Refractive Errors

On this page we explain the different types of refractive errors. An illustration shows what happens to light rays entering the eye and how this can result in impaired vision or what we call refractive errors.

Normal Eye

Normal Eye

Normal Eye

In the normal eye, light rays entering the eye are accurately focused on the retina and a clear image is formed. Most of the bending or focusing of the light rays occurs at the cornea, with the natural lens inside the eye being responsible for fine adjustments. If light is not focused on the retina then the eye is said to have a refractive error. Common refractive errors include: myopia or shortsightedness, hyperopia or longsightedness, astigmatism, and presbyopia.

Myopia

Myopic Eye

Myopic Eye

In Myopia, or shortsightedness, light rays come to a focus in front of the retina. This results in blurred vision, especially when looking at objects far away. Myopia results from the length of the eye being too long or the cornea being too steeply curved.

Hyperopia

Hyperopia Eye

Hyperopia Eye

In hyperopia, or longsightedness, light rays are focused behind the retina. This results in blurred vision especially when looking at objects that are close. Hyperopia results from the length of the eye being too short or the cornea being too flat.

Astigmatism

Astigmatic Eye

Astigmatic Eye

In astigmatism, the cornea, or window of the eye, has an irregular curvature being shaped more like a rugby ball than a soccer ball. Light rays are focused at different points. Astigmatism is often associated with myopia or hyperopia.

In Presbyopia the problem is not a result of the cornea being incorrectly shaped or irregular compared to the length of the eye, but is due to an aging process occurring in the natural lens of the eye. As we age the lens becomes harder and less pliable and is unable to change its shape to focus (accommodate). After we reach forty years of age we begin to experience problems with focusing close, for example when we read. This is when most people begin to use reading glasses. Because presbyopia is not caused by an abnormal shape of the cornea, but by aging of the lens, it cannot be treated by the excimer laser.

In myopia, hyperopia, and astigmatism the cornea is incorrectly shaped or irregular compared to the length of the eye. The Excimer Laser is able to accurately re-sculpture the cornea to make the curve of the cornea match the length of the eye. Light rays are then focused normally on the retina. In myopia the cornea is flattened, whereas in hyperopia the cornea is made more curved. With astigmatism the surface of the cornea is re-sculptured to a regular curvature.