ted toward or away from the axis depending on the relativesizes of the focal lengths for the edge and central zones. The graphical representation of zonal variation of E. F. L,. isshown in Fig. 2. Prolong the rays to P/, P^ , P^ y and P/ back-vv^ard into the lens till they meet the rays P^, Q^, P^… … FlG. 4.—Diagram illustrating image by lens free from spherical aberration, but having zonal variation of E. F. L. sponding zones, and the surface on which all these points lie is the principal surface of the lens. lyCt h be the height at which the incident ray strikes the lens and u be the angle…
Lenses and optical systems
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