Aberrations

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Aberration, spherical

(Science: optics) a lens defect whereby image forming rays of one colour, passing through the outer zones of a lens come to focus at a different distance from the lens than do those of more central rays.

With a simple spherical (or plano-spherical) lens the outer rays always meet the axis closer to the lens than do more central rays and the lens is uncorrected or undercorrected. When the reverse is true the lens has been overcorrected.