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Focus Adjustment on OM Zuiko 50 f1.8 made-in-japan
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PostPosted: Wed May 23, 2018 11:08 pm    Post subject: Focus Adjustment on OM Zuiko 50 f1.8 made-in-japan Reply with quote

Hi all,

I need help adjusting the focus on this lens, specifically because I lost infinity focus.

I adapted this lens to Nikon F using the Pixco/Ebay ZuikoOM->NikonF adapter (http://a.co/dmvUjcK) ground-down to a plate. It works GREAT!!

But, I lost infinity focus. My calculations said I would be "close" after the adaptation, but my margin of error was around .5mm, which is enough to blow the hyper focus.

Anyone know how to adjust this on this lens?

NOTE: I did have luck with adjusting the rear element; it screws on and even had some inked marks on it (and a matching mark on the inside of the body) suggesting that is how the adjustment was made at manufacturing, but I'M NOT 100% SURE so I'm hoping for some help. I can't re-use these mark because: 1) The marks match up on every full turn and I don't know how many turns out they supposed to be in the first place and 2) It doesn't matter how it lined up for OM, I'm on Nikon F and have to re-calibrate for sure.

What I think is that the rear element assembly is spun on its threads until infinity focus is achieved, then lightly adhesived in place. If this is indeed the way to do it, I also need suggestions on what compound to secure the threaded elements in place that is acceptable for lens usage. Super glue? Thread Locker?

To confirm, I DID get infinity working like this, with a few turns of this element, but need to confirm this is corrrect and secure it in place.

Thanks!


PostPosted: Thu May 24, 2018 6:42 pm    Post subject: Reply with quote

No, what I'm doing is not entirely correct. I think I'm on the right track, but I need to move all of the rear elements as a group, not just the last element. I know this is wrong because unscrewing this element actually loosens the doublet in the next group.

The rear convex element and the next doublet seem to be in one modular configuration. I think, when disassembling the lense, I managed to disassemble this module, but left part of it threaded in the lens body. I think this part is actually cemented in the lens, likely to set inifinity focus as I described above.

So I need to somehow break the adhesive holding this retaining part, then reassemble the rear elementS (plural), then re-thread that whole thing back in, and adjust infinity that way, then adhesive in-place again so it doesn't creep.

All of this requires a bit of knowledge about how lenses focus in general. This lens has 6 elements in 4 groups, it seems, arranged symmetrically around the diaphragm. It seems the focus ring moves all of the back elements (as one big group) to acheive focus. Does this make sense with respect to lens design in general? That the elements behind the diaphragm can be moved to adjust focus? Just wondering because I'm new to this.

Any thoughts about safely breaking-free glued threads? I thought maybe some light heating and a wooden dowel to try and spin it, then some careful clean-up with some solvent.

Then, of course, what should I use to re-glue it? The stuff currently used is clear and hard and smooth. I think thread-locker and superglue are both to messy when dried to use in a lens. Maybe this is some sort of epoxy?


PostPosted: Thu May 24, 2018 9:06 pm    Post subject: Reply with quote

Cart before horse.

Ground down plate is too thick.


PostPosted: Tue May 29, 2018 5:53 pm    Post subject: Reply with quote

Good thinking, unfortunately it's already as thin as I feel comfortable, I think around 1mm or less, and I think it would need to be 0 thickness to get where I need.

The material was already very thin (cheap?) and thinner that the plate it replaced.

However, even with the same size plate the Nikon F flange distance is .5mm farther than the OM, so to compensate, a thinner plate would do the trick.

I have this, and this is part of my calculation I mentioned, that even though Nikon pushes the distance out, my plate is roughly .5mm thinner than the original, so it should be a wash.

It's ALMOST there, and I don't think the plate can get any thinner, or at least not enough.

I needed 2-3 full turns of the element mount to get in the ballpark, and that is around .5mm I think, maybe a little less, but I don't have that much material to remove from the plate.

Another update:

It doesn't look like the element housing can be removed, only the part that I already have unscrewed can really come off, so it looks like there is no way to adjust the focus on this lens, it must be just a precision thing, for OM.

Does this sound reasonable? Or, in the greater experience in the group, is there ALWAYS a way to adjust? Or is it common to have non-adjustable lenses?

I looked closer and there is a groove in the original mount plate around the edge, and a ridge on the lens body, that when fitted gets the plate just a hair closer. If I can duplicate this groove on my plate, which I should anyway so it fits exactly the same, it might just give me the little distance I need.

So, moving on from adjusting elements (unless someone here can insist it is the right path):
1) Compare thickness again of original and new plate, just to get a good diff. Maybe I can take off some material to make it EXACTLY .5mm thinner, just to be exact and compensate for the F/OM flange distance difference.
2) Work on adding the circular groove to my plate, to fit over the ridge in the body, to get just a little smidgen closer
3) Hope that works