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Steve_s
Tinkerer
Username: Steve_s
Post Number: 39
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 26, 2007 - 10:04 am: | 
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Every Meyer lens I have met has its rear element assembly located with 3 radial grub-screws which allow for some adjustment, clearly set up at the factory. I don't know what the adjustment is for, but I have worked out a system to make sure everything is put back in exactly the same position as when I found it.
The rear element assembly on Zeiss lenses is always held by a slotted-ring with a spacer underneath it. This seemed odd to me - why not just make the retaining ring deeper - and why does everything have slots, even the parts that don't unscrew? Yesterday I was reassembling a Pancolar and I realised that the bore of the spacer was not coaxial with the outside diameter. This means that rotating it would move the rear element, but only in a fixed circle. On further investigation it looks as though the lens housing itself is also eccentric, so rotating both components would give a full range of adjustment.
I haven't seen anything similar on other makes of lens, but maybe I just haven't looked hard enough.
Does anyone know what this adjustment is for? Does it compensate for errors in the metal, or in the glass? And have I ruined all the Zeiss lenses I have taken apart by losing this setting? I will be marking everything very carefully when I get to the next one! |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 42
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 01, 2007 - 02:01 pm: |
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In case anyone out there is interested (I'm a bit surprised nobody seems to be - maybe I am the only one who doesn't already know about this!) here are the results of some further checks.
The next lens out of my overflowing fungus box was a little Zeiss Tessar 2.8 manual click-stop (aluminium), which I marked carefully before stripping it. The difference between the thickest part of the spacer and the thinnest was .009", meaning an offset of 4½ thou, or just over .1mm. The difference between thickest and thinnest on the lens mounting sleeve, unsurprisingly, was the same. This means that by rotating the parts independently you can adjust the rear element radially from central, up to 9 thou offset, at any angle.
On the lathe I turned up an accurate mandrel which registered on two of the plain internal diameters in the front part of the barrel. This enabled me to rotate the barrel on its axis and check accuracy. I hoped to be able to find out whether the offset adjustment was provided to correct for machining errors. I set a dial-gauge on the internal diameter at the rear where the spacer fits. This lens must have been made on a good day because there was no measurable run-out here at all. I fitted the spacer and sleeve in their original positions and the retaining ring and checked at the diameter where the lens element fits - no run-out again, so obviously this particular lens didn't need any offset and the spacer and sleeve were arranged to cancel each other out. This makes the result a bit inconclusive, but you could take it to partly confirm my original theory that the offset is to correct for machining errors ie. there were no errors on the barrel of this lens, so it didn't need any offset.
On a lens where this offset setting has been lost, and the parts are assembled at random, this could make any error much worse. For example, if this Tessar had, say, a 2 thou error on the rear element, which probably wouldn't matter much, and on assembly the parts happened to fall together with a maximum offset in the wrong direction, you would have an 11 thou error which might matter quite a bit. Obviously the best thing for a lens where the setting is lost is to set up the spacer and the sleeve so that their offsets cancel out. This leaves any error uncorrected, but as long as the error is small it should be OK, so this is what I will do to the Zeiss lenses I have previously stripped and assembled.
I have checked a Flektogon (35mm/2.8 "striped" version) and a Pancolor f2 which I have stripped down previously, losing the offset setting. On both these the difference between maximum and minimum thickness on each of the eccentric parts is .008", again giving a total possible offset of about 0.2mm. There is no relationship between the offset axis and the adjusting slots. I have set up these lenses so that the offsets cancel.
I hope this is useful to someone, because it is a pretty fundamental bit of knowledge if you are dealing with a Zeiss lens! |
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