Eyeglasses have been used since antiquity and are still the most common method of correcting vision. The science of optics is complex, but the basic principle governing how glasses work is simple. The curvature of the lenses in the glasses compensates for the refractive error of your eye. If you are nearsighted, you will need a concave lens, which will adjust the focal point of your eye so that it is on the retina rather than in front of the retina. If you are farsighted, a convex lens will refract the light so that the focal point is on the retina rather than behind it. A cylindrical lens corrects astigmatism by sharpening the blurred image caused by the uneven cornea. For some people with astigmatism, cylindrical lenses cause problems of distortion, but adjustments in prescription and the way the glasses are fitted to the face can usually overcome these problems. If not, contact lenses usually are effective. Simple magnifying glasses–the reading glasses you might buy off the rack in a pharmacy–are helpful for many people with presbyopia. However, when presbyopia is combined with nearsightedness or farsightedness, correction becomes more complicated. For some nearsighted people, the disorders cancel each other out, and they can read comfortably without correction. (Distance vision, however, still must be corrected.) Bifocals and trifocals are other options to make the corrections necessary for middle-aged and elderly eyes. Bifocals correct for distance and near vision. The top and outer rim of the lens corrects for distance vision, while a smaller lens set into the inside bottom quadrant of the frame–the section you usually look through for reading or other close work– corrects for near vision. However, bifocals can be custom designed for a user’s specific needs. For example, a reversed design may be best for an airline pilot who may need to read through the upper part of the lenses when looking up at an instrument panel. Some people, particularly as they grow older, need a third level of correction for intermediate, arm’s length work (2 to 4 feet) that allows them to work comfortably at a computer terminal or read the prices on the supermarket shelf. If you find yourself straining your neck as you peer through the bottom part of your bifocals to read something that will not focus with your distance correction, you may need trifocals. Trifocals allow a person to focus on objects that are beyond reading distance but are closer than about 3 feet. Traditionally, a line has separated the lenses in bifocal and trifocal glasses. This necessitates a jump, a quick change in focus, when you move from one section to another. Some people adjust to this very well, but others find the jump disorienting. Progressive lenses offer an alternative and are becoming increasingly popular. In progressive lenses, also called multifocals, there is no visible line between the sections. A narrow and invisible hand of gradual change separates the sections. Some people find that this seamless change translates into better vision; others prefer progressive lenses for cosmetic reasons, believing that the line in the lenses of traditional bifocals and trifocals is an unattractive sign of aging. For some conditions, such as crossed eyes, prisms are incorporated into the lens. Prisms play a role in helping align the eyes. Clip-on or stick-on prisms can be tried before having the prescription ground into the lens. Lenses in glasses are made of glass or plastic. Although glass is stronger, government regulations require that all lenses pass a test for impact resistance; they must be able to withstand the impact of a 5/8-inch steel ball dropped from a height of 50 inches. Plastic lenses are more likely to scratch but they are 30 to 40 percent lighter in weight than lenses. Coatings for lenses are available for several purposes. Coatings can reduce reflective glare, block ultraviolet light and make the lenses scratch-resistant. These are all useful features to protect your eyes and your eyewear. Sunglasses are recommended for anyone spending extended periods of time in bright sunlight. Dark sunglasses absorb as much as 80 percent of ultraviolet rays, which are known to cause tissue damage and may contribute to eye disease. Try to choose sunglasses that provide protection from both ultraviolet A (UVA) and ultraviolet B (UVB) light. The higher the UVA or UVB rating on the glasses, the more protection they offer. The fit of glasses is important for the best correction for your refractive problem. Lenses sitting too far from or too close to your face can make glasses ineffective. When you get glasses, be sure to get them from a qualified optician who has experience in fitting glasses and offers a wide selection of frames. Protective eyewear is manufactured for many purposes. A plastic called polycarbonate is used in lenses when athletes, people in certain workplaces or people who have already injured one eye need special eye protection. Polycarbonate is extremely tough and impact-resistant; however, it is more expensive than is the material used to make conventional lenses. Safety glasses made of polycarbonate provide side shields to protect the eye and have other safety features. Glasses work well for most people and are easy to maintain and relatively inexpensive. The disadvantages of glasses are that these may restrict peripheral vision and may interfere with some physical activities. Some people also do not like the way glasses change their appearance; others are bothered by the weight and presence of glasses on the face, and they are easy to lose or damage. Last Updated: 1/13/2003
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation.
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