Astigmatism
Astigmatism is a refraction error of the eye
characterized by an aspherical cornea in which one axis of corneal
steepness is greater than the perpendicular axis. Astigmatism
causes difficulties in seeing fine detail, and can be often
corrected by glasses with a cylindrical lens (i.e. a lens that has
different radii of curvature in different planes), contact lenses,
or refractive surgery.
Astigmatism occurs when either the cornea or the lens of the eye is
not perfectly spherical. As a result, the eye has different focal
points in different planes. For example, the image may be clearly
focused on the retina in the horizontal plane, but in front of the
retina in the vertical plane.
Aspherical cornea (top,
exaggerated) and normal eye below.
Types of Astigmatism
- Simple
-- Simple hyperopic astigmatism
-- Simple myopic astigmatism
- Compound
-- Compound hyperopic astigmatism
-- Compound myopic astigmatism
- Mixed astigmatism
Simulated vision of a person with
uncorrected astigmatism. The oblong shape of the cornea causes
opposing radial portions to be blurred.
