Focus (of a fixed lens)
The point of intersection of the axis of the lens (2-2-115) and the median horizontal plane through the belt. A ray of light emitted by a point source situated at this point is re-directed by an element of the lens into a direction normal to the axis of the lens. (Figs. 16b and 18)
Note: The term "focus" in its more general and correct sense is given in 2-1-125.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM