QLpC it is 7.5mm at infinity. I attempted to confirm this via an email to Leica, but haven't had a reply.

I've checked both my Jupiter-8s and though I haven't proved yet that their infinity focus is correct, I get figures of 7.53mm and 7.56mm. So if I average these out with wolves3012's results, between us we get pretty close to the correct result!
 
My Leitz has a flange>RF contact distance of 7.5mm, just like my I-50. The differences you talk about are hardly going to have any bearing on the infinity focus and I don't think you should get too precious about this.
 
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