aberration, spherical

Definition: Defect in image caused by rays of light from edge of lens tending to be focused nearer lens while rays entering lens near axis are focused further away. * That is, focal length depends on distance between the lens axis and point of entry of the light. * Spherical aberration is negative (undercorrected) when rays from the outer zone is focused closer to the lens than rays passing through the central zones; vice versa for positive spherical aberration (over-corrected). Direct aberration that affects entire image field. Worsens with square of the diameter of the lens and as lens focuses nearer. * It is improved by stopping down.

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