Elementary optics and applications to fire control instruments (1922)
United States. Army. Ordnance Dept · Washington, Govt. Print. Off. · leaf 42
ck lens has three focal lengths, the equivalent focal length(E. F. L.) the back focal length (B. F. L.), and the front focal length(F. F. L.). The equivalent focal length is the focal length of athin lens which will form an image the same size as that formedby the thick lens provided that the… … Fig. 42.—Method of locating the image formed by a thick lens. In figure 41 the principal points are shown at P^ and F^. Parallel rays from the leftare brought to afocus at Fj, similarrays from the rightat ¥,. The distance P^Fjor PjFi is the equiv-alent focal length.The back focallength is VaF,, i.…
Optics · Optical instruments · Fire control (Gunnery) -- Optical equipment
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