The cornea and lens of the eye focus rays of light by bending them in a process called refraction. The light rays form an image on the retina in the back of the eye, much as a camera lens focuses images onto film.
The figure above shows an ideal eye with perfect focus. All the rays of light traveling through the eye focus as a single image on the retina. When the path through an eye contains imperfections, light is refracted onto the retina abnormally, and the resulting image is distorted or blurred. This condition is known as a refractive error. Many people with refractive error need corrective lenses or laser vision correction to help them see more clearly.
The three most common refractive errors are:
• Myopia (nearsighted)
• Hyperopia (farsighted)
Nearsightedness occurs when the cornea is too steep or the eye is too long. Light passes through the eye but focuses before it reaches the retina, and it is out of focus on the retina, causing blurred distance vision. Patients are usually able to see objects at near but not at distance. Glasses, contact lenses, or excimer laser reshaping of the cornea diverge light rays to focus on the retina, bringing distant object into focus.
Astigmatism occurs when the cornea is more curved in one direction than the other, shaped like a football rather than a basketball. Light passes through the eye and focuses in more than one plane, causing distorted vision. In one plane, light is focused on the retina but not in another. Patients with significant astigmatism often have blurred vision at both distance and near. Most patients have some astigmatism associated with the myopia or hyperopia. Glasses, contact lenses or the excimer laser correct astigmatism by bringing the astigmatic plane into focus on the retina.
Farsightedness occurs when the cornea is too flat in relation to the length of the eye. Light passes through the eye and comes into focus past the retina and is out of focus on the retina, causing blurred distance vision. Patients with mild hyperopia are able to see both distance and near when young. As they age, they first lose near vision, then distance vision. Glasses, contact lens or excimer laser reshaping of the cornea converge light rays to focus on the retina, bringing distant objects into focus.
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