About Schneider center filters

[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.