astigmatism—An aberration affecting the sharpness of images caused by the fact that for objects off the axis the rays passing through different meridians of the lens come to a focus in different planes.   Thus, an extra-axial point object is imaged as two mutually perpendicular short lines located at different distances from the lens.   See also aberration of light (definition 2)

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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