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Zeiss MK II distagons - loose focus ring


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#1 Gregg MacPherson

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Posted 20 April 2012 - 06:32 AM

I'm looking at some (S16) Zeiss MK II distagons. Ex German TV, quite cheap, optically quite clean and looking good through the viewfinder and hitting the focus marks. But on a couple of them there is some mechanical wear or loosness in the focus ring or what I assume are cams inside. I will show them to a good lens tech in a few days, but I'm really curious to get some ideas about it.

The 12mm has what feels to the fingers like a fraction of a mm backlash in the focus ring. The little thud that you feel at either end of this is quite distict. The grease is fairly viscous so it's hard to tell if there is inertia involved or not. Like maybe something heavy is being rotated over that fraction of a degree.

The 25mm has what feels like a very loose focus ring. If you look at the edge of the focus ring you see almost a mmm of completely free backlash in the focus movement (about 3/4 of a degree). You can wiggle the focus ring about the vertical and horizontal axis so you see over a mm of movement at the edge (a bit over a degree)

Both these lenses look fine through the viewfinder while working the focus ring. You could actually use them just like that.

Any thoughts? Dom?

Cheers,
Gregg.
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#2 Dom Jaeger

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Posted 20 April 2012 - 09:01 AM

Hi Gregg,

from your description it's most likely a very easy fix. The tiny grub screws that secure the focus ring have probably just come a bit loose. With a small jeweller's screwdriver you can nip them up. If they're over-tightened they put pressure on the helical thread within and make the focus stiff, so they should be only lightly tightened and then fixed with locking varnish or paint. On some lenses there is a second ring under the focus ring, secured in the same way, so it could also be that one that's loose.

If there's rotational play between the front section of the lens and the mount (which can feel similar to just play in the focus ring) then it's a different story and the lens needs a service.
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#3 Gregg MacPherson

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Posted 20 April 2012 - 11:15 PM

Hey Dom,
Thanks. The 25mm with the wiggly focus ring is all fixed. The scews were very loose. I tried to screw them all in roughly the same amount. The tightening torque I did wwas very light, just a light feel on my finger tips with a 10mm diameter handle on the screwdriver.

The 12mm does in fact have some rotational movement of the front of the lens relative to the mount. It is so small I'm not sure I can see it, but you can feel it easily with your finger tips. So that one may need the doctor.

Cheers,
Gregg.
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#4 Tom Jensen

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Posted 21 April 2012 - 12:38 AM

If you take off the ring there are are little holes that that grub screws fit into, If they don't line up, the lens should be adjusted,
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#5 Gregg MacPherson

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Posted 21 April 2012 - 09:04 PM

If you take off the ring there are are little holes that that grub screws fit into, If they don't line up, the lens should be adjusted,


I didn't do that yet. If the "holes" are about 2mm diameter then what I saw on the 25mm could make sense. The screws were sitting loosly in the holes and became progressively constrained as they were screwed in. They actually were sitting about 2 or 3 turns loose.

Thanks,
Gregg.
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#6 Tom Jensen

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Posted 21 April 2012 - 10:35 PM

I always used a little blue Locktite to keep them from backing out. You can also dab some model airplane paint over the holes.
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#7 Gregg MacPherson

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Posted 22 April 2012 - 03:42 PM

I always used a little blue Locktite to keep them from backing out. You can also dab some model airplane paint over the holes.


Thanks Tom,
I was going to cheap nail polish for that. Smells like old school nitrocelulose laquer.
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#8 Gregg MacPherson

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Posted 26 April 2012 - 03:56 AM

I took the lenses to Stephen Spooner at Panavision in Auckland for a quick lookover. I described in an earlier post a small ampont of "backlash" in the focus ring of the 12mm. He showed me that the whole front of the lens has a mall amount of movement. He prescribed a re gtease and maybe the internal sliders need some fixing. Maybe a couple of hours work so not major.

We put all the lenses on the projector and they all looked really sharp and had accurate focus marks, except the 12mm lens. All except the 12mm could show a clean 200 line pairs/mm. I think the 12mm was about 140 lp/mm, the 200 lp/mm block didn't look as crisp.

Then in a few seconds the machine told us the shim thickness needed to correct the 12mm at a distance of 4', 8/1000mm. Way cool. Interesting and fun.

Cheers,
Gregg.
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