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Russian Lenses on a Leica- Computing the Shim.
Old 10-01-2011   #1
Brian Sweeney
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Russian Lenses on a Leica- Computing the Shim.

FSU lenses are a nominal 52.4mm, as per the data sheets with the J-3 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 close-up and wide-open on a Leica, you need to increase the thickness of the shim as per:

Jupiter-3 and Jupiter-8

Infinity

1/(focal length)= 1/(distance)+ 1/(backfocus)

1/52.4mm= 1/infinity+ 1/(backfocus)

1/52.4mm= 1/ (backfocus)

back-focus= 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: 55.2976-52.4 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)

back-focus= 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 J-3 and J-8 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.
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Old 10-08-2011   #2
Brian Sweeney
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bump, test of Flickr BBcode, and to show how it worked with this J-3...


after the rain by me, on Flickr

So the BBCode on Flickr works here- something that I have not tried before. I used to grab the absolute URL, now the generated code on "Share This" works.

Last edited by Brian Sweeney : 10-08-2011 at 05:52.
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Old 10-14-2011   #3
David Hughes
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Thanks Brian,

The J-3 and J-8 are copies of the Zeiss Ikon lenses for the pre-war Contax and so I was wondering if this shim would work on the copies of the old Leica lenses like the Elmar and Summar in 5cm?

And the second query is, there was a Tessar for the Contax in 5cm and f/3.5 aperture, as Contax had a longer vesion of the nominal 5cm focal length, then would that be the Industar 22 or 50?

Or had the Soviets started to move away by then and so were producing their own designs?

Lastly, presumably the shim is only needed for close work; meaning I can make one to fit between the lens and camera body and only fit it when needed? And focussing adjust slightly?

I'm sorry to lumber you like this but your article started me thinking about things. Usualy I'm wary of using a 50mm lens for close shots and haven't bothered too much about it. And f/2 is seldom needed when I'm playing with the things or giving them a work out or check.

Regards, David
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Old 10-14-2011   #4
Brian Sweeney
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I would think the pre-war lenses were built to the Leica standard, but have never had one. I believe the post-war lenses were built to the Zeiss standard as a result of bringing the equipment and parts back to Russia. I have shimmed a couple I-50's in the same fashion for a Leica, and one I-22.

On the close-up/distance work: for a lens that uses a rotating Cam, such as a later J-8, I-22, or I-50- you could hack an Indexed-Cam by building up only the portion of the cam for close-distance. Example: use a layer of Copper tape on the portion of the Cam from 0.9m to 4m. Once past 4m (or so), you are back to focus collimated for infinity. Building up the RF Cam of a Normal (5cm) lens has the same effect as thickening the Shim between the optics module and the focus mount.
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Old 10-14-2011   #5
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Do Jupiter 12s require the same adjustment, or does the fact that it's a wide angle with a maximum aperture of f2.8 make this difference negligible?
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Old 10-14-2011   #6
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I adjusted the J-12 for the Leica. The rear module unscrews, then the front comes out. Shim is underneath. I used copper tape to build it up.

But- that is really pushing it for "required" vs "I know it's there".
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