Definition: Optics and optometrics: aberration of optical system which causes image detail in one axis to focused at a different plane from image detail in another e.g. a spoked carriage wheel can have its spokes rendered sharp but not the wheel rim, or the rim can be sharp but the spokes will be out of focus. * In primary astigmatism, the axial separation of the tangential and sagittal image planes. * In radial astigmatism, the aberration increases with increased radius or field angle: a large circle centred on the optical axis is less sharp than a smaller circle centred on the optical axis.

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