Eyeglasses and Contact Lenses

Lenses to correct or improve vision

There are two types of lenses prescribed to correct or improve vision. These include:

• Eyeglasses. These are the most common form of eyewear used to correct or improve many types of vision problems. Eyeglasses are a frame that holds two pieces of glass or plastic that correct refractive errors. Refractive errors can include difficulty seeing far away (nearsightedness or myopia), difficulty seeing close up (farsightedness or hyperopia), and blurring due to an unevenly shaped cornea (astigmatism). Eyeglasses work by adding or subtracting focusing power to the cornea and lens.

• Contact lenses. Contact lenses are worn directly on the cornea. Like eyeglasses, contact lenses help correct refractive errors. They do this by adding or subtracting focusing power to the cornea and lens.

How to read an eyeglass prescription

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

O.D. Oculus dextrus means the right eye (sometimes the abbreviation RE is used).

O.S. Oculus sinister means the left eye (sometimes the abbreviation LE is used).

The eyeglass prescription may also have the following measurements:

Sphere	This number shows the extent of the nearsightedness or farsightedness.
Cylinder	This number means the amount of astigmatism. Astigmatism is an unevenly shaped cornea that causes blurring.
Axis	This number describes the astigmatism in degrees from the horizontal axis. Most left and right eyes have a similar axis of astigmatism.

Bifocal prescriptions often have an additional measurement listed on the prescription as "add." This shows the strength of the lens for near vision. Bifocals are used to correct both nearsightedness and farsightedness. 

What are the different types of eyeglass lenses?

The type of lenses used in eyeglasses depends on the type of vision problem. They may include:

• Concave lenses. These are thinnest in the center. They are used to correct nearsightedness. The numerical prescription in diopters is always marked with a minus (-) symbol.

• Convex lenses. These are thickest in the center. They are used to correct farsightedness. The numerical prescription in diopters is always marked with a plus (+) symbol.

• Cylindrical lenses. These curve more in one direction than in the other. They are used to correct astigmatism.

Facts about contact lenses

More than 30 million Americans wear contact lenses. Of those, 80% use daily wear soft lenses. Currently, there are four types of contact lenses: 

• The soft, water-absorbing lens

• The rigid, gas-permeable lens

• Other rigid lenses

• Other flexible, non-water-absorbing lenses

Reading a contact lens prescription

The prescription for contact lenses includes more information than what is available in the prescription for eyeglasses. Special measurements will need to be taken of the curvature of the eye. In addition, the eye care provider will determine if the eyes are too dry for contact lenses. They check if there are any corneal problems that may prevent a person from wearing contact lenses. Trial lenses are often worn for a period of time to make sure they fit correctly.

The contact lens prescription often includes this information:

• Contact lens power (measured in diopters, like eyeglasses)

• Contact lens base curve

• Contact lens diameter

• Contact lens manufacturer and brand

• Expiration date

Eye care providers are required by federal law to give you a copy of your contact lens details.