JUPITER 8 Focus tweak on M8 - Help needed

[nasdak]

Hi everybody,

Here's my first post at the RFF forum 🙂
I'm a french man really interrested in RF, i own a M6, M8, Fed2 and some glasses.

I recently bought a Jupiter 8 for my M8.
It was out of focus and the focus wasn't smooth so I've opened it to clean and relube it.

The hardest is now to have a proper focus...

I understand the only thing to change is the distance from the optical bloc to the focus bloc with some shims.
that's what i did (ading 2 papers shims) to get good focus at F/2 at 1 meter ; but now it front focus at 3 meters and infinity.
If i put only one shim, it's ok at 3 meters, but back focusses at 1m....

So i'm a little bit confused... I could try another thread when screwing the focus block to the lens block but the one i'm using seems fine.

Could someone help?

PS : my M8 RF is perfectly aligned with my other lenses (35 summicron and Industar 61)

thanks !

 

[LeicaFoReVer]

Brian is the pro for those! I am sure he is going to notice the thread and help you out.

To my knowledge the jupiters are designed for contax lenses so it wont focus if you dont tweak it in order to use them on Leicas.

 

[nasdak]

well, waiting for brian 😀

 

[Phil_F_NM]

If I recall correctly, many of the classic Sonnar designs have a focus shift at wide apertures that is definitely noticeable on the M8, M9 and R-D1. The lens can be easily optimized for a focus range at aperture, but they aren't perfect at every stop & the whole focal range. But that's the cool thing, since it's easy to collect a few stellar J-8 samples and have one for each purpose.

 

[ampguy]

Brian should be answering soon. I had him adjust my J8, and I'm pleased with the focus now. Before it had front focused wide open.

I think he uses foil instead of paper. There is some shift in the design, so if you optimize it for f2, you may be off at f4 and beyond.

However, if you mainly use it at F2, you'll want it adjusted there so you don't have to do the nod back or focus on air, or get softer images.

 

[Sonnar Brian]

Lunch Break...


I've found it necessary to get within 0.02mm for "best" agreement, and the paper shims are probably thicker than this. SO: plan B. Use one paper shim, and use the variable stand-off ring for fine adjustment. Undo the set screw on the variable ring, and unscrew it slightly. Then use 1 paper shim. Get the focus so that it is optimal for close-up and wide-open. The J-8 has a slightly longer focal length than the Leica, so it is going to front-focus at longer distances. Stop down a bit, to F2.8 or F4- the Sonnar focus shift will push the focus back.

 

[nasdak]

Thanks brian 🙂
i don't have the proper screwdriver to get the tiny standoff ring screws, but i used 2 papers shims (plus the 3 alloy shims already in place!). This way i get good focus at F2/ 1mt.
But it's really off at 5 meters... even at F/4 i can still see it's front focussing.
May i have a wrong helix jupiter 8 ?

 

[nasdak]

another question : the only way to fix the focus is via shims? Nothing to do with the focusing tab ?

 

[Sonnar Brian]

Some J-8's and J-3's are on the "long-side" of 52.4mm, and some are on the "short-Side" which is closer to the Leica standard.

You can try another copy of it, or you can try moving the rear optics group in closer. The latter will shorten the focal length, which is the real problem. The older J-8's have a rear optical fixture that unscrews as a unit. You have to polish a little bit off the the main optics fixture so that the rear can be screwed in a little deeper. I have done this on three J-3's, never had to do it for a J-8.

Lunch Break is Over--- will check back later.

 

[nasdak]

Thanks a lot for the help 🙂
i'm not that good with technical english ; would you be kind enough to tell me wich part i have to polish :


Uploaded with ImageShack.us

(part 2 is upside down)


(shot with summicron 35 at F/2 - focus right on! )

 

[Sonnar Brian]

I have several spare J-8 modules, this one is like yours.

1) Remove the Optics Module from the focus mount.

2) Take the rear optics module out.

3) I would remove all the glass for this next step: You need to polish down the threads on the main optics fixture.

The idea is to reduce the spacing between the front and rear group by ~0.4mm to reduce the focal length.

