[mst]

Well the simple substraction formular posted above of course contains a small error: normally the manufacturers post the physical length of a lens including mount (to my knowledge at least). Theoretically one would have to substract the depth of the mount from the physical length of the lens.



However, for MFD (or (usually) 1:1) physical length and MFD is all you really need to know to get a good idea about the working distance. The only thing you usually don't know is the depth of the hood, which should be substracted, too. In the field, though, you can simply leave the hood in the bag to increase the effective working distance.



As is written in the text Klaus linked to: calculating the working distance for other reproduction ratios requires knowledge about the lens that is usually not available.



-- Markus