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.
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