The hard part is keeping the metal filings from getting into the lens. I have stuffed lens cleaning paper against the aperture, which works many times. Other times I have flood cleaned the aperture after some got through. This is a "last resort" operation for me.

After polishing down the rear, screw the rear optics module back in, place into the focus unit, and adjust the main shim. The focal length is "ever so slightly reduced", and should help with maintaining focus at both close and far distance. It will be better, not perfect.

This is one of those "Try at your own risk" hacks. I only do this with my own lenses, as there is risk of destroying the lens.

 

[nasdak]

you're by far the best.
I will try that this WE as i'm a bit of a DIY personn
i'll keep you in touch !

this jup8 already performs not so bad 🙂

 

[Sonnar Brian]

The 1950s KMZ J-8's are first rate, I can tell yours is one of those by the focus knob. I'll look for some up-close/distance shots with my modified J-3. All I have in the gallery are close-up shots.

 

[nasdak]

it's a '55 one, really beautiful with that focussing knob (type III from the jupiter 8 service pdf)

 

[nasdak]

The idea is to reduce the spacing between the front and rear group by ~0.4mm to reduce the focal length.

I just realised that 0.4mm is really small.
I do not own the specific tools to measure nor to polish ; i think i could simply gently rub the lens unit on fine sand paper placed on a flat surface ?

 

[Sonnar Brian]

Yes- I use sand-paper layed flat in a box, and use circular motions over the paper. Do a LITTLE bit at a time, and check the results using a through-the-lens viewer. You might also use a marker around the portion to be polished, mark off the amount to polish, and take it down until the marks are gone. AND: the 0.4mm is "nominal", some lenses require more, others less. This is a "hack a little and test", repeat if necessary.

 

[nasdak]

Yes- I use sand-paper layed flat in a box, and use circular motions over the paper. Do a LITTLE bit at a time, and check the results using a through-the-lens viewer. You might also use a marker around the portion to be polished, mark off the amount to polish, and take it down until the marks are gone. AND: the 0.4mm is "nominal", some lenses require more, others less. This is a "hack a little and test", repeat if necessary.

Thanks Brian.
I tried it yesterday ; i checked the result directly on the M8 (what's a true-the -lens viewer?).

I'llt try to explain everything so you can tell me what to do.

Lens untouched :
3 alloy shims
1 meter : focus is 30 cm behind (back focus)
10 meters : didn't check.

BEFORE polishing :

3 alloy shims + 2 paper shim
1 meter : focus is good
10 meters : focus is 1 meter before (front-focus)


AFTER some polishing :
3 alloy shims+ 1 paper shim
1 meter : focus is good
10 meters : focus is still in front ; maybe a little less


So after polishing i need only one paper shim to get proper focus at 1 meter, but focus at 10 is still of.
Should i keep on polyshing? I don't know how much i polished, maybe 0.2 mm or so

Pierre

 

[Sonnar Brian]

You lens was WAY off when you started. I'm used to them being a little off, like 3cm at 1m. I've had a couple really far off- some required an extra 0.5mm shim.

Try polishing it down a little bit more. It is going in the right direction. You will probably not get it to be perfect at distance. That is where stopping down the lens helps: the Sonnar focus shift moves the best point of focus backwards. SO: the optimal becomes Close-up and Wide-open, Distance shots stop down to F4.

 

[nasdak]

Thanks,
i polished a little more, i cannot do more, the distance between the 2 lens module can't be shortenned more.

I now need 2 shims to be spot on at 1 meter, but it's still off at 10 meters and infinity ; i think i'm gonna look for another one jup8 !

 

[Sonnar Brian]

Some of them are on the long side and cannot be brought under control. I took several millimeters off of a 1950 J-3, could get it better- but never correct. When I compared the optics fixture against later ones, it was easy to understand. It was just WAY too long. I ended up parting out the lens to fix two others.

1955 saw a change to the manufacturing of the J-8, I know the design of the focus mount changed. Yours is one of the first with the three screws holding the mount together. 1955 is also around the time that the German glass was used up, and production was switched over to Russian glass. I think it took some time to get the new forumulation down.