The science and practice of photography : an elementary textbook on the scientific theory and a laboratory manual (1920)
Roebuck, John R. (John Ransom), 1876-1965 · New York ; London : D. Appleton and company · leaf 173
lenses 147 brought to a focus at a point which is called the principal focus, see Fig. 25. The distance of this point from the lens is called the focal length of the … lens. In this connection the two points N and N' are important, see Fig. 24. They are called nodal points and are so placed that rays of light entering Fig. 26. one from its side of the lens will appear to have come from the other as it emerges from the lens.
Photography
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