Lens Types for Glasses

Once eyeglass lenses only came in glass, but today a variety of plastics (as well as innovative lens shapes) are available. Lenses are thinner than was previously possible, more difficult to break, or capable of changing colors, among other advancements. Talk to your optometrist about lens choices before you buy your next pair of glasses.

Multi-focal Lenses

• Many people with glasses are either nearsighted or farsighted, but some people, especially as they age, have trouble seeing at both ranges. Bifocal lenses split horizontally down the middle. They're like two different lenses spliced together. The top section is for distance vision, and the bottom section is for close-range vision, used for tasks such as reading.

  Trifocal lenses are like bifocals, but they have a third section in the middle for distances farther than reading but nearer than long range vision.

  Progressive lenses are similar in concept to trifocals, but instead of being split into discrete sections, each lens has a continuous gradient. Looking through the top of the lens you can focus on things that are far away, and the focus comes closer as you move towards the bottom of the lens.

Polycarbonate Lenses

• Polycarbonate lenses are impact-resistant plastic. They're more difficult to break than glass lenses, so they're particularly good for children. Polycarbonate lenses are also used in safety glasses and sports goggles. Glass is very hard, so when it breaks, it shatters. Polycarbonate, on the other hand, is more pliable and does not break easily. However, the same property makes polycarbonate easier to scratch if you're not careful. The polycarbonate material naturally blocks harmful ultraviolet rays, so there's no need to get an additional UV-blocking coating.

Photocromatic and Tinted Lenses

• Tinted lenses come in sunglasses, and can be either glass or plastic. Only buy sunglasses that block ultraviolet light. Cheap sunglasses that don't block UV radiation can actually harm your eyes, because the dark tint causes your irises to open and let more UV radiation into your eyes. Photochromatic lenses may also be either glass or plastic, but the special color treatment causes them to change from clear to tinted when exposed to sunlight.

High-Index Plastic

• Usually, the stronger your prescription, the thicker your glasses need to be. High-index plastic lenses come in a material which bends light more efficiently than normal glass or plastic, so the lenses can be made thinner. High-index plastics are usually for people with strong prescriptions who don't want bulky glasses.

Aspheric Lenses

• Normal lenses have a regular spherical curve, but aspheric lenses have complex optics that gradually flatten out towards the center of the lens. This enables strong prescriptions to be made with thinner lenses. For this reason, aspheric lenses are often made of high-index plastic. Because the lens is thinner and doesn't curve as dramatically as a normal lens with the same prescription, the effect on the look of the wearer's face is minimal. Strong glasses for farsightedness, for example, often give a person a "bug-eyed" look. Aspheric lenses, however, have little of this effect.