>>3246897You can read the specs straight out of the factory or measure the actual focal distance on the bench.
Russian lenses and cameras were made to the (contax) 52.4mm standard, ±0.5mm. Leica (Canon, Voigtlander) lenses were made to the 51.6mm standard.
Maybe you got lucky with manufacturing tolerances erring to your favour and getting a Jupiter close to 51.9mm or lower (52.4-0.5). The camera body and rangefinder calibration simply expect a slightly different focal length,
To optimise it for close distances (or all distances) you need to shim it. In 2 places, if you want the focus not to shift across the range, even though most people optimise for close distance and let DoF take care of it when focusing further away.
Good instructions here:
>http://pentax-manuals.com/repairs/j3service.pdf 