Quote Originally Posted by Jim Noel
To answer your question, you need to measure from lens to subject to determine magnification and exposure compensation.
If that were the question, he'd have to measure from the rear nodal point of the lens. That may be inside the lens, but doesn't have to be. Most macro lenses are symmetrical however, so the front and rear nodal points will be at the same distance from the aperture. Exact data should be available from the manufacturer.