What Do Prescription Numbers Mean in Eye Glasses?

Written by kate bruscke
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What Do Prescription Numbers Mean in Eye Glasses?
Millions of eyeglass prescriptions are written every year. (Image by Flickr.com, courtesy of Flóra Soós)

Eyeglasses correct many different types of refractive errors, or sight problems, including nearsightedness, farsightedness or astigmatism. Trying to understand the numbers on an eyeglass prescription requires understanding the measurements optometrists use when fitting you for corrective lenses.

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Dioptres

The numbers on your prescription are given in dioptres, abbreviated "D," which measure the focusing power, or amount of correction, that each of your eyes require.

O.S. and O.D.

Optometrists use the Latin abbreviations O.S. and O.D. to differentiate between your eyes. O.S. stands for oculus sinister, and refers to your left eye, while O.D., or oculus dextrus, refers to the right eye.

Positive and Negatives

Generally, your prescription will show a number for O.S. and O.D. (given in dioptres). The farther this number is on a number line from zero, the more correction your eyeglasses should provide. A plus sign means the eye is farsighted, while a minus sign means the eye is nearsighted.

Spherical Value

This positive or negative number for each eye is called the spherical value, or the degree of nearsightedness or farsightedness you have. It is abbreviated as "S."

Cylinder

If you have an astigmatism, the optometrist will also measure its degree and severity. The higher the number (given in dioptres) marked as the cylinder of astigmatism, (abbreviated "C"), the more astigmatism there is in that eye.

Axis

An astigmatism is also measured in terms of its axis, which is a number between 0 and 180 that refers to the orientation and placement of the astigmatism on that particular eye.

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