Explanation of Eyeglasses Prescription

A typical eyeglass prescription form.
A typical eyeglass prescription form. (Image: Image by Flickr.com, courtesy of Danny Hope)

If you have ever glanced at your eyeglass prescription and wondered what all the boxes and numbers meant, here's a quick summary. You'll find a prescription for your right eye, sometimes labeled O.D. (oculus dexter) and one for your left eye, sometimes labeled O.S. (oculus sinister).

Sphere Power

This indicates the refractive error of the eye. Generally, a negative number indicates myopia (nearsighted) while a positive number indicates hyperopia (far sighted).


Cylinder indicates the amount of astigmatism correction. It denotes lens curvature adjustments needed to compensate for an irregular shape of the eye's cornea or crystalline lens.

Axis (written in degrees)

Think of the 180 degrees in a semi-circle. This number tells the lens technician where to locate the power adjustment for the astigmatism correction.

Prism and Base

When the eyes are not well aligned and working in tandem the doctor adds prism to a lens to pull the errant eye back into place. The prism number indicates the strength of the correction while the base indicates the direction the eye will be drawn (in, out, up or down).

Add Power or Near

After age 40, we begin to notice a decreased ability to focus up close (called presbyopia). This number shows how much power must be added to the reading portion of the lens.


Don't assume that an "Add Power" of +2.00 on your prescription correlates to a pair of +2.00 Diopter off-the-rack reading glasses. The proper prescription for reading glasses is an algebraic combination of the distance and near prescriptions.

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