A Guide to Eyeglass Lenses

By Giansante, Helene

Eyeglasses can be prescribed for a number of conditions: myopia (nearsightedness), hyperopia (farsightedness), astigmatism (a defect in the eye that causes blurred vision), or age-related presbyopia, which robs your eye's lens of elasticity and makes it tough to focus on nearby objects.

Your vision problem determines the shape of the eyeglass lens. Nearsightedness can be corrected by lenses that curve inward (concave); farsightedness can be corrected by a lenses that cures outward (convex). A cylinder-shaped lens is used to correct an astigmatism, and bifocals, trifocals or progressive lenses are used for presbyopia.

Beyond the changes required by your eye problem, other options in choosing glasses can be confusing: Glass or plastic? Tinted or clear? Coated or non-coated?

Once, all "glasses" consisted of just that: glass. But technological advances have provided lightweight plastic lenses for nearly any type of refractive error. They can add a tint, resist impact, block ultraviolet light, diminish glare - and make you look good, too. Glass lenses are more resistant to scratches than plastic lenses.

A baby boomer's "PAL"

Traditionally, bifocals were a presbyopic person's only option. These glasses split the lens into two prescriptions - one for near vision (bottom half), the other for distance (top half). Other than an adjustment period and a fairly visible midline, bifocals have few disadvantages.

The progressive addition lens (PAL) is another option. For some people, PALs may offer advantages over bifocals. The PAL provides a gradual, invisible change in lens power from top to bottom, without the bifocal midline.

With PAL lenses, there is always some place in the lens to focus on the distance you need.

Most wearers adjust to PALs more easily than to bifocals because there is no line to jump across. But you must learn to hold your head in the best position for the distance at which you want to see. When reading with PALs, you need to move your head from side to side; with bi- or trifocals, you can scan reading material, because the correction is the full width of the lens.

Progressive lenses cost about twice as much as bifocals. PALs can fit any frame, but they shouldn't be so small that they don't include the entire spectrum of near and distance prescriptions.

A third option is the trifocal lens. Similar to the bifocal, it adds a middle range to aid intermediate vision.

Glass or plastic?

Plastic has become the lens of choice. Made of hard resin, these lenses are lighter, easier to tint and more shatter-resistant than standard glass. They're slightly thicker and more scratch-prone, however, so experts recommend a scratch-resistant coating.

The newest material is the plastic high-index lens, better for high-power prescriptions because it's lighter and thinner than glass or plastic. Polycarbonate lenses are a special type of high-index lens recommended for children and sports. Made of the same material as bulletproof windows, these lenses are four to five times more impact-resistant than other materials.

Tints

Tints screen out a variety of light, including dangerous ultraviolet (UV) rays. Gray, green or brown are good choices for sunglasses, which also provide maximum protection against UV radiation. Glass and plastic absorb some UV rays without a coating, but experts recommend the UV coating if you spend a lot of time outdoors and don't want to buy separate sunglasses. Photochromatic lenses provide the best of both worlds: They gradually darken when you go outside. Plastic photochromatic lenses lose their ability to darken after several years, according to the American Academy of Ophthalmology; glass lenses don't lose that ability. One drawback of photochromatic lenses is that they take much longer to lighten up than they do to darken. This presents a problem when going from a lighter area to a darker area.

If you're bothered by headlights and indoor glare, anti-reflective coating might help you see better.

Still confused? Talk with your eye doctor about your lifestyle, work, sports and hobbies. The doctor can help you choose the lenses that are right for you.

Medical Reviewer: [Fincannon, Joy RN MN, Hammersmith, Kristin, MD] Last Annual Review Date: 2008-02-28T00:00:00-07:00 Copyright: Copyright Health Ink & Vitality Communications

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