High index lens materials are lighter in weight and thinner than their regular glass or plastic counterparts. The benefit is that if you have a strong prescription, you are no longer forced into unsightly Coke-bottle lenses. Just about everyone can get attractive eyeglass lenses that complement their appearance, rather than detract from it.

High index lenses are made of materials that are compressed, or denser, so the same amount of visual correction is taking place using less lens material than traditional plastic or glass requires. "High index" means that the lenses are constructed of a plastic or glass material that has a higher index of refraction. The "index of refraction" refers to the speed that light travels as it passes through the lens material.

Why High Index Lenses Are Thinner

Eyeglass lenses work to correct vision by bending light as it passes through the lens. The amount that the light must be bent in order to give you good vision is determined by the prescription that your eye doctor provides. The higher the number in the prescription, the stronger that prescription is, and the more the light must be bent in order to correct your vision. (For example, if your prescription is -5.00, you are very nearsighted and need a stronger lens than someone with a -2.00 prescription, who is less nearsighted.) So people with stronger prescriptions traditionally have had thick lenses - that is, their lenses provided more visual correction than the thin lenses worn by others.

For people with high myopia or high hyperopia, regular lenses can be heavy...

...but aspheric lenses are lighter because they're thinner. Drawings thanks to Essilor Airwear.

But lens materials with a higher index of refraction (or IOR) allow light to pass more quickly through the lens, so less of the actual lens material needs to be used. That's why lenses with a higher index of refraction, or high index lenses, can be thinner than traditional lenses.

The Higher the Number, the Thinner the Lens

High index lenses are available in either glass or plastic (CR-39, for example). Different manufacturers make different high index lenses, and what sets each lens apart from the others is its index of refraction. The higher the IOR, the denser the material. All things being equal, a 1.66 IOR material will result in a thinner and lighter-weight lens than a 1.57 IOR material will.

Generally, the higher the IOR, the higher the cost of the lenses will be. The thinnest, most lightweight lenses are typically the most expensive. With high index lenses, it seems that the less you get, the more you pay.

Regardless of the index of refraction, plastic high index lenses tend to be lighter in weight than glass high index lenses, simply because plastic weighs less than glass to begin with.

What Are Aspherics?

Just like high index lenses, aspheric lenses are also thinner than regular lenses. But the thinness is accomplished with the lens design, not just the lens material. Aspheric lenses use flatter base curves in their design. In plus lenses for farsighted people, the curves flatten away from the center, for a lens that doesn't bulge out as much. It also doesn't magnify the eye nearly as much as a regular lens design would, so the eye looks more natural.

In minus lenses for nearsighted people, the curves steepen away from the center, toward the edge of the lens. This results in a thinner lens edge, even for a strong prescription. Newer aspherics contain the base curves on the back side of the lens, closer to the eye, rather than on the front side. These can provide even thinner lens edges that will fit more attractively in thinner eyeglass frames.

Besides these cosmetic benefits, aspherics also provide better optics than regular lenses. This is because they correct more of the distortion that can occur when the eyes move from the center toward the periphery of a lens.

How to Get Better Looking Eyeglasses

Complicated mathematical formulas are used in conjunction with the optical prescription to determine the exact thickness of a lens in a particular frame for a particular patient. But generally, the way to make sure that the resulting lenses are as thin as possible is to use the highest index lens material available, use an aspheric lens design if appropriate, and keep the eyeglass frame as small and as round as possible. The portion of the lens edge that is left exposed outside the frame can also be polished clear to make it unobtrusive.

These steps, along with applying an antireflective coating, will result in the best-looking prescription eyeglasses that modern science can provide. If your optician doesn't offer all of these options to you, make a point of asking about them.