A condensed course in motion picture photography ([1920])

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THE NATURE OF LIGHT lenses, perpendicular to the surfaces of the lens, as in diagrams No. 22 and No. 23. 2d. The optical center of a lens is the point from which focal measurements are made. This does not refer to a photographic objective which (in other than single view lenses) is a combination of lenses and quite another matter for the reason that a combination may have its optical center at a number of places according to the circumstances under which it is employed. The Fig. 22. NodaJ point within the lens. positive Optical center of a lens is determined by its form as follows and shown in diagrams No. 22 and No. 23. Draw two parallel radii AB and ab one from each center of curvature, and both inclined to principal axis ; then connect the two points B and b at which they touch the curved surfaces of lens. The point O, at which the line connecting B and b cuts the principal axis, is the optical center. In most cases the optical centre is within the lens itself but in some cases as with telephoto combinations and single meniscus lenses it may be some distance outside the lens. Such an example is shown in Fig. 23. 3d. Conjugate foci. If a lens which has been carefully focused upon a distant object be then directed toward one comparatively near at hand, the nearer object will be found to be 49