The normal cornea – the front surface of the eye – has approximately the same spherical curvature in all meridians. Astigmatism is usually the result of distortion of the cornea with steeper curvature in one meridian and flatter curvature in the opposite meridian. The usual comparison is between a spherical football and an astigmatic rugby ball. Rays of light in the different meridians focus at different points on the retina. Astigmatism can be associated with either short or long sight and is corrected by means of a toric contact lens – simply put, a two curve lens for a two curve eye.
Astigmatism will appear on your prescription as the cylinder (cyl) and will usually have a minus sign although it can be written in plus form. The meridional direction of the astigmatism is referred to as the axis and is designated by numbers between 0o and 180o.
Even high degrees of astigmatism can be corrected by either hard or soft contact lenses.
Illustration © and reproduction courtesy Simon Levy.