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Understanding Your Eye Glasses Prescription

This information will help you decipher those numbers and abbreviations on your spectacle prescription.

A spectacle lens tries to focus a clear image onto the retina. The doctor’s prescription is due mainly to the shape of your cornea. If you are only nearsighted (myopic) or farsighted (hyperopic), then you have what is called a “spherical” prescription. This means a simple spherical lens (like a magnifier or minifier) is used, and there is only one focal point.

The cornea is normally spherical, like a basketball. No matter how you slice it, vertically, horizontally or at an angle, the curve is always the same. The focus power of the presxcription would be a single number such as 2.25. It looks like dollars and cents, but the units are called “diopters” (D). The number is the “refractive error,” or the number of degrees you are away from normal vision.

If your cornea is shaped more like the oval surface of a football, two focal points exist. This is because the curve around the center of the football is different than the curve from one tipÊto the other. That condition in the eye is called “astigmatism.” A lens correcting an eye with astigmatism, therefore, would have two diopter numbers.

The “football” can be vertical, horizontal, or at an angle. A line from end to end, going though the center of the football, is called the “axis.” It’s angle is based on the direction of the axis and identified in degrees, as on a protractor, with 0 degrees to the right and 180 degrees to the left. The angle number is the third number in a prescription for people with astigmatism. Such a prescription is called spherocylindrical.

Most doctors use a plus sign to indicate the need for a convex lens to correct farsightedness. A minus sign indicates the need for a concave lens to correct nearsightedness.

The term “plano” (pl) means there is no refractive error.
OD stands for “oculus dexter,” or right eye. OS stands for “oculus sinister,” or left eye.
DS stands for “diopter sphere.” If this appears in the 2nd and 3rd columns of your prescription, or if the columns are blank, you have no astigmatism.

So a spectacle prescription for one of your eyes can look like this:
OD: “+1.25 -2.50 x 85”
Meaning your right (OD) eye is:
1. slightly farsighted, needing a focus power of +1.25
2. has 2.50 units (diopters) of astigmatism
3. the angle of the axis is tilted slightly to the right (at 85 degrees on the protractor)

Or it might read like this:
OS: “+1.25 DS DS” (or “+1.25 ___ ___”)
Meaning your left eye (OS) is slightly farsighted, needing a focus power of +1.25, and no astigmatism is present.

Finally, remember that the prescription has nothing to do with how well you see. That is your “acuity.” Someone with this prescription could measure better than 20/20 acuity, while someone else with the same prescription could measure 20/200.

More information about glasses and visual acuity:
How Visual Acuity Is Measured
Computer Vision Syndrome and Computer Glasses
Sunglasses and Macular Degeneration
Snellen Chart

Roy Cole, O.D., F.A.A.O. (Director of Vision Program Development, Jewish Guild Healthcare)
How To Read Eyeglass Or Contact Lens Prescription