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Can anyone provide a simple explanation as to how the focal length of a lens is measured? Where is measured from? What makes a 17mm lens a 17mm lens? What makes a 400mm lens a 400mm lens. My initial thought was that if you held up a negative 400mm from your eye, you might get a similar angle of view to a 400mm lens. But that doesn't seem quite right, and doesn't account for fish eye lenses etc. A quick google search didn't shed much light. Does anyone know?