Naturally, now that I need to link to it I can't find a fairly recent discussion on center filters in which a poster said that Schneider's aren't sandwiches but have something dark sputtered on a surface.
I can't address all of the sizes etc. of CFs that Schneider made, but here https://web.archive.org/web/20200403...r/401990882024 is one that's separated and showing Newton's rings. It has to be a sandwich.
Dan, my impression is that older Schneider CFs are sandwiches but recent ones have sputtered coatings. I've had the experience of buying an older Schneider CF on eBay and having to return it to the seller because of Newton's rings that weren't documented or shown clearly in the listing. But my late-model ones, in addition to being optically pristine, are sufficiently thin that it doesn't seem plausible that they're sandwiches. But I too would be interested to see more definitive documentation on this.
Center filters are a coating applied in a vacuum in the required pattern. They are expensive because they are difficult to make and many were thrown out as not to required spec.
Some early ones were airbrushed between glasses.
Dan, I have two center filters of the same age as the one you linked and one of them has the same problem with the balsam going yellow and separating. I recently took it out of it's ring and I can confirm that it's two pieces of glass cemented together. I had planned to re-cement it this weekend, only I'm now wondering if that will work if it's actually airbrushed in the center. Hopefully it's a sputtered coating between the glass that won't be damaged by acetone.
Jim
Jim, I think Bob is mistaken.
If your filter is a sandwich it should be two mating pieces of glass of the same refractive index, one clear, the other tinted more-or-less neutral gray. What do the edges look like?
As I mentioned in my CF article, the first optical CF seems to have been a sandwich -- clear and yellow glass -- from Rodenstock that came to market before WW I. The first CF was mecanical, Goerz' Hypergon's star.
The edge of my center filter looks as if two regular clear filters were cemented together, no indication of any nd glass anywhere and as far as I can tell by looking at the edge both are completely flat. I suspect there is a thin film applied the the inside of one or both of the pieces of glass, but I won't know until I separate them.
Jim
Dan,I watched a little old man at the Heliopan factory airbrush them as well as their glass, rectangular graduated ND filters.Originally Posted by Dan Fromm
Unfortunately he never trained anybody else to do this and once he died they had to stop offering the glass rectangular grads and then had to switch how the center filters were made.
Heliopan did many things a bit different.
I watched them take a lump of black silicon rubber, put it in a heated press and outcome one of their rubber hoods. But their was not a smooth rim to the hood. The raw hoods then went into a room where women mounted them to a wheel the rotated quickly. The women at each wheel then took a tapered stick and used it to smooth the rim of the hood.
The sticks were cut from a tree behind the factory and then The end was contoured To the desired shape by hand.
Bob, thanks for the Heliopan story. What process did Heliopan switch to after the little old man died? And, how did Rodenstock make theirs?
Rodenstock and Heliopan cooperated.
Rodenstock latest technique involves concave and a convex pieces of glass bonded together, one is clear and the other a gray color. This technique eliminates the magenta shift when shooting, say an aluminum clad building in bright light.
Thanks again. That was the sandwich -- one glass plano-concave, the other plano-convex -- that I referred to. The filter's outer surfaces are flat.
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