Shimming a Ground Glass

[samoore]

I'm working with an Ebony 23S that has some issues with being a little farsighted at intermediate distances (like a portrait or interior shot). So the GG needs to be shimmed out a touch, and I'm wondering what would be the appropriate material to make shims out of? Edit: I'm using a pair of Horseman 6x9 120 backs and usually shooting with a Schneider APO Symmar 120mm lens (also use a 65mm, and 210mm).

[EricWHiss]

If you have a vernier caliper with depth gage you can measure the depth of the ground glass in the frame.  If you have a combination ground glass and fresnel it can get a bit more complicated depending on wether the fresnel is in front of the glass or behind it.   For a simple ground glass you need the diffuse side facing the lens and it should be positioned at the same point as the film in the holder would be:
Here's a table of film holder specifications http://home.earthlink.net/~eahoo/page8/filmhold.html

I've been told that if you have the fresnel in between the ground glass and the lens then you'd want the film plane to fall about 2/3rd the way into the fresnel but I haven't confirmed this myself.   If you have a BosScreen you'd want the film plane to match up with the wax layer. 

But either way once you know where the screen should be placed you can measure how far off it is and then look for a material that is the correct thickness.  I have used things like heavy card stock that I cut in my rotatrim but you can buy shim stock in very precise thicknesses from places like McMaster Carr.  The material isn't that important, but plastic shim stock will not change thickness with changes in humidity. If your screen is only off by a little bit you can even use tape. You can buy very narrow (1/4 inch and finer) tape which is available for industrial and artistic applications.

[samoore]

Thanks for the advice! I should have clarified that the Ebony has a combination glass / fresnel. I uses some strips of 120 film as shims, and measured the depth with a dummy roll, it didn't take much, but I'm going to shoot another test, and fingers crossed we should be in business...