What does it mean to have astigmatism in your eye?

Astigmatism is the lack of the eye having a single focal point. In an astigmatic eye, irregularities in the cornea or lens cause there to be a range of focal points, e.g. the eye might be -3.00 Diopters nearsighted vertically, but -4.50 D. horizontally. We would say this eye has 1.50 D. of astigmatism. Astigmatism can occur in both nearsighted (myopic) and farsighted (hyperopic) eyes. It can even be a little of both (mixed astigmatism), e.g. +1 D. vertically and -2.50 D. horizontality:

(│+1│+│-2.50│ = 3.50 D.) astigmatism, some of it nearsighted and some farsighted!

Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic