Astigmatism is due either to the irregular shape of the cornea, the clear front cover of the eye, or sometimes the curvature of the lens inside the eye.

This causes blur to the visual image at distance and near and glare and halos around lights. An eye with astigmatism does not have the focus of light fall sharply onto the retina, the part of the eye that send the visual image to the brain. Instead it is has more than one focus point.

Toric lenses in glasses or contact lenses can change the focus of light back to a fine clear point on the retina again, restoring clear vision.