lido
Established
Hello,
I got a nice 80's J8 that I love. But there is an issue of focus accuracy close up, it appears to focus behind a subject. If I focus on anything 2-3 meters and farther, it is fine.
Is there anything that can be done on J8 to remedy this, or would shimming the lens to fix close up focus mess up focus at infinity. I really love the results I got from this lens and would love to fix it up so that it works well accross the whole focus range.
I got a nice 80's J8 that I love. But there is an issue of focus accuracy close up, it appears to focus behind a subject. If I focus on anything 2-3 meters and farther, it is fine.
Is there anything that can be done on J8 to remedy this, or would shimming the lens to fix close up focus mess up focus at infinity. I really love the results I got from this lens and would love to fix it up so that it works well accross the whole focus range.
Stu W
Well-known
I think that Brian Sweeney is the resident Jupiter expert. Search for his posts. Stu
Kim Coxon
Moderator
Hi Doug,
Which version of the J8 do you have. Is it the later black one or an earlier chrome one with 3 screws arounf the mounting flange?
Kim
Which version of the J8 do you have. Is it the later black one or an earlier chrome one with 3 screws arounf the mounting flange?
Kim
lido
Established
it's the black one, it says jupiter-8 2/50 on it, and the serial # is 7722702
Kim Coxon
Moderator
Hi,
As Brian says, you can change the main shims in the lens so that it will focus close up but then the infinity focus will be slightly out. Because of the greater depth of field when focused at infinity, this will be easier to hide with depth of field. The problem stems from a mismatch between the real focal length of the lens and what it should be. This is down to poor manufacturing tolerances.
I have been very lucky so far, both my black J8 and the J3 focus correctly throughout the range on my Bessa R. You may be able to "change" to true focal length by adjusting the postion of the rear lens group. I haven't tried this yet as I haven't had to but I have 4 "parts" J8's on the way so I play around with this problem. The theory goes: Unscrew the lens unit and remove it from the focus module. I would then slacken off the rear group and shim it slightly with paper. Put it back together and check the collimation. If this makes the situation better, shim a bit more until you have solved the problem. If it make it worse, the only solution is to shave a little of the metal off the front unit so the rear group is moved forward. Again do it in very small stages and check each time. If you do go too far, you would have to shim it back. Now for the health warning: You could solve the situation and there is no harm in trying the first stage. However, if the second stage goes wrong, you could end up wrecking a good lens that was usable over most of it's travel.
Kim
As Brian says, you can change the main shims in the lens so that it will focus close up but then the infinity focus will be slightly out. Because of the greater depth of field when focused at infinity, this will be easier to hide with depth of field. The problem stems from a mismatch between the real focal length of the lens and what it should be. This is down to poor manufacturing tolerances.
I have been very lucky so far, both my black J8 and the J3 focus correctly throughout the range on my Bessa R. You may be able to "change" to true focal length by adjusting the postion of the rear lens group. I haven't tried this yet as I haven't had to but I have 4 "parts" J8's on the way so I play around with this problem. The theory goes: Unscrew the lens unit and remove it from the focus module. I would then slacken off the rear group and shim it slightly with paper. Put it back together and check the collimation. If this makes the situation better, shim a bit more until you have solved the problem. If it make it worse, the only solution is to shave a little of the metal off the front unit so the rear group is moved forward. Again do it in very small stages and check each time. If you do go too far, you would have to shim it back. Now for the health warning: You could solve the situation and there is no harm in trying the first stage. However, if the second stage goes wrong, you could end up wrecking a good lens that was usable over most of it's travel.
Kim
lido
Established
Thanks guys, I might try to find another 50 LTM lens before I attempt to fix this one. If things go wrong, I'll be left without a 50, and I do use it a lot.
Was looking at 50 'cron collapsible on eveil site tonight, the auction just ended and it sold for $279. I almost bid on it, but decided that I would rather have 50/1.5 CV.
Again, thanks for the advice.
Was looking at 50 'cron collapsible on eveil site tonight, the auction just ended and it sold for $279. I almost bid on it, but decided that I would rather have 50/1.5 CV.
Again, thanks for the advice.