Eyeglasses, the most common form of eyewear used to correct or improve many types of vision problems, 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.
Types of Eyeglass Lenses
The type of lenses used in eyeglasses depends on the type of vision problem and may include:
- 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.