Focusing problems are not usually due to disease of the eye, but to the relationship of the size of the eye to the strength of its lens system. The eye consists of a focusing or refracting component (the cornea and the lens) and a light sensitive membrane in the back of the eye (the retina with its rods and cones). The only variable element in this system is the lens, which changes its shapes, and therefore its strength, in response to focusing effort. When the refracting mechanism brings light rays to a sharp focus on the retina, it is possible to see clearly. When the focal point is not sharp, or when it lies in front of or behind the retina, the image is blurred and the eyes are said to have a refractive error.
Most children are normally somewhat farsighted (hyperopic), which means that their lens system, in its relaxed state, is focused better for distant than for near objects. At the same time, children have a large reserve of focusing power, which allows the eyes to adjust to the demands of seeing at close range. Therefore, their moderate degree of farsightedness is not a problem. With increasing age the eyes grow larger and many children eventually become nearsighted (myopic), meaning that their eyes are now better focused for objects that are near, while at a distance, objects are blurred. A third kind of refractive error, astigmatism is a condition in which images do not come into sharp focus because of an irregular curvature of the cornea. Astigmatism may coexist with either nearsightedness or farsightedness. It cannot be overcome by focusing the eyes, but can be improved with glasses.
When refractive errors account for decreased vision, glasses can be used to complement the lens system of the eye. While the glasses provide clearer vision, they do not change the eyes in any physical way. Wearing glasses does not eliminate refractive errors; they are simply neutralized while the glasses are worn. Shatter-resistant glasses are recommended for all children.
Reproduced with permission and thanks to PREVENT BLINDNESS AMERICA®.
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