Eyeglasses
Eyeglasses are a frame that holds two pieces of glass or plastic, which have been ground into lenses to correct refractive errors. Refractive errors can include nearsightedness or myopia (difficulty seeing far away), farsightedness or hyperopia (difficulty seeing close up), and astigmatism (blurring due to an irregularly shaped cornea). Eyeglasses perform this function by adding or subtracting focusing power to the eye's cornea and lens.
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 eyeglasses prescription, you will see the following abbreviations:
- O.D. - Oculus dextrus 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 contain the following measurements:
- Sphere - the extent of the nearsightedness or farsightedness
- Cylinder - the amount of astigmatism in the eye
- Axis – description of the astigmatism in degrees from the horizontal axis (most left and right eyes have the same axis in astigmatism)
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. By federal law, an eye doctor is required to provide an eyeglass prescription following an eye exam at no extra cost. In addition, under federal law, you have the right to a copy of your eyeglass prescription, so you can shop for the best value in eyewear.
Types of Eyeglass Lenses
The type of lenses used in eyeglasses depends on the type of vision problem and may include:
- Concave lenses (thinnest in the center) to correct nearsightedness (myopia)
- The numerical prescription in diopters is always marked with a minus (-) symbol
- Convex lenses (thickest in the center) to correct farsightedness (hyperopia)
- The numerical prescription in diopters is always marked with a plus (+) symbol
- Cylindrical lenses curve more in one direction than the other and are often used to correct astigmatism