Russian Lenses on a Leica Computing the Shim.
FSU lenses are a nominal 52.4mm, as per the data sheets with the J3 that I have seen. Leica RF's are set for a normal lens of 51.6mm. The Russian lenses are set in their mount so that Infinity agrees on a Russian camera and on a Leica camera.
SO: of you want the Russian lens to agree closeup and wideopen on a Leica, you need to increase the thickness of the shim as per:
Jupiter3 and Jupiter8
Infinity
1/(focal length)= 1/(distance)+ 1/(backfocus)
1/52.4mm= 1/infinity+ 1/(backfocus)
1/52.4mm= 1/ (backfocus)
backfocus= 52.4mm
at 0.9m
1/52.4mm= 1/(900mm)+ 1/(backfocus)
0.01908mm= 0.001111mm+ 1/(backfocus)
0.07969mm= 1/(backfocus)
Backfocus= 55.6514mm
Travel from Infinity to 0.9m: 55.297652.4 and is 3.2514mm
Leica:
Infinity
1/(focal length)= 1/(distance)+ 1/(backfocus)
1/51.6mm= 1/infinity+ 1/(backfocus)
1/51.6mm= 1/ (backfocus)
backfocus= 51.6mm
at 0.9m
1/51.6mm= 1/(900mm)+ 1/(backfocus)
0.019380mm= 0.001111mm+ 1/(backfocus)
0.0182688mm= 1/(backfocus)
Backfocus= 54.7380mm
Travel from infinity to 0.9m: 54.7380 51.6 and is 3.1380
The difference in travel between the FSU lens and the Leica is 3.2514 3.1380, 0.1134mm.
An FSU lens collimated for Infinity on the Leica needs to be 0.1134mm farther from the film plane than what the Leica is calibrated for. I usually end up increasing the shim by ~0.12mm in most of the J3 and J8 lenses that come my way.
There is a lot of variation in the Russian lenses, about 80% of the ones that I've seen need the shim increased about 0.1mm~0.15mm. Agrees with the math.
You can use the same formula to compute the required change in thickness of the shim after measuring how far the focus is off on your camera. After taking some pictures of an object at a known distance, substitute the measured distance using the RF and the actual distance in focus in the image. Usually a ruler is a good candidate for this. The difference in the Back Focus between the actual distance and measured distance gives you the required change in thickness of the shim.
