LTM sonnar will not focus at infinity

dexdog

dexdog

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I recently acquired an 5cm/1.5 Sonnar in LTM with serial number in the 279 series. The lens shows up in Hartmut Thieles's book, but unfortunately production dates are not available due to missing records (Beleg fehit). Lens is not in bad shape overall, but lotsa shallow "cleaning marks" on front lens element. I have tried out the lens on one of my Sonys, and have found that it will not focus to inifinity, i.e., focus maxes out at about 50 meters or so. Lens is a decent performer within 50 meters, the cleaning marks dont appear to affect contrast or sharpness all that much. Unscrewing the thing showed that the optical block does not have a variable stand-off ring, although there are tiny divots in the threaded area where the ring would usually be.

Photo of the optical blocks of the 279 series and a 280 series Sonnar also in LTM
 
Many of you are familiar with buying old lenses off eBay, often a crap shoot- sometimes you get a good lens and sometimes not. I am interested in working on the 279 series lens to try to bring up up to infinity focus if possible. The lens helicals appear to be put together correctly, lens indexes properly, focus ring and aperture work smoothly. The distance scale is off the mark, for instance showing about 7meters against a measured 5 meter distance. Any suggestions on where to start?
 
BTW, the 6 digit partial serial number on rear lens group matches the serial number on the name ring, so I think it unlikely to be a franken-sonnar built outta spare parts. Incidentally, I have a junker KMZ 1951 J-3 optical block that I will donate to Sonnar Brian in the future, and am thinking that I could use the variable standoff ring from the J-3 on the Sonnar.
 
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The lens will not focus to infinity- meaning the optics are not close enough to the image plane. No variable retaining ring- so no possible reduction of the shim. This looks like an irregular production lens.

The easy fix: unscrew the rear triplet to get it closer to the image plane. If it focus is so far off that the variable retaining ring had to be removed- the focal length is probably too short. The other possibility, the innermost helical is misassembled, and can be screwed in 1/2 turn more than it is. I found one Jupiter-3 mount had this problem, and corrected it. Not sure of the Zeiss inner helical can be adjusted the same, would have to go 1 full turn deeper.
 
Brian, thanks for the help. I unscrewed the rear lens group a couple of times, one full turn was too much, 1/2 turn was still too much, about 1/3 of a turn (120 degrees) was just right. The lens now focuses at infinity, which in my neighborhood is tree tops about 300 meters away as judged by Google Earth. Close focus still works just fine after the change. I brought the camera back inside to my measured 5 meter testing point, scale on lens mount indicated just a smidge over 5 meters. Close enough. I wrapped a bit of plumber's teflon tape around threads of rear lens group before screwing it together to keep the new set point in place. Thanks again!
 
A bit late to the show, but yeah I found that understandably the QA for these "post war" LTM Sonnars is quite spotty. From my experience from 2.79x million onward. Aka your lens is just on the threshold where the issues start.

This doesn't mean these are not worth buying (cheaply) - I have some amazing performers in the 2.8x range - however it is definetly something that should not be bought with the idea of a guaranteed of a successful repair. I had another 2.85x whose focus helical was so badly constructed that the focus range was between 2meters and 10 meters.... it now sits in a Jupiter-3 mount. But since the glass is also poorly it's a soft-focus portrait lens now.
 
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