Casey Eye Institute at OHSU, Portland, Oregon

Where Healing, Teaching and Discovery Come Together

Improving Vision with Glasses

Eyeglasses are the most common form of eyewear used to correct or improve many types of vision problems. They are made of two pieces of glass or plastic, ground into lenses that correct refractive errors by adding or subtracting focusing power to the eye's cornea and lens. Refractive errors can include nearsightedness, or myopia (difficulty seeing at a distance), farsightedness, or hyperopia (difficulty seeing close up), and astigmatism (blurring due to an irregularly shaped cornea).

How to Read an Eyeglass Prescription

The lens power of eyeglasses is measured in diopters. This measurement reflects the amount of power necessary to focus images directly on to the retina. When looking at an eyeglass prescription, you will see the following abbreviations:

eh_improve_lenses_300pxO.D. – Oculus dextrus simply refers to the right eye (sometimes the abbreviation RE is used).

O.S. – Oculus sinister refers to the left eye (sometimes the abbreviation LE is used).

In addition, the eyeglass prescription may also contain the following measurements:

Sphere This number measures the extent of the nearsightedness or farsightedness.
Cylinder This number measures the astigmatism (an irregularly shaped cornea which causes blurring) in the eye.
Axis This number measures the astigmatism in degrees from the horizontal axis.

Bifocal prescriptions, which correct both nearsightedness and farsightedness, usually have an additional measurement listed on the prescription as "add," to indicate the strength of the lens.

What are the Different Types of Eyeglass Lenses?

The type of lenses used in eyeglasses depends on the type of vision problem.

Concave lenses are thinnest in the center. Used to correct nearsightedness (myopia), the numerical prescription in diopters is always marked with a minus (-) symbol.

Convex lenses are thickest in the center, like a magnifying glass. Used to correct farsightedness (hyperopia), the numerical prescription in diopters is always marked with a plus (+) symbol.

Cylindrical lenses curve more in one direction than in the other and are often used to correct astigmatism.