Thin eyeglasses are attractive; thick ones aren’t. Light eyeglasses are comfortable; heavy ones aren’t. So it’s no surprise that most of us want the thinnest, lightest eyeglasses possible. Most eyeglass wearers are nearsighted, and require the basic physical property of lenses with edges that are thicker than their centers. The stronger the prescription, the thicker the edges. Hi-index-Eyeglass lenses are able to correct vision because they bend light as it passes through the lens. The amount of light-bending (or refraction) that’s needed to provide good vision is determined by the eyeglass prescription provided by your eye doctor.
Because of the ability to bend light more efficiently, nearsighted lenses made of high-index materials have thinner edges than the same prescription made from conventional plastic materials of the same prescription power. Lighter, Thinner edges require less lens material, which reduces the overall weight of the lenses. Lenses made of high-index plastic are lighter than the same lenses made in conventional plastic, so they’re more comfortable to wear. High-index glass lenses also have thinner edges, but high-index glass is heavier than conventional glass, so there is not as much weight savings with glass as there is with plastic lenses.
Lightweight lenses are even more of a benefit for farsighted prescriptions, which can make conventional lenses very heavy. And most high-index lenses also have an aspheric design, which makes them flatter and reduces the magnified “bug-eye” look that conventional lenses cause in strong farsighted prescriptions.