Jupiter 8 adjustment

[wolves3012]

Continuation of a couple of posts in a J-9 related thread...

I want to adjust a Jupiter 8 to focus properly on a Leica IIIC (Kim: note Jupiter EIGHT not nine).
At infinity or medium distance the focus is fine anyway, probably due to DOF. I've tried it close up (1m) and wide open and the true point of focus is slightly further away than the RF is showing (is that back or front-ocussing then?). Carelessly I didn't use a ruler to find out just how far out it is, but it's a few cms. Assuming the lens scale is correct, it says about 1.1m when the RF is set on a target at 1m.

So, the question is: do I need the move the rear group forwards by removing metal or do I shim it backwards? Presumably I'll need to thin the main shims under the optical block in order to keep infinity correct? Suggestions on how much to adjust please...

 

[wolves3012]

Hold fire on this post - Kim Coxon has PMed me about it!

 

[Kim Coxon]

The lens is back focussing if the focus is correct at infinity. My comments on the J9 apply equally to the J8. However, it is very rare to find that the J8's have a problem. I would suspect that it is also out at Inf but not noticeable under normal shooting. Much will depend on which version of J8 it is but shortening the optical block, if that is what is needed could be difficult. The differences and tolerances are much less in the J8 due to it's shorter focal length. I would be tempted in this case to adjust the lens for close focus using the main shims and then see what the effect is at infinity. In this case, you need to increase the shimming. If the lens is focussing beyond the subject, it needs to be moved frther forward to focus in the correct place. 😉

Kim

Continuation of a couple of posts in a J-9 related thread...

I want to adjust a Jupiter 8 to focus properly on a Leica IIIC (Kim: note Jupiter EIGHT not nine).
At infinity or medium distance the focus is fine anyway, probably due to DOF. I've tried it close up (1m) and wide open and the true point of focus is slightly further away than the RF is showing (is that back or front-ocussing then?). Carelessly I didn't use a ruler to find out just how far out it is, but it's a few cms. Assuming the lens scale is correct, it says about 1.1m when the RF is set on a target at 1m.

So, the question is: do I need the move the rear group forwards by removing metal or do I shim it backwards? Presumably I'll need to thin the main shims under the optical block in order to keep infinity correct? Suggestions on how much to adjust please...

 

[wolves3012]

Right, I did some quick measuring...based on the lens scale being correct I need to shim the main block out by 0.1mm - would that sound about right? That would leave infinity focussing at about 15m, assuming (!) the lens was correct initially.

Kim, I think I may just leave this to your expertise!

 

[Kim Coxon]

I use the distance marked on the lens purely for a guide to DOF. I do not believe it is as accurate as an RF. 😉 As a result I teld to collimate to a known rangefinder rather than distance scale. For me an accurate diastance scale would only be important in macro cases or when using such things as Close Up lenses, flash etc.

Kim

 

[mike goldberg]

Hi...
I hope this helps someone; I've never opened a lens in my life 🙄

Early in 2007, I bought a J8 from Brian Sweeney, collimated or modified to the Leica standard. In his tests, he found it to be as good as his $175- Nikon LTM 50. Without a doubt, this is my favorite LTM lens... accurate at Infinity and close focus.

In an exchange of emails at the time, Brian said," If a J8 is back-focusing, most of the time a 1/4 turn to the left of the [rear] standoff ring, is the fix."
Good luck guys,
Mike