What Do Astigmatism Measurements Mean?

After an eye exam your doctor will hand you a prescription. But have you ever tried to read it?

Ophthalmologists and optometrists sometimes use different measurements for astigmatism. Their prescriptions may appear different depending on what type of provider you see. The optical shops who make the glasses or contacts have no problem understanding the prescription no matter who is providing it.

Astigmatism is measured in diopters. A perfect eye with no astigmatism has 0 diopters. Most people have between 0.5 to 0.75 diopters of astigmatism. People with a measurement of 1.5 or more typically need contacts or eyeglasses to have clear vision.

Of the three numbers on your contacts or glasses prescription, the last two refer to astigmatism:

• Spherical indicates whether you are nearsighted or farsighted. A plus sign indicates you are farsighted, a minus sign indicates you are nearsighted. The higher the number, the stronger your prescription.
• Cylinder measures what degree of astigmatism you have, or how flat or irregular the shape of your cornea is. The more your eye looks like an American football (instead of a basketball), the more astigmatism you have. 
• Axis is measured in degrees, and refers to where on the cornea the astigmatism is located. Axis numbers go from 0 to 180. If you think of the eye as a map hanging on a wall, the 90 degree axis or line, runs up and down (or north to south) on the eye. The 180 degree line runs across the eye, east to west.