A routine eye exam begins by measuring visual acuity and performing a refraction to determine your best vision. How well you see at different distances (visual acuity) depends on how your eyes bend light to focus the picture properly (refraction). Eye doctors use the Snellen chart (a chart of letters or numbers of different sizes) to measure visual acuity. The degree to which you do not have normal vision is your refractive error. Your eyeglasses and contact lens prescriptions correct your refractive error. Common types of refractive errors are Myopia (nearsightedness), Hyperopia (farsightedness), and Astigmatism (the roundness of the eyeball).