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?