A Guide to Eyeglass Lenses
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. This robs your eye's lens of elasticity and makes it tough to focus on nearby objects. They may also be prescribed for strabismus. This is an eye muscle imbalance that causes double vision.
Your vision problem determines the shape of the eyeglass lens. Lenses that curve inward can correct nearsightedness and lenses that curve outward can correct farsightedness. A cylinder-shaped lens is used to correct astigmatism. Bifocals, trifocals, or progressive lenses are used for presbyopia. Prismatic lenses are used for strabismus.
Beyond the changes required by your eye problem, other choices in choosing glasses can be confusing. Glass or plastic? Tinted or clear? Coated or noncoated?
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, reduce 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 the only choice for a person with presbyopia. These glasses split the lens into 2 prescriptions—1 for near vision (bottom half), the other for distance (top half). Other than an adjustment period and a visible midline, bifocals have few disadvantages.
The progressive addition lens (PAL) is another choice. 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's 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 range of near and distance prescriptions.
A third choice 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 prone to scratch, so experts recommend a coating that is resistant to scratching.
The newest material is the plastic high-index lens. This lens is 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 4 times to 5 times more resistant to impact than other materials.
Tints screen out a variety of light, including dangerous ultraviolet (UV) rays. Gray, green, or brown are good choices for sunglasses. They also provide maximum protection against UV radiation. Glass and plastic soak up 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. One drawback of photochromatic lenses is that they take slightly longer to lighten up than they do to darken. This presents a problem when going from a lighter area to a darker area. In addition, photochromatic lenses don't get as dark inside a car as they do outside a car.
If you're bothered by headlights and indoor glare, anti-reflective coating might help you see better.
Still confused? Talk with your eye healthcare provider about your lifestyle, work, sports, and hobbies. He or she can help you choose the lenses that are right for you.