Leica Adapting a Jupiter-3 in Contax mount for optimum wide open performance


In the past I had a 1962 Jupiter-3 in LTM, that was impossible to focus consistently (front focus issieus well known because of different Leica body register). I had adapted it myself through some twisting to work superbly wide open. Unfortunately I sold it. I regretted it the day after. The day after blues. Have not gotten over it.

Then I bought an Amedeo adapter and a J-3 in Kiev mount. This is a superb lens, at F2 I can enlarge to 50x60 and see all sharpness I need. Absolute stunning for portraits. See next post. .
However I would like to have one [also] with full opening F1.5 focus at 1.5 meters.

So. I bought a new J-3 lenshead.
Question: how can I disassemble it? Then what should I turn, and how securely lock it in the right position once I find a good plane of focus?
I know I should better leave it to the professional. But I can try.

Now i tried to turn it at the outside: did not move.

Looking at the side I saw that the metal has been stamped at four places. Is this A primitive way of locking the barrel? Then it will be hard to open up . .
I definitely do not want to open the innner ring holding the rear parts of the lens.
There should be a set screw holding the optics module in place, near the base of the mount. Once this is out, the lens should unscrew.

If you are still having problems, later today I will post an image.


Yes, one single screw, about 2 mm from the bottom.
Then, if I turn it loose, I expect the head will be able to turn loose from the filter ring up front.
I also expect that if I only unscrew the lens by say 90 degrees the focus will be optimum at F 1.5. Are my expectations too high ?
Then I should make a new hole in the inner part, screw back the lens and presto.
One full revolution of the barrel in the mount is 0.5mm. A 90degree turn is 0.125mm, and is usually just right.

You would be best to mark the position of the barrel and lens for the 90degree turn, and make a paper or aluminum foil shim to hold the lens in place, to make sure the lens is firmly in place. The set screw trick will work, BUT- the lens will be somewhat loose in the mount.


I have used brown baking paper in the past (it is coated) and it can be easily cut (to the contrary of aluminium foil). This is measuable to be about 100n for a layer.
So I should take the head out completely, add the little paper ring, place it behind the existing shim for sturdiness, and place the lens back.
The making of the new recess for the screw is hardest to do.
Then light a candle.

The good thing - the effort is non-destructable and can be reverted easily if not satisfactory.
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Thanks. I did it, worked out fine.

Apart from that I to turn it off, I held the diaphragm ring also in the hand, and that slipped a bit so that is off-center now, display does not show 1.5 but just before that. :-((
Stupid me. But the black ring on the outside is sooo small. Should have used some device like a very small rubber band.
Anytime that you change the shim- the aperture must be re-indexed, it will not line back up with the marks until you do so. With Contax/Kiev lenses yu can either undo the three set screws that hold it in place, turn the aperture ring, then put them back in after making new Taps OR just cover over the old index mark and make yourself a new one. I use a silver Sharpie (Marker) to fill the old Index mark and Red nail polish to make a new one.

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