Steve, E.F. is what we now call focal length and it has the same meaning. It is the distance from the lens rear nodal point to the film plane when the lens is focused at infinity. Extension in general has little to do with that, depends on the lens' mechanical design. For many lenses the rear node is close to the diaphragm, i.e., close to the back of the shutter, so many of us have fallen into thinking that extension when the lens is focused at infinity is focal length. Not so.
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