Light rays enter the eye through the clear cornea, then through the pupil and the lens. These light rays are focused onto the retina, a light-sensitive tissue lining the back of the eye. Signals from the retina are sent through the optic nerve to the brain where they are interpreted as the images we see. Seventy percent of the eye’s focusing power comes from the cornea. When people have problems seeing clearly, it is frequently because of a refractive error. There are four main types of refractive error: myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia. People with refractive errors might have one or more combinations of these conditions. Myopia is also known as nearsightedness. This occurs when the distance between the cornea and the retina is too long. Light rays entering the eye are focused in front of the retina causing the image that falls on the retina to be blurred. Hyperopia is also known as farsightedness and occurs when the distance between the cornea and retina is too short. Light rays entering the eye have not yet come into focus when reaching the retina so again the image is blurred. Astigmatism is a condition in which the cornea is curved unevenly. A cornea that is curved the same in each direction is shaped like a basketball while a cornea with astigmatism is more curved in one direction than the other like a football. Light passing through this uneven cornea is not properly focused on the retina. Presbyopia is a normal condition associated with age that causes problems with our near vision. When we are young, the lenses in our eyes are flexible and are able to change focus easily between near and far objects. As we age, this flexibility gradually decreases. At around age 40, many people begin to need corrective lenses for reading.