Having trouble reading your ophthalmologist's prescription? Here's some information to help you decipher it.

First of all, it’s important to know that your right eye and your left eye don’t necessarily require the same correction; therefore, your prescription distinguishes between “OD” for your right eye and “OG” for your left eye.

After this abbreviation comes the spherical power expressed in diopters, preceded by a + or – sign.

The "-" sign indicates that you need a negative or concave lens.
Example: OD -1.00

The + sign indicates that you need a positive or convex lens.
Example: OD +1.00

If this designation is followed by another number—often in parentheses—it means you need toric lenses. This second number indicates the cylindrical power of the lens, which is necessary to correctastigmatism.

A third number, ranging from 0° to 180°, indicates the cylinder axis.
Example: OD -1.00 (-0.75) 45°

These numbers indicate the strength of the corrective lenses; they do not provide information about your visual acuity and, unless otherwise specified by your eye care professional, indicate the strength needed to correct distance vision.

It’s also important to note that these numbers always refer to the prescription strength of eyeglass lenses. The prescription strength of a contact lens used to correct the same vision problem would be different.

It is theoptometrist's job to determine the power of the contact lens needed to correct the vision problem.

Often, an additional notation appears on the prescription to indicate your near vision needs. This is called the add. It indicates the difference in prescription strength between distance and near vision.