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 170
PHOTOGRAPHY 146 foci, the first a real focus and the second an imagi- nary or virtual focus. The straight line through the centers of the spheres of which the lens surfaces are a part, is called the axis of the lens. If one of the lens surfaces is a plane, the axis meets it per- pendicularly while… … When a lens is formed of more than one piece of glass, as with the majority of photographic lenses, the centers of all the spheres should lie on the one line; a lens otherwise perfect may be very much impaired in definition of image if the makers of the mount have not been careful to have this…
Photography
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