Focus (of a fixed lens)

From International Dictionary of Marine Aids to Navigation
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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.

Ch2 Fig16b.jpg

Ch2 Fig18.jpg

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