A question about unscrew off the front cell of Schneider Super Symmar 80mm XL lens

[Mark J]

Multiply that value by about 20x, Arri. The out-of-roundness of a lens cell alone would be typically 0.005mm !
( edit : maybe you were thinking of 1/2000th of an inch ? )

[diversey]

I found some info in the picture below regarding super symmar XL lenses including 80mm lens. You should never try to remove or unscrew the front cell by yourself.

[Mark J]

Interesting. I have often wondered how shutters are set up for front/rear concentricity. There are a number of parts, with their own tolerances, between the front thread and the rear thread. I would therefore expect the build-up to cause at least 40µm error just with random assembly, whereas a reasonable target for the lenses would be less than 20µm. Is anyone familiar with the internal construction, and aware of any adjustments ?

[arri]

Multiply that value by about 20x, Arri. The out-of-roundness of a lens cell alone would be typically 0.005mm !
( edit : maybe you were thinking of 1/2000th of an inch ? )

I learned by an old mechanic, born in 1927, he explained me how to set the concentricy of shutters and he always adjusted it plain and roundness in the tolerance of 1/1000- 3/1000. I have adjusment tools for some shutter sizes, high precision tools, made by Compur/Zeiss, hardenes steel, all ground.

Ok I wrote 1/2000, please excuse my mistake. But the more accurate it is the more better ist the optical quality.

In the best setup the shutter has the same concetricity than the lens elements in each cell has and than we are in a range of < 1/2000mm not inch.

[Mark J]

I'm sorry Arri, but this is just not correct. I've been designing lenses for over 35 years, including two years at Zeiss in Oberkochen. I have probably spent nearly half of my design time on the computer evaluating and specifying lens tolerances, or discussing them with manufacturing engineers. Normal photographic optics ( eg. large format lenses ) require in the range 20 to 35µm component centering.

A tolerance of 1/2000th mm is 0.5µm, which is wavelength of green light. This is the sort of tolerance that my colleagues in the Microlith department were trying to achieve in 97/98. These sort of dimensions have to be measured interferometrically, or with a Talysurf, not with ground steel tools. Even picking up a steel tool with your warm hand would put it several times out of that specification in a few seconds.

[Tin Can]

Many lenses have a penciled distance on the lens edge

Seems to vary with shims

carry on

I found some info in the picture below regarding super symmar XL lenses including 80mm lens. You should never try to remove or unscrew the front cell by yourself.

[Mark J]

Yes, that's a very important thing to note, if there is a shim. I spoke to Schneider about that recently when getting some info about the Componon-S 210.
Shimming any symmetrical or near-symmetrical lens will adjust the field curvature ( or more technically, it adjusts the astigmatism, so changing the curvature of the 'T' focal surface ). I'm sure some people have practical experience of this with the older lenses.

[xkaes]

I'm sure a lot of us have removed lots of front and rear groups from shutters -- for a lot of reasons (cleaning, shutter swapping, glass swapping, convertible lenses, etc.) -- and reinstalled them without any apparent problems at all.

Is that pure luck or have we been screwing things up without noticing it?

[reddesert]

I wouldn't interpret the Schneider cautions about precision to mean that lens cells must never be touched. There isn't any mechanical degree of freedom to adjust when a lens cell stops as you screw it into the shutter, except by adding shims to limit where it stops. So if you unscrew it and re-screw the cell, it should come back to the same place. Additionally, it's not clear how Schneider could have made the original adjustments, except by adding shims to control the depth. There isn't a degree of freedom to control centration of the front cell with respect to the rear. (For example, you can imagine a mount where one element is free to rotate and then locked in place with a setscrew; but typical lenses don't use this, and it invites its own problems.)

Now, maybe what Schneider meant is that they don't want you unscrewing cells and losing the shims or cross-threading the cell (they specifically refer to cross-threading). That's perfectly justifiable, but also these are mistakes that a user can avoid by being careful.

[Mark J]

Yes, i agree with you. You wouldn't be able to access any adjustment screws from the side once the shutter housing is in place, anyway.
I think the shutter manufacturers just set up the bore-to-bore centering very accurately during assembly.
The way the threads, fit & tapers of those cells are designed, I'd not be surprised if they went in repeatably to 5µm or so. ( On modern lenses, anyway ).