Refraction is the bending of light as it passes from one medium to another. In an eye, light passes from the air, through the cornea at the front of the eye, the lens, into the aqueous humor inside the eye, and onto the retina at the back of the eye, from which the image is transmitted to the brain.
The cornea provides about 2/3 of the eye's optical power, but it is fixed-focus. There are muscles attached to the lens to fine-tune its focus, ideally producing a sharp image on the retina. These muscles change the shape of the lens depending on the distance of the focused upon object, much like an autofocus lens on a camera.
Refractive errors occur when the lens does not focus the image on the retina. They can be caused by changes in the length of the eyeball, changes in the shape of the cornea, or aging of the lens.
The most common types of refractive errors are:
- Myopia (nearsightedness): Close objects are clear, while distant ones are blurry. It is caused by the lens focusing the image in front of the retina.
- Hyperopia (farsightedness): Distant objects are seen more clearly than close objects. Some people may not notice any blurring, especially when they are young. As you age, vision at both distance and near becomes blurred.
- Astigmatism: The cornea is misshapen, causing blurriness at all distances.
- Presbyopia: As the eye ages, the lens cannot change shape enough to focus on close objects. It affects most people over age 40.
Refractive errors are diagnosed by standard vision tests. Presbyopia is corrected with reading glasses, or bifocal eyeglasses that have higher focusing power in the lower part of their lenses, so that the lower part can be used for close work, and the upper part for distance vision.
Myopia, hyperopia, and astigmatism are corrected with eyeglasses, contact lenses and with LASIK refractive surgery. LASIK permanently changes the shape of the cornea, allowing the cornea-lens system to focus the image sharply on the retina without the need for eyeglasses.