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Leica Jupiter 8 and backfocus

mike3996

Regular
Apr 2, 2018
178
Hi everyone

So I received my very own Jupiter 8 (it's a PT3110 so very late in the game). Gorgeous condition on this specimen, aperture and focus both smooth, very smooth. There's some oil on the aperture blades but to my understanding that's not a problem.

But yeah, it backfocuses a bit on my M240. At first I hoped it'd be the adapter, because I had trouble getting it lock to the body, but no, there is some considerable backfocus.

My poorly conducted test shows around 2 inches, shot near minimum focusing distance of 0.9m (a large picture behind link)

I wasn't able to test infinity just yet, because of the weather today. And work of course. By the time I get home, it'll be too dark to conduct any kind of a test outside. Let's hope I can get it done Friday.
 

BrianS

Super Moderator
Apr 3, 2013
3,849
On the Jupiter-8: the optics barrel simply unscrews from the focus mount. You may need a rubber mat, like a mouse pad. You will see a "shim" that controls the stand-off between the optics and the focus mount. The lens is back-focusing, meaning that the distance between the optics and image plane must be increased. The shim needs to be increased About 0.15mm or so. Try two layers of aluminum foil, or paper.
 

mike3996

Regular
Apr 2, 2018
178
On the Jupiter-8: the optics barrel simply unscrews from the focus mount. You may need a rubber mat, like a mouse pad. You will see a "shim" that controls the stand-off between the optics and the focus mount. The lens is back-focusing, meaning that the distance between the optics and image plane must be increased. The shim needs to be increased About 0.15mm or so. Try two layers of aluminum foil, or paper.
you know I might be just crazy enough to try it

and thank you. I knew I could count on your knowledge :thumbsup:
 

mike3996

Regular
Apr 2, 2018
178
Hmph, I removed the lens from the camera I noticed the adapter was again actually unfastened. Could it be a problem, should I first buy a tried-n-true one before attempting to open this otherwise pristine copy?

And a dumb question, I think I read it somewhere before but the focus point doesn't align to 12 o'clock when I have the lens on the camera... should it?

Btw, now I'm glad I bought the expensive Summicron with the body. At least there's a solid foundation I can come back to. If I had begun my journey with M with old M39 lenses that misfocus badly, I'd have had a very rough start!
 

BrianS

Super Moderator
Apr 3, 2013
3,849
The focus index on many lenses comes up near the viewfinder, at about "2'Oclock" as you look at the camera.

Inexpensive M-Mount adapters often are not "exact", but this will not affect the RF/actual focus agreement on a 50mm lens. It does adversely affect calibration on a wide-angle or telephoto lens.
 

mike3996

Regular
Apr 2, 2018
178
Ok, now I got the adapter to fasten properly -- no effect on the focus as you said.

My first attempt at disassembling the lens was not successful at all. I unscrewed the three tiny screws near the mount, but that didn't bring anything open -- all it enabled was for me to screw the distance scale. I adjusted it back and did manage to rescrew everything. Everything is as it was, only now the focusing is not smooth anymore throughout the range (on an occasion).

Lots of cold sweat -- this sort of tinkering never has been a part of my passions! :(
 
Last edited:

BrianS

Super Moderator
Apr 3, 2013
3,849
With the later J-8's: no need to take out the set screws on the focus mount, the optics barrel simply unscrews.

Make sure the distance scale is lined up correctly, and that the set screws are tightened evenly. Best to tighten each one a little bit at a time, and move from screw to screw. If you tighten one down all the way, the ring will not be centered correctly and can drag on the focus ring.
 

mike3996

Regular
Apr 2, 2018
178
With the later J-8's: no need to take out the set screws on the focus mount, the optics barrel simply unscrews.

Make sure the distance scale is lined up correctly, and that the set screws are tightened evenly. Best to tighten each one a little bit at a time, and move from screw to screw. If you tighten one down all the way, the ring will not be centered correctly and can drag on the focus ring.
Oh yeah, that's right. I didn't read your instructions carefully enough :)

Second attempt at the lens, some time later. I have a good feeling about it.
 

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