Leica M4-P and Jupiter-12 black focusing problem

[zanxion72]

I have tried a black Jupiter-12 on my Leica M4-P with a cheap M39 to M adapter and I have a strange problem. Focusing seems fine in the rangefinder down to 1m when I mount the lens and turn the focusing ring for the first time. After I focus at infinity, going back to 1m it seems that the rangefinder does not follow the focusing of the lens from 3-5 meters and closer. In the range from close to 3-5 meters the double image does not move at all. From 5 to infinity it does and it is pretty accurate.
I have also tried with an Industar 26 and the focusing is perfectly accurate from close to infinity.
Could it be that the rangefinder sensor looses contact with the back of the J-12? Is it possible that the J-12 is not compatible with my M4-P?
I thought that the rangefinder was out of alignment, but then it would be for the other lens too, am I correct?

 

[antmar]

J12 is fully compatible with your camera. Make sure that the helicoid is following the focusing ring at all focusing range. A small screw holds the focusing ring to the helicoid and this can be loose. Αν μπορώ να βοηθήσω μου λες...

 

[Ko.Fe.]

J-12 are strange lenses. It is known to have RF focusing issues with them at LTM FSU cameras.
It was acting weird on my FED-2. I have to bend RF arm, which is known method.
I never did it with M (where are less problematic and not so expensive old LTM 35mm lenses), but you could try same method of inspection.
With shutter open on B, look through the back (back door with pressure plate is removable) and see what it going on during focusing.

 

[zanxion72]

I have thought of that too. I have opened the back and tried to watch what is going on while focusing, but that damn rear element covers almost anything. The rear of the lens where the rangefinder arm is supposed to rest seems to rotate all the way. I think that the rangefinder is in a tight space and touches something preventing it from moving smoothly from near to infinity and versa.