aspheric (aspherical) lens

A lens element that changes shape across its surface as opposed to one having a smooth continuous arc. Generally, an aspherical lens deviates slightly from an exactly spherical shape, and is relatively free from aberrations. Light rays are bent more at the edges of a conventional spherical lens than they are at the center, causing them to come into focus before the film plane or sensor plane. A lens made with aspherical elements focuses all the light rays passing through it on the film/sensor plane.

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