About Schneider center filters

[Dan Fromm]

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/20200403180302/https://www.ebay.com/itm/Schneider-Kreuznach-Center-Filter-E-49-fur-das-8-75-Centerfilter/401990882024 is one that's separated and showing Newton's rings. It has to be a sandwich.

[Oren Grad]

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.

[Bob Salomon]

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.

[aphcl84]

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.

[Dan Fromm]

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.

[aphcl84]

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.

[Bob Salomon]

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.

Dan,I watched a little old man at the Heliopan factory airbrush them as well as their glass, rectangular graduated ND filters.
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.

[Dan Fromm]

Bob, thanks for the Heliopan story. What process did Heliopan switch to after the little old man died? And, how did Rodenstock make theirs?

[Bob Salomon]

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.

[Dan Fromm]

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.

[Alan Klein]

I just bought a Schneider IV for a Nikkor 90mm f/4.5. I've been told I should open 1 1/2 stops when using it. Also is there anything special I should know when using it or how it's made?

[Bob Salomon]

I just bought a Schneider IV for a Nikkor 90mm f/4.5. I've been told I should open 1 1/2 stops when using it. Also is there anything special I should know when using it or how it's made?

You must stop down at least 2 stops for it to work. Then add the filter factor. After adding it you still have to be stopped down at least 2 stops from wide open.

[Dan Fromm]

Yes and no.

Yes, you need to check the lens to know how far to stop down when using the CF. Most LF wide angle lenses suffer mechanical vignetting at large apertures because the barrels occlude the exit pupil. Center filters more-or-less offset optical vignetting (cos^4), do nothing for mechanical.

To check, open the diaphragm as wide as it will go. Then hold the lens at arm's length with the shutter open, rear cell facing you. Rotate it slowly (around a vertical line through the diaphragm) until the exit pupil touches the edge of the barrel. Stop down 1/2 stop. Repeat. Eventually the exit pupil will be so small that it will just touch the edge of the barrel when rotated through approximately half the angle the lens is supposed to cover. Use that stop or smaller when using the lens with its CF.

Bob gave you a rule of thumb, not a test procedure. Testing is better.

No, how the density gradient is created is irrelevant to how the CF should be used. But and however, if you think the gradient is due to a variable density layer of paint or something on the outside of the filter, be very careful when cleaning that side of the filter. My Schneider CF III seems to have something on the rear surface, its front surface is clean. Best not to rub the rear surface.

[Alan Klein]

Thanks for that info. I've asked this elsewhere. But I've been told it's best to use 88mm BW contrast filter and polarizer filters between the lens and CF. Of course that leaves grad ND filter (square) on a holder outside where the CF is 105mm vs 82mm for the lens side. Any recommendations or considerations?

[Dan Fromm]

Thanks for that info. I've asked this elsewhere. But I've been told it's best to use 88mm BW contrast filter and polarizer filters between the lens and CF. Of course that leaves grad ND filter (square) on a holder outside where the CF is 105mm vs 82mm for the lens side. Any recommendations or considerations?

Alan, CFs are designed to sit as close to the lens as possible. If you put anything between the CF and the lens you risk mechanical vignetting. If you put anything in front of the CF it has to screw in, otherwise, again, you risk mechanical vignetting.

If you don't use movements you may be able to get away with doing what you want. Its worth trying, but be prepared for it not to work well.

[Bob Salomon]

Thanks for that info. I've asked this elsewhere. But I've been told it's best to use 88mm BW contrast filter and polarizer filters between the lens and CF. Of course that leaves grad ND filter (square) on a holder outside where the CF is 105mm vs 82mm for the lens side. Any recommendations or considerations?

And be aware, if you will be photographing anything with large expanses of sky from edge to edge that areas of the sky are naturally polarized. If you use a polarizer with a wide angle under that condition you will get banding in the sky from those naturally polarized areas.

[Corran]

Dan, my impression is that older Schneider CFs are sandwiches but recent ones have sputtered coatings.

I've always suspected there was some kind of physical differences between new and old Schneider CFs. Also, multicoating perhaps?

A data point: I at one time had an older Schneider 3B CF and a newer one at the same time. The older one flared horribly in many conditions and it seemed to cause some loss in contrast. The newer one was unnoticeable in use. Some time ago I gave the old one away, else I could look at it more closely.

[Alan Klein]

Dan and Bob, Thanks for the tips. So how do you use CF's and other filters with 90mm and 75mm lenses?

[Bob Salomon]

Dan and Bob, Thanks for the tips. So how do you use CF's and other filters with 90mm and 75mm lenses?

You put them in front of the CF. Thin mount brass ones are best.

[Alan Klein]

You must stop down at least 2 stops for it to work. Then add the filter factor. After adding it you still have to be stopped down at least 2 stops from wide open.

Bob, I'm confused. Why would I stop down? Don't I have to open the lens 2 stops for the CF?

Let's make this simple for me. My meter indicates the exposure should be F22 at 1/4 sec.
1. What should my setting be with the CF mounted?
2. What should my setting be with the CF dis-mounted?
3. What do you mean I have to be stopped down two stops from wide open?

[Alan Klein]

Yes and no.

Yes, you need to check the lens to know how far to stop down when using the CF. Most LF wide angle lenses suffer mechanical vignetting at large apertures because the barrels occlude the exit pupil. Center filters more-or-less offset optical vignetting (cos^4), do nothing for mechanical.

To check, open the diaphragm as wide as it will go. Then hold the lens at arm's length with the shutter open, rear cell facing you. Rotate it slowly (around a vertical line through the diaphragm) until the exit pupil touches the edge of the barrel. Stop down 1/2 stop. Repeat. Eventually the exit pupil will be so small that it will just touch the edge of the barrel when rotated through approximately half the angle the lens is supposed to cover. Use that stop or smaller when using the lens with its CF.

Bob gave you a rule of thumb, not a test procedure. Testing is better.

No, how the density gradient is created is irrelevant to how the CF should be used. But and however, if you think the gradient is due to a variable density layer of paint or something on the outside of the filter, be very careful when cleaning that side of the filter. My Schneider CF III seems to have something on the rear surface, its front surface is clean. Best not to rub the rear surface.

Dan: Can you clarify better: Rotate it slowly (around a vertical line through the diaphragm) until the exit pupil touches the edge of the barrel."

[Bob Salomon]

Bob, I'm confused. Why would I stop down? Don't I have to open the lens 2 stops for the CF?

Let's make this simple for me. My meter indicates the exposure should be F22 at 1/4 sec.
1. What should my setting be with the CF mounted?
2. What should my setting be with the CF dis-mounted?
3. What do you mean I have to be stopped down two stops from wide open?

If you have a 5.6 lens you must be at least f11 for the center filter to work.
So, put a filter factor of 1.5 into your meter and use any aperture that is 11, 16 or 22. If you stop down beyond 22 you will be in diffraction.
If you don’t use the cf then just shoot away. But best results will always be between 11 and 22.

[Dan Fromm]

Dan: Can you clarify better: Rotate it slowly (around a vertical line through the diaphragm) until the exit pupil touches the edge of the barrel."

No. Play with it. You'll figure it out.

[Drew Wiley]

As long as movements aren't extreme and you're reasonably stopped down, it's fine to use a contrast filter between the CF and primary lens. Just look thru the corners of the groundglass to check for mechanical vignetting. I did it frequently and have always been known for very precise images. Bob is technically correct, but more nitpicky than practical in this case. Giant filters way out front are just going to put more wt and potential vibration on the front standard of a lightwt field camera. Reasonable compromises need to be considered.