Focusing in the eye occurs at the level of both the corneal (the external clear covering of the eye) and the natural human lens. The cornea is responsible for the great majority of the overall focusing power of the eye. The cornea and the human lens work together to focus images precisely on the retina. Various conditions exist which prevent the cornea and lens from precisely focusing the entering rays of light onto the retina.
Nearsightedness, or myopia, is the most common focusing problem, affecting over 25% of the Americans. Nearsighted people can see better up close than distance. This is usually because the eyeball is too long, preventing the cornea and lens from being able to focus rays of light onto the retina. In such a situation, the rays of light converge onto a point which is infront of the retina instead of on the retina. The result is blurry distance vision. Myopia is very often corrected with lasik or with refractive surgery.
Farsightedness, or hyperopia, occurs in shorter eyeballs in which the convergence of light occurs beyond the retina rather than directly on it. The result is problems with near vision more so than distance vision. Farsightedness has varying effects depending on the age of the patient. Younger patients with farsightedness typically have fairly reasonable vision for both distance and near. However, as we age the human lens becomes more inflexible and hyperopia can cause increasing problems with reading. Later in life, nearly all focal ranges may be unclear. Farsightedness can be corrected with refractive surgery or clear lens extraction.
Astigmatism is a general inability of the eye to focus clearly at any distance because of uneven curvatures of the cornea, the eye’s front window. The normal cornea is shaped spherically ie: like a basketball. In astigmatism, the cornea is shaped like a football with different curvatures in its various meridians. This means that light rays are focused on several different areas instead of precisely one area of the retina. Astigmatism can cause distortion of vision and double images. Astigmatism is very often corrected with with contacts, glasses, or refractive surgery.