Sixty percent of the 161 million Americans who wear prescription eyewear choose eyeglasses. Wearing eyeglasses is one of the simplest ways to correct vision problems. To see images clearly, light rays must focus directly on the retina, the light-sensitive nerve layer that lines the back of the eye. There are different kinds of focusing problems, called refractive errors, which may require corrective lenses. In the case of myopia or nearsightedness, the eye is too long.
Light rays focus before reaching the retina and images appear blurry.
In hyperopia or farsightedness, the eye is too short, so light rays have not yet focused when they reach the retina. Astigmatism describes an eye with a cornea that is oval shaped instead of round, causing light rays to hit the retina in more than one place. Eyeglass lenses compensate for an eye that is too long or too short by adding or subtracting focusing power. The lenses create just the right amount of focusing power so light rays focus directly on the retina. If a second and third number are present in the prescription, they indicate astigmatism.