Refractive errors are the most common types of eye disorders, according to the Foundation of the American Academy of Ophthalmology. Light entering the eye bends as it passes through the cornea--the curved dome that covers the front of the eye--and the lens behind it. The refracted light beams fall on the macula, the central point of vision on the retina. Refractive errors occur when light entering the eye isn't focused directly on the macula.
Myopia
Myopia, or nearsightedness, is one form of refractive error. A longer than normal eyeball or sharper curve of the cornea focuses light in front of the retina rather than on it; close up objects are clear, but objects far away are blurry. Myopia in children generally worsens until the child reaches full growth, at which point vision stabilizes. Nearsightedness is often inherited. Myopia increases the chance of several more serious eye disorders, such as retinal detachment and glaucoma, high pressure in the eye that damages the optic nerve. People with myopia need glasses with a concave lens. Laser eye surgery can eliminate the need for glasses.
Hyperopia, or farsightedness, is the exact opposite of myopia. The eyeball is shorter than normal, or the cornea is less curved, and light falls behind the retina so that close-up objects are hard to see and faraway objects are clear. Hyperopia is often inherited. People who are farsighted need glasses with a convex lens. Laser eye surgery can correct farsightedness.
Astigmatism
If the cornea is irregularly shaped, astigmatism occurs. Light is focused unevenly or on two separate points; both close and distance vision is blurred or distorted. Two-thirds of people with myopia in the United States also have astigmatism, says Froedtert & The Medical College of Wisconsin. Astigmatism is common and often so mild that it may be noticeable. Twenty-eight percent of children in a recent study conducted by Dr. Karla Zadnik of the Ohio State University School of Optometry were found to have some degree of astigmatism. Laser eye surgery or corrective glasses can improve astigmatism.
Presbyopia
Presbyopia, another common refractive error, is caused by the stiffening of the lens with age. The lens loses its ability to accommodate, or change shape to adjust to close or distant objects. Many people older than 40 develop presbyopia, which worsens until around age 60 and then stabilizes, the Mayo Clinic says. Myopics who develop presbyopia may find it easier to read without their glasses.
