There are 2 types of lenses prescribed for correcting or improving 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 2 pieces of glass or plastic. These have been ground into lenses to 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 do this by adding or subtracting focusing power to the cornea and lens.
Contact lenses. Contact lenses are worn directly on the cornea of the eye. Like eyeglasses, contact lenses help to correct refractive errors. They do this by adding or subtracting focusing power to the cornea and lens.
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 eyeglasses 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).
In addition, the eyeglass prescription may also have the following measurements.
This number shows the extent of the nearsightedness or farsightedness.
This number means the amount of astigmatism in the eye. Astigmatism is an unevenly shaped cornea that causes blurring.
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" to show the strength of the lens for near vision. Bifocals are used to correct both nearsightedness and farsightedness.
The type of lenses used in eyeglasses depends on the type of vision problem and 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 and are used to correct astigmatism.
Over 30 million Americans wear contact lenses, 80% of whom wear daily wear soft lenses. Currently, there are 4 types of contact lenses:
The soft, water-absorbing lens
The rigid, gas-permeable lens
Other rigid lenses
Other flexible, non-water-absorbing lenses
The prescription for contact lenses includes more information than what is available on 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 or 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
Eye care providers are required by federal law to give you a copy of your contact lens specifications.