In normal vision, the cornea (in front of the eye) and the lens (behind the pupil) work together to focus light rays onto the retina at the back of the eye. The retina converts light impulses into nerve impulses that travel along the optic nerve to the brain. The brain interprets nerve impulses and allows you to "see". Four primary types of optical focusing errors,
or refractive errors, may occur. These are nearsightedness (myopia), farsightedness (hyperopia), astigmatism, and presbyopia.