What are refractive errors of the eye?

The eye’s ability to refract or focus light sharply on the retina may be affected by a variety of conditions. When the eyeball is too long, visual images are focused in front of the retina, and myopia, or nearsightedness, results. When the eye is too short, images are focused beyond the retina. This causes hyperopia, or farsightedness. If the cornea is not perfectly round, then the image is refracted or focused irregularly, resulting in a condition called astigmatism. Similarly, an irregularly-shaped natural lens or problems with the way it functions also can cause focusing problems.

These various focusing problems can cause light rays to bend or refract at odd angles, leading to blurry or distorted vision. This inability to achieve sharp focus is called refractive error.

Although refractive errors are called eye disorders, they are not diseases.

There are a number of methods used to correct refractive errors, including eyeglasses, contact lenses, and refractive surgery.

Refractive error is a condition that causes blurred vision when light rays entering the eye meet in front of or behind the retina rather than directly on it.

Normally, light rays are bent (refracted) by the cornea and lens and focus directly on the retina, the nerve layer at the back of the eye, which results in clear vision.

In most cases, refractive errors represent a natural variation from normal vision and are not considered a disease. Nearsightedness, farsightedness, astigmatism, and presbyopia are common refractive errors.

Vision usually can be corrected with glasses or contact lenses or, in some cases, surgery.

The most common refractive vision problems are nearsightedness (myopia); farsightedness (hyperopia); astigmatism and presbyopia.

In nearsightedness, the image from distant objects gets focused in front of the retina instead of right on it. This happens usually when the eyeball itself is too long; it can also be caused by too much focusing power in the lens system. The result is that the person can see objects fine close up, but objects at a distance are blurry. The problem can be corrected by using a concave lens - this spreads out the light so that it comes to focus on the retina as it passes through the lens system.

In farsightedness, the light gets focused in back of the retina instead of right on it. This happens when the eyeball is too short or for example when the focusing power of the lens system is too weak. A person can see distant objects fine, but in this case close-up objects appear blurry. The problem is corrected with use of a convex lens to concentrate or converge the light so that it comes to focus on the retina when the image passes through the lens system.

In astigmatism, the shape of either the cornea or the lens is distorted; the light comes into two focal points. For example, the lens is egg-shaped instead of spherical so that light comes over the top and bottom edges bringing it to a different focal point than when light comes in over the right and left sides. This problem can be corrected by a lens that is shaped to rearrange the distorted shape of the eye's egg-shaped lens system.

When the cornea and lens of the eye become less stretchy, they cannot change shape readily and cannot bring light to a focus on the retina. This is called presbyopia. When people reach their 40's, this is a common problem. When a person has presbyopia, there is trouble focusing light on the retina from objects both near and far. A pair of bifocal lenses can correct this: the top part allows clear vision of far objects and the bottom part for near objects.

Refractive errors refer to a category of sight problems (hyperopia, myopia, presbyopia, and astigmatism) in which the eyes have trouble focusing light properly on the back of the retina. Luckily, glasses, contact lenses, or laser surgery can help correct these problems.

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When the eye sees normally, light focuses directly on the retina, producing a clear image. But in some people, the optics are faulty, and images appear blurred because the eye focuses the image either in front of or behind the retina. These problems are not eye diseases but common conditions known as refractive errors of the eye. Although laser surgery procedures such as laser-assisted in situ keratomileusis (LASIK) have become increasingly popular as a way to correct refractive errors, such techniques are usually appropriate only for people younger than 50. For that reason, refractive errors in older adults are most often corrected with eyeglasses and contact lenses.