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Question about helicoid on Minolta MC-X 135/2.8
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PostPosted: Sun Nov 19, 2017 1:52 pm    Post subject: Question about helicoid on Minolta MC-X 135/2.8 Reply with quote

Hi,

I am trying to repair a 5 dollar Minolta MC-X 135/2.8 lens which still has very good and clean glass but had multiple other problems:
* It had severe filter ding, fixed.
* The built in lens hood moved to easily, fixed.
* Front part of lens wobbling, fixed.
* Stop down lever stuck, fixed.

The remaining thing is the helicoid, I need to regrease it. I have bought Helimax XP and I took the helicoid apart as slow as I could but before I had the chance to mark the position it suddenly came apart. I took some images during the take apart process, so from them I think I have it where it should be by counting visible amount of threads at a certain position on the lens IRL and compare to the pictures how it looked before de-assembly.

Test images comes out fine both at infinity and close up.

This lens has one big thread and a smaller finer one. Does anyone here know what the smaller, finer thread is meant for, what does it do in the focusing process?

Thanks and best regards from a grey autumn day in Stockholm/Sweden!
/Anders


PostPosted: Sun Nov 19, 2017 2:45 pm    Post subject: Reply with quote

I don't offhand know what the finer threaded part is for. When it comes to those double helicoids, I find that often it is best to just look at what it does to figure out what is intended. Just watch it as you move the focusing collar or the helicoids if the collar is removed. It will probably make obvious what its purpose is. I seem to vaguely recall -- I may be wrong about this, but see if I'm right -- the other set might be used so that the front element stays fixed and does not rotate.

I too have marked many a lens only to find it come apart unexpectedly, then try to mark it again where I think it came apart at. I'm almost always having to get it into the right threads by trial and error, observing it at infinity focus to see if things line up correctly.


PostPosted: Sun Nov 19, 2017 2:56 pm    Post subject: Reply with quote

This is how it looks:



The finer thread is on the right of the bigger visible one, under the aluminium ring which has the mount attached to it.
The focusing ring sits grabs in the brass ring.
All lenses seems to sit together as one package.
Both the bigger thread and the finer thread moves at the same time. The finer thread much less distance when going from close up to infinity.
The rotation is locked by two L-shaped brackets in the back that glides in slots.

/Anders


PostPosted: Sun Nov 19, 2017 5:39 pm    Post subject: Reply with quote

If those finer threads are associated with an element group, it could be that the lens design requires that group to move independently from the group controlled by the big helical. It's hard for me to say without having the lens in front of me.

I have a late MD 135/2.8. It's quite compact, so probably a different design from yours. I don't use it because the rear element has what looks like a bunch of tiny bubbles in the coating. Impossible to remove without removing the coating. So I have no qualms about dismantling it to see if it resembles yours at all. Probably doesn't, but the same sort of mechanicals might still be involved.


PostPosted: Mon Nov 20, 2017 9:30 pm    Post subject: Reply with quote

This Minolta is unit-focusing, meaning there's only one lens assembly with all the elements in it. The aluminum helicoid of the lens assembly glides in the aluminum counterpart of the focusing ring. The lens assembly translates and doesn't rotate, but the focusing ring (practically) only rotates - that's what the fine brass helicoid is for. The ring actually translates, too, but only by a fraction of a mm.

There should be be a gap about 0.25 mm wide between the focusing ring and the lens base at infinity. When focusing close, this gaps widens by about 0.25 to 0.75 mm (depending on the focus throw of a lens). That's from the fine brass helicoid. If you screwed that in wrong (if that is possible at all - not sure), you either have a gap between base and focsuing ring which is a little too wide or a little too tight. Either way, that's only critical if resistance goes up when focusing at either limit of the scale. If not, it should be fine but infinity focus is probably slightly off. If the gap is as wide as before disassembly and the focus is spot on, all is good.

Then, there's the aluminum helicoid. If you screwed that in wrong, infinity focus might be far off - or not. I'm not sure how many turns are in the thread (even number -> position is irrelevant). However, as long as the infinity focus is spot on, you don't need to worry about this one either.

To sum it up: Infinity focus good -> all good. Infinity focus bad -> one of the threads may be misaligned.


PostPosted: Tue Nov 21, 2017 4:52 am    Post subject: Reply with quote

VLR wrote:
This Minolta is unit-focusing, meaning there's only one lens assembly with all the elements in it. The aluminum helicoid of the lens assembly glides in the aluminum counterpart of the focusing ring. The lens assembly translates and doesn't rotate, but the focusing ring (practically) only rotates - that's what the fine brass helicoid is for. The ring actually translates, too, but only by a fraction of a mm.

There should be be a gap about 0.25 mm wide between the focusing ring and the lens base at infinity. When focusing close, this gaps widens by about 0.25 to 0.75 mm (depending on the focus throw of a lens). That's from the fine brass helicoid.


Just checked mine and a few other lenses. Can confirm! Interesting.


PostPosted: Tue Nov 21, 2017 3:35 pm    Post subject: Reply with quote

The brass piece is what actually connected to the focusing ring, movement of the fine thread in the aluminum base matches it rotation and translates it to the multistart thread inside the brass piece which in turn moves the inner part of the helicoid which holds optical assembly.

Often times it is that fine thread that need to cleaned and regreased. Often times it takes lighter grease than the main helicoid.