I wanted to know the focus distances for my Holga with and without my
#2 close-up filter. So I rigged this test on the basis of a
suggestion from Randy at Holgamods.com.
Instead of holding ground glass in the film plane, I cut a strip of vellum to the same height as 120 format film and wound it onto two spare spools. The left spool is loose and could fall out, so it's held there with masking tape.
The shutter is held open with a locking cable-release.
My Holga 120S on a tabletop tripod with the back off and a strip of vellum on the film spools. The vellum is held taut in the film plane by the takeup reel on the right and masking tape on the left.
There is a candle on the table about 14" away.
Candle light only, 25 sec exposure in D70.
There is a step by step here: www.toycamera.com/tech/macro.cfm