Originally posted by WPRESTO OK, but FYI: The Asahi No. 1 was intended to take the 50mm f1.4 from it's nearest focusing distance down to much closer. Consequently when attached to any lens regardless of FL, when that lens is focused to infinity, the front-of-filter-to-subject focus distance is about 18 inches. I think that should work for a large number of museum cases. And also BTW: the shortest-possible-filter-to-subject focus distance is unpredictable because it depends on the nearest focus distance of the lens.
Very good info, thanks! FYI I'm not looking for 'macro' shots per se (given highly unfavourable lighting, no tripod, and my own lack of interest in macrophotography as something I need to try), but I want to be able to frame in as close as possible; the 'naked' jupiter 9 MFD is ~31", which isn't going to do the job on smaller specimens. I've got a couple of close-focusing zooms which generally perform quite well, except they are too slow.