How are lenses measured?

[Hillsilly]

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?

 

[Loswr]

Focal length is 'measured' as the physical distance between the lens' exit pupil (an optical point that may or may not be within the physical space occupied by the lens, depending on the lens design) and the image plane (where the in-focus image from the lens is projected), with the lens focused at infinity.