An ocular refractive error in which a point object cannot be brought to one sharp retinal focus by a single spherical correction. Regular ocular astigmatism has perpendicular principal meridians of unequal power; irregular departures require a more general description.[ref]
Astigmatism by the National Eye Institute. Overview and correction
The two principal meridians have different powers and are perpendicular. A paraxial pencil forms two perpendicular line foci at distinct depths. A cylindrical lens or toric lens can supply the required meridional correction.[ref]
Physiology of Astigmatism by the American Academy of Ophthalmology. Regular astigmatism
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