Jupiter 3 Focus Question

[Classique]

I recently purchased Jupiter 3 from RFF member (pleasant transaction btw) and took a risk on whether it will focus correctly on canon P, leica LTM.

I've been reading up on the focal length difference between FSU ltm lenses and leica standard and was ready to add some shims to compensate for that. However, what I discovered was that the lens actually front focuses! (distance scale show focus at 1.25m when lens is in focus at 1m)

From what I've read, I need to somehow make the lens optical group closer to the film plane. The original shim was 1mm thick and even without any shim, it front focused about 0.1m. There were few threads that mentioned that some lenses show this behavior and only way to solve it is to reduce the distance between lens element groups to make the effective focal length shorter. Unfortunately, all the images associated with those threads were no longer available and I am actually lost on how as to achieve that.

If you have any similar experience or remedy, it is greatly appreciated.

 

[gb hill]

Mine was front focusing & I had to reshim it to work on my Bessa R. You know how to unscrew the lens already I assume because you know the shim inside is 1mm. Unscrew the lens & adjust the section that contains the rear glass which is threaded. Unscrew it until it stops if I remember correctly. (been a while) What you are doing is adjusting the rear element close to the film plane. It may already be set. I had to exchange my 1mm shim for a paper thin shim. This will also bring the rear element closer to the film plane. I made my shim out of plastic. works great. Sorry I can't remember about any screws. Here is a photo after I fixed it. It wasn't hard. Good luck with the fix.

 

[wolves3012]

The man to ask would be Brian Sweeney but he's not using this site at present, sadly. You can find an article he wrote on the J-3 here:
http://pentax-manuals.com/repairs/j3service.pdf

That may help, although is does not appear to cover reducing focal length. I've never had a J-3 apart but I believe it's similar to the J-8, in which case the rear element(s) separate from the front. You can reduce the focal length by removing metal from the front piece, reducing the groups' separation. Sand it down, very carefully, by a rotary action using fine emery cloth laid on a sheet of glass. You need to keep checking how far you've altered things since (obviously) if you take too much off it's hard luck! I take no responsibility if you attempt this, proceed with caution!

Also, you'll need to remove the metal dust from the elements, something that's inevitable in the process. Take great care just to use something like canned air, since the dust will easily scratch the coatings if rubbed.

 

[gb hill]

I think all you need to do is to replace that 1mm shim with a thinner shim. I would try that first before I sand any metal. Try a shim as thin as a milk jug. I have a J8 that I bought that was shimmed for a Bessa R. I looked at the shim when I got it & it's made of thin plastic. That lens is nice & sharp too.

 

[Ron (Netherlands)]

Currently I have a little collection of J3's. Only the ones before 1959 seem to be out of focus on the Leica, the others are ok - but I guess since tolerances are not tight with FSU lenses, that there are J3's as from 1959 may be out of focus

 

[Classique]

My Jupiter 3 is from 1961 and from the front focusing nature, a rare one with opposite problem to what is usually done. I took out all the shim, screwed in as far as I can and it is still not focusing far enough, which leads me to believe that I can't solve this problem with shim adjustment. It is unfortunate because the optics are pristine.

Do you know which part I need to sand down? I was able to take out the rear optical group and all I can see that may be sanded is a bump that holds the rear group in place.

I think I also read about sanding down some of the threads where the optical module screws into to get the module closer to the film plane but not too sure if that would work.

 

[wolves3012]

Do you know which part I need to sand down? I was able to take out the rear optical group and all I can see that may be sanded is a bump that holds the rear group in place.

I think I also read about sanding down some of the threads where the optical module screws into to get the module closer to the film plane but not too sure if that would work.

Unfortunately, I'm not familiar enough with the J-3 to give you good advice on exactly which part to sand. On the J-8 it's the front piece containing the front optics, I think the J-3 is similarly constructed but can't guarantee that. Whatever you do, be sure that you're doing the right thing before removing metal, it's not a reversible process and you might junk an otherwise useful lens - and not the cheapest of FSU glass either.

 

[Classique]

No problem. Thank you for all your support.

I think I am almost there on fixing this lens! Luckily I found the posts about close focus mod and it helped me get the optics close enough to have almost perfect focus. Now, all I need to do is reduce the depth of coupling ring to make it match the rangefinder. I am so excited!

 

[Classique]

Quick update for anyone that may be interested (for the future).

I have done the close focus mod, which brought the optical unit closer to the film plane and somehow that resulted in distance scale matching up with the lens's actual focus.

After that was done, I noticed that the rangefinder coupling to the camera was off, which was expected from screwing in that section further in than normal. Although I am not for messing with vintage goods, I decided that this lens can be a great user (most likely I will keep for some time) and sanded a bit of the coupling portion of the lens.

Many hours later... the lens is ready!
Now, the lens focus is right on with the distance scale AND the coupling is accurate also. The weird thing is that, after all this was done, I had a whole new lens. When I was testing the lens on my GF1 to see what I can expect on film, before I noticed a lot of glow (sort of hazy look) when the lens was wide open and also when stopped down to F/2. However, after all this modification, the lens displayed MINIMAL glow (there is a slight amount which will greatly benefit portraits) and became sharper!! I am amazed that this happened and I can't explain how... but I am glad it did. (All I have left is to file down small amount of the mount so that my Canon P coupling arm can follow the lens down to what I'm guessing is around 0.75-0.8m)

I am going to test on film to see what I get from this. I hope it's a gem 😀