How to get thinner eyeglass lenses

Quick answer: High-index lenses (for example 1.60, 1.67 or 1.74) bend light more efficiently, so they are noticeably thinner and lighter than ordinary lenses. They are recommended mainly for medium to high myopia or hyperopia — the higher the index, the thinner the lens.

If you want your eyeglasses to have thinner and lighter lenses, you will need to choose high-index lenses so that your glasses look as attractive and comfortable as possible.

High-index lenses are especially recommended if your prescription includes a fair amount of myopia, hyperopia or astigmatism. But no matter how strong your prescription is, high-index materials can make your glasses noticeably thinner, lighter and more attractive.

Most eyeglass wearers are short-sighted, which calls for corrective lenses that are thin in the centre but thicker at the edge. The stronger the prescription, the larger those edges become.

Most of today's modern frames are made of plastic or metal with rims thinner than the lens itself. Popular rimless designs also leave the lens edges fully exposed. In both cases the edges are very visible, and thick edges can make your glasses look less flattering.

How high-index lenses differ from ordinary lenses

Eyeglass lenses correct refractive errors by changing the direction of light (refraction) as it passes through them. The amount of light-bending power needed for clear vision is exactly what your eyeglass prescription from your eye doctor specifies.

Refractive errors and the lens power needed to correct them are measured in units called dioptres (D). Mild myopia might read -2.00 D; strong myopia might read -8.00 D. If you are long-sighted you need "plus" (+) lenses, which are thicker in the centre and thinner at the edge.

Ordinary lenses — glass or plastic — can become quite thick and heavy at high powers. Fortunately, chemists have created a range of new "high-index" plastic materials that bend light more efficiently, so less material is needed to correct the same error. That makes high-index lenses considerably thinner and lighter than conventional glass or plastic.

The advantages of high-index lenses

Thinner. Because they bend light more efficiently, high-index lenses for myopia have thinner edges than conventional-plastic lenses of the same power.

Lighter. Thinner edges require less lens material, reducing the overall weight. High-index plastic is lighter than the same lens made in conventional plastic, so it is more comfortable to wear.

High-index glass lenses also have thinner edges, but high-index glass is heavier than conventional glass, so the weight saving is smaller than with plastic.

The weight difference is even greater for hyperopia, and grows with your prescription. Most high-index lenses for hyperopia also have an aspheric design, giving a slimmer, more attractive profile and reducing the eye-magnification effect that plus lenses can cause.

Many high-index options

There is a wide variety of thin, light high-index lenses based on how efficiently they bend light — that is, how high the refractive index is. This "refractive index" is the ratio between the speed of light in air and its speed through the lens material. The more the light is slowed and bent, the higher the index, and the thinner the lens for a given power.

Put simply: the higher the refractive index, the thinner the lens.

SEE ALSO: How to clean your glasses without scratching the lenses!

Conventional plastic lenses have an index of about 1.50; crown glass is 1.52. Any material with an index higher than that is considered high-index. High-index plastics today range roughly from 1.53 to 1.74; lenses of 1.70 or higher are typically at least 50 percent thinner than conventional plastic.

Generally, the higher the index, the higher the cost of the lens. Your prescription also determines which high-index material suits you best — the highest indices are used mainly for the strongest prescriptions. Most popular designs, including progressive and photochromic lenses, are available in high-index materials.

SEE MORE: Features and prices of the eyeglass lenses for your glasses.

Anti-reflective coating: essential for high-index lenses

Every lens material reflects some of the light hitting it, creating ghost images and reducing night-vision clarity. Conventional glass or plastic reflects about 8 percent of the light that would otherwise reach the eye, while high-index lenses reflect up to 50 percent more.

For better vision and appearance, an anti-reflective (AR) coating is a very good idea. High-index lenses with an AR coating transmit 99.5 percent of the light to the eye for optimal vision, and because reflections are virtually eliminated the lenses look almost invisible — people see your eyes, not your lenses. Studies show that AR-coated lenses provide clearer night vision with less glare, a real advantage for night driving.