Astigmatism
A visual defect caused by non-spherical, asymmetric curvature of the cornea or lens, preventing rays in different planes from focusing at a single point.
Governing formula
Power difference = P_vertical - P_horizontal
SI unit
Diopters (D)
In depth
Vertical and horizontal light rays focus at different focal planes, causing blurred vision at all distances. Corrected using cylindrical lenses.
Examples in the real world
Prescribing eyeglasses with cylindrical power ($+1.5 D cylindrical @ 90^°$) to correct corneal astigmatism.