Infinity focus stop adjustment

[nussbusch]

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I have the following focus problem. 10+ Leica/Zeiss/Voigtländer lenses have a perfect infinity stop on my M11 Monochrom. Only my slightly used “new” APO Summicron-M 2/75 stops at infinity at a distance of about 150-200m. You can also clearly see a very small stray circle on star images. Stopped down, this is not noticeable, but at open aperture it is disturbing. A technician has now returned the optics as “in tolerance” without having done anything (after a 2-month turnaround). The Rangerfinder focus fits within the usual accuracy of a 75/2 at portrait distances anyway. Has anyone adjusted the focus stop themselves? How is this realized internally? I'm quite confident that I can do this myself, but of course it would be helpful to get instructions or an explanation. Another side issue: how is the ease of movement of the focus ring adjusted? I have quite different resistances on my Leicas.  Thank you!

Edited May 8 by nussbusch
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[Al Brown]

Meiner bescheidenen Meinung nach wäre es viel besser, Fragen auf Deutsch an den deutschsprachigen Teil des Forums zu stellen. Die Erfolgsquote der Antworten könnte höher sein.

[sinjun]

Not directly relevant to your problem, but I have an issue with a 35mm Summilux pre-asph. The lens has good rangefinder calibration at short distances but achieves infinity focus with 10 m on the scale, and at the hard stop is clearly out of focus at infinity even at f8. After a lengthy spell away at Wetzlar it was returned as 'within specification' when it is clearly not. It does not inspire confidence in Leica technicians.

[adan]

4 hours ago, nussbusch said:

The Rangerfinder focus fits within the usual accuracy of a 75/2 at portrait distances anyway. Has anyone adjusted the focus stop themselves? How is this realized internally? I'm quite confident that I can do this myself, but of course it would be helpful to get instructions or an explanation. Another side issue: how is the ease of movement of the focus ring adjusted?

The 75 APO-Summicron has a floating-element system, one of the first designed for Leica's M lenses (2005).

When being focused, the last two elements (or cemented group) slide in and out independently of the rest of the lens, on a focus helical of their own. The optical formula actual changes slightly for close-focusing, as the element spacing changes. (The intention being to maintain the excellent image quality all the way down to 0.7m, even as the mathematics of bending light rays changes in the close range).

However that system has been noted for both stiffness (friction from the additional moving parts) and for occasional differential focus problems (off at infinity, or off at 0.7m). Especially among early units, made before the move to the new, improved factory in Wetzlar. Bottom line - the floating element helical can "derail" (entgleisen) and make the optics no longer the exact design Peter Karbe intended.

It is not common, but it is not exactly rare either. There was a time, ~2007-2014, when I tried five different used 75 APO-Summicrons, and they all had inconsistent focusing, as well as other evidence of optical positioning problems (abnormally high color fringing, soft corners, etc.) And the one I sent to Leica, with photo examples, also came back with the problem not fully cured, but marked "within tolerances."

Adjusting it will not be a matter of a simple infinity stop. Your lens may well be at the correct stop - but the optics themselves (fixed and floating) may not be aligned and spaced as they should be at that stop.

Edited May 8 by adan

[spydrxx]

Not sure where you reside, but if the lens was mine, I'd contact either DAG or Sherrie Krauter.

[dpitt]

Although he has a job queue as long as Wetzlar, I would recommend contacting Will van Manen in the Netherlands. Kamera Service
In the past 20 years he saved some of my most difficult collection items with issues, one even after a failed attempt by an other lab.

Edited May 9 by dpitt