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I happened to run across the unmarked front and rear mounted cells of a large old lens and, what the heck, for $12.50 including the brass shade, I didn't even argue over the asking price. A cursory examination makes me guess that it was about an 18 inch f/8ish RR. Does anyone have one mounted (Jim? Ole?) who could tell me the spacing? Or, would anyone have a formula or even a guess as to the proper spacing range before I go into barrel making mode?
just from experience with similar lenses I'd say about 70mm. You would have to measure the exact same lens to make sure.
Since they are unmarked, how about sticking the cells in some sort of plastic plumbing tube temporarily and see what results? Not very scientific, but it could vbe interesting.
The formula is here (equation 1.16 or 1.17):
http://www.mellesgriot.com/pdf/001.8-1.10.pdf
Uli, I ran the numbers and, Wimpler, 70mm is as good as anything. Minor variances just change the combined focal length a little. I made a barrel out of a cardboard mailing tube and mounted it in an extra board I had. I just tested it on the camera and it is now a 19 ½ inch f/8 lens. Looks pretty good on the ground glass with just a little "glow". I'll give it a try wide open then decide if I want to work out some stops for it. Thanks for the input.
Oh, and Ernest, I checked out the plastic plumbing tubes and, as the lens threads are a bit larger than 3 inches, I thought it would be easier to rough out a 3 inch mailing tube than the heavy plastic.