= Optical aberration
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An <optical aberration> is a departure from ideal <image formation>: <light rays> from one object point may miss their intended common image point, or image positions may depart from the intended mapping. <Spherical aberration>, <coma>, and <astigmatism> describe different ways of spreading a point's <light rays>; <optical distortion> can change image shape without spreading each point. In <physical optics>, these departures can be described by <wavefront error> relative to a reference <wavefront>. <Diffraction> also gives a finite <point spread function> in a perfect <optical instrument>, so an extended image alone does not establish an <optical aberration>.
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