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Tel33
Tinkerer
Username: Tel33
Post Number: 1
Registered: 07-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 20, 2012 - 12:36 pm: | 
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alright, i successfully disassembled and clean my old vivitar - main purpose was to adjust infinity focus which is off by a few mm. In order to do that i screwed off the lens group and wanted to re-insert it on a different start in the thread. But this doesnt work as expected. In fact i find NO position to screw it back it just doesnt stick. if it does it gets stuck. So basically i cannot find the right position on this multi-start thread - what now? are there any tricks for this kind of thread? |
M_currie
Tinkerer
Username: M_currie
Post Number: 296
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 20, 2012 - 09:14 pm: |
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So you're not taking apart the focus helical, but just unscrewing a lens element? Focus helicals can be a huge hassle, but lens elements generally just screw back in until they hit bottom. If there's printing anywhere, you would want it to line up, but I don't think it's critical otherwise. However, the threads are incredibly fine, and even a slight cocking will lead to frustration. Make sure you're not getting any distortion in vision, glasses, etc. A suction cup on the front element may help in getting the angle right. An undamaged thread shouldn't fight back much if you engage it right. You may just have to try and try until you feel it going together right.
I may be wrong, but the usual way to make fine focus adjustments would be to loosen the outer collar of the lens, (screws may hide under rubber), so that the outer collar, on which the focus markings are, rotates a bit relative to the actual focusing element of the lens. You then focus the lens at infinity, and move the outer ring accordingly before screwing it tight again.
If you are putting together a lens for which thread catching is critical, and have no reference, about the only thing you can do handily is put a piece of ground glass on the film plane and try focus at infinity until it comes right. If you go too far, back the thread out while keeping a little pressure on it, and it should jump noticeably to the next thread back. |
Tel33
Tinkerer
Username: Tel33
Post Number: 2
Registered: 07-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 20, 2012 - 11:57 pm: |
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actually no, I didnt touch the lens group but the helical. Yes I know I should have marked its positions but its not that I cannot find the correct entry, I cannot screw it back in at all, not matter what i try. I can feel when it tries to catch a thread but then it gets stuck. |
Glenn
Tinkerer
Username: Glenn
Post Number: 1037
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, July 21, 2012 - 03:43 am: |
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Wear can play havoc to helical threads. Make sure you have not bent any of the wafer thin starts and then really clean out everything before trying every start position. As you progress around the start positions, check that you have not damaged any of the thin first portions - if you find this happening gently remove the very end of that thread with a very fine file. |
Tel33
Tinkerer
Username: Tel33
Post Number: 3
Registered: 07-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, July 22, 2012 - 02:12 pm: |
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ok, update. after some minor tweaking with the file and some more cleaning I finally managed to reassemble everything in correct position. Thanks Glenn. |
M_currie
Tinkerer
Username: M_currie
Post Number: 297
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, July 22, 2012 - 05:13 pm: |
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Well done. Helicals can be a real nuisance. |
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