Focal length: The effective focal length (EFL) is the distance from the
principal point to the focal point. The back focal length (BFL) is the
distance from the vertex of the last lens to the second focal point. The
front focal length (FFL) is the distance from the first lens surface to
the first focal point.
Focal plane: A plane (through the focal point) at right angles to the
principal axis of a lens or mirror; that surface on which the best image
is formed.
Focal point: That point on the optical axis of a lens, to which an
incident bundle of parallel light rays will converge.
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The Photonics Handbook is another useful guide for practitioners of
optics.
<http://www.photonics.com/directory/hb/XQ/ASP/QX/index.htm>