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In an optical system, astigmatism occurs when light rays associated with two perpendicular meridians focus differently. For an off-axis point in a rotationally symmetric system, tangential and sagittal bundles can form line-like images at different axial positions rather than one common point image.

Move a screen through focus while imaging an off-axis point: the blur can pass from a line in one direction through a more compact patch to a line in the perpendicular direction. This differs from Petzval field curvature, which concerns the shape of the best-focus surface. A detector placed at one compromise focus does not remove the underlying astigmatism.

Petzval field curvature is an optical aberration in which the preferred image surface is curved rather than planar. Even an otherwise sharply imaged flat object can therefore have its center in focus on a flat detector while its edges are out of focus.

A useful test images a flat pattern and refocuses separately for central and peripheral features. If the best-focus detector position changes with field position, field curvature contributes. Unlike optical distortion, it changes where sharp focus occurs rather than simply displacing image positions. Additional optics can flatten the useful image surface, subject to other optical aberrations.