A routine eye exam includes a “refraction test” (or vision test), which can determine what lens prescription (if any) is needed to bring vision to optimum levels. “Refractive errors” (nearsightedness, farsightedness, and astigmatism) occur when light rays entering the eye cannot focus exactly on the retina, resulting in blurred vision. To get the needed prescription, the tested individual looks through a “phoropter,” an instrument that utilizes various lenses to measure the refractive error of each eye. As the individual looks through lenses of different strengths, he or she will be asked which of the choices looks clearer or more blurry. An appropriate prescription needed for glasses or contacts will be determined on the basis of the answers. Individuals who have “perfect” 20/20 vision are able to read letters that are three-eighths of an inch tall from 20 feet away.