How to correct for back-focus

[tlitody]

Linhof also made vacuum film holders as did Hoffman. But the sag in film is well within the depth of focus of the lens. Linhof rather quickly discontinued their 45 vacuum film holders as impractical in practice. But the did continue to manufacture their vacuum 126mm (5") aerial roll film back for many years.

Before Kodak came out with their instant film they surveyed the major camera manufacturers to see what the actual gg positioning and film plane position was in the various cameras. After they completed the survey they contacted us to let us know the results of the survey. What they discovered was that only two of the manufacturers that were contacted had set the film plane and gg position to 0. Neither Linhof nor Sinar were one of those two. Camera manufacturers know film sags as do lens manufacturers. That is one of the major differences between digital and analog view cameras and analog and digital view camera lenses. Digital sensors have no sag and digital sensors have no depth like film emulsions do.

The Contax RTS III had a vacuum film pressure plate too.
I'm not sure that the depth of focus and depth of field are sufficient when using wide apertures to stop a noticeable shift in the focus plane if the film is out of place. The argument seems to be that moving the GG 0.2mm will fix it (assuming that figure is correct). Well if that 0.2mm move in GG will fix it then a 0.2mm displaced sheet of film is enough to cause the problem which means film moving in holder has to be considered as one possible problem.

[Brian C. Miller]

I read throught the whole thread, and there's something that everybody missed.

I use a Linhof MT 3000 with a Cooke f/4.5 229mm lens. I have tested the focus four times from three different distances using the LensAlign testing setup. In all cases, the camera/lens combination back-focuses about 2". I can compensate, of course, by focusing 2" in front of the plane I want in focus. Is there anything else I can do?

Yeah, check it with a different lens.

All of this is centering around the Cooke being a soft-focus lens. How about checking the camera with a sharp-focus lens? Any lens will do for that. Is the issue really with the lens, or is it really with the camera? You can pick up a Calumet lens for cheap. I can send you something that works to try to nail the problem down.

My Super Graphic came with a different ground glass, which threw off the focus. After I realized that was the problem, I ordered a replacement fresnel/gg combo from
Brightscreen and shimmed it into position with some film. (stable, thin, and it works) Since you have a brand new camera, you can have this done for you by the factory. But first, just test whether a different lens works for you. It should be a very quick test.

[David Solow]

Thanks, Brian.

When I get back to North Carolina in February, I will check the camera with another lens. I have a 210mm Rodenstock that should do nicely.

David

[David Solow]

To everyone who has contributed to this thread,

I have started a new thread, "Back-focus Images", that show my LensAlign tests as well as a shot of a friend that is representative of all of the shots I have done with this camera and lens.

David