Originally posted by switters Thanks for the link. One thing I don't understand is the difference between focusing distance and working distance in the extension tubes tables.
The focusing distance is the distance from the lens node, a techie term meaning the center of a single element lens, a piece of glass shaped like this () with the same focal length as the actual lens. The working distance is the distance from the front of the lens barrel to the subject. They are identical with the single element lens, but not with a compound lens.
My 400 mm lens is only about 305 mm long, so the node is actually in front of the lens out in mid air 95 mm in front of the lens furthest tip (without hood, of course). That is the definition of a telephoto construction, by the way. If it wasn't telephoto construction, the lens focal center would be 400 mm from the film/sensor and the barrel would be something over 400 mm long. This means that the working distance of the lens is 95 mm more than the focal distance.
A telephoto design macro lens exaggerates this to no end. At 1:2, my 100mm macro has 273 mm of working distance. The focal distance is about 200 mm, so I get an extra 73 mm between the lens and the subject.
Fun ain't it?