Eyeglasses

Eyeglasses correct vision problems, such as nearsightedness, farsightedness, astigmatism, and presbyopia by focusing light more appropriately on the retina.

The type of vision problem that you have determines the shape of the eyeglass lens. For example, a lens that is concave, or curves inward, is used to correct nearsightedness, while a lens that is convex, or curves outward, is used to correct farsightedness. To correct astigmatism, which is caused by distortions in the shape of the cornea, a cylinder-shaped lens is used. Presbyopia requires bifocal or multifocal lenses.

What are multifocal lenses?

People who have more than one vision problem often need glasses with multifocal lenses. Multifocal lenses, bifocals, trifocals, or progressive lenses are lenses that contain two or more vision-correcting prescriptions.

• Bifocals: Bifocals are the most common type of multifocal lens. The lens is split into two sections; the upper part is for distance vision and the lower part for near vision. They are usually prescribed for people over the age of 40 whose focusing ability has declined because of presbyopia.
• Trifocals: Trifocals are simply bifocals with a third section for people who need help seeing objects that are within an arm's reach.
• Progressive: Progressive lenses have a continuous gradient (inclined) lens which focuses progressively closer as one looks down through the lens.

What types of lenses are available?

In the past, eyeglass lenses were made exclusively of glass; today, however, most lenses are made of plastic. Plastic lenses are lighter, do not break as easily as glass lenses, and can be treated with a filter to keep out ultraviolet light, which can be damaging to the eyes. However, glass lenses are more resistant to scratches than plastic ones.

As technology advances so, too, do eyeglass lenses. The following modern lenses are lighter, thinner, and more scratch-resistant than the common plastic and glass lenses:

Polycarbonate lenses: These lenses are impact-resistant and are a good choice for people who regularly participate in sporting activities, work in a job environment in which their glasses may be easily scratched or broken, and for children who may easily drop and scratch their glasses.

Photochromic and tinted lenses: Made from either glass or plastic, these lenses change from clear to tinted when exposed to sunlight. This eliminates the need for prescription sunglasses.

High-index plastic lenses: Designed for people who require strong prescriptions, these lenses are lighter and thinner than the standard, thick lenses that may otherwise be needed.

Aspheric lenses: These lenses are unlike typical lenses, which are spherical in shape. Aspheric lenses are made up of differing degrees of curvature over its surface, which allows the lens to be thinner and flatter than other lenses. This also creates a lens with a much larger usable portion than the standard lens.

If you have questions about which type of lens is right for you, talk to your eye doctor. He or she can help you choose the lenses that are best for you based on your lifestyle and vision needs.

Caring for your eyeglasses

Always store your eyeglasses in a clean, dry place away from potential damage. Clean your glasses with water and a non-lint cloth, as necessary, to keep them spot-free and prevent distorted vision.

How often should I change my glasses?

Generally an eyeglass prescription is good for a year, sometimes longer. Some circumstances may lead to a need for new glasses at a shorter interval. They include:

• Increasing nearsightedness in the teen years
• Presbyopia in midlife
• Developing cataracts
• Surgery
• Onset of diabetes

If your vision is decreasing in one or both eyes, you should check to see if you need new glasses or to be sure that there is no significant disease that may require treatment.

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