Astigmatism usually is an imperfection in the curvature of your cornea — the clear, round dome covering the eye’s front surface. If you wear a contact lens, the lens sits on your cornea.
Sometimes, more rarely, astigmatism is an irregularity in the shape of the lens of your eye, which sits behind the pupil.
An eye without any astigmatism is basketball-shaped (round), while an eye with severe astigmatism is football-shaped (oblong).
-1.0 Astigmatism, -2 Astigmatism Prescription: What Do These Numbers Mean?
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 correct their astigmatism in order to maintain clear vision.
Glasses or contacts correct astigmatism by counteracting uneven curvatures of your cornea and lens. Treatment such as LASIK can treat astigmatism as well.
A person with the eye condition known as kerataconus, in which the cornea becomes thin and develops a cone-like bulge, can have astigmatism of up to 20 diopters. In such extreme conditions, a corneal transplant may be necessary to restore the spherical shape of the cornea. Ophthalmologists and optometrists sometimes use different measurements for astigmatism. Optical shops — who actually make the glasses or contacts to correct vision — have no problem understanding the prescription, no matter who is providing it.
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 your cornea is shaped.
- “Axis” is measured in degrees, and refers to where on the cornea the astigmatism is located. Axis numbers go from 0 to 180.
Astigmatism Prescription Measurement Machines
Your eye doctor may use several machines to measure you for astigmatism:
- An auto refractor shines light into the eye and then measures how it changes as it bounces off the back of the eye.
- A topography machine (also called a keratometer) focuses a circle of light on the cornea and measures its reflection. This determines the curvature of the cornea and helps the doctor properly fit your contact lenses.
- A phoropter measures how a lens must be shaped and curved to correct your vision to a normal state. The doctor has you look through a series of lenses in front of your eyes, and asks which ones make your vision better. Based on your answers, the doctor determines the lenses that provide the clearest vision.