Choosing the best lenses for your glasses

The type of lenses in your glasses play a big role in your vision, comfort and safety when wearing eyeglasses. And buying the best lenses for your glasses is not an easy task. There are many different types of eyeglsass lens materials and designs, and each has its own features, benefits, and cost.

The lenses you choose for your glasses influence four factors:

• Vision
• Comfort
• Appearance
• Safety

The following information will help you buy eyeglasses lenses wisely. It applies to prescription eyeglass lenses that correct nearsightedness, farsightedness and astigmatism. It will also help you choose the best progressive lenses or other multifocal lenses if you have presbyopia.

Glass lenses

In the early days of vision correction, all eyeglass lenses were made of glass. Glass lenses offer exceptional optics. But hey are heavy and can break easily, potentially causing a serious eye injury or even loss of an eye. For these reasons, glass lenses are no longer widely used for eyeglasses.

Plastic lenses

The first plastic eyeglass lenses were developed in the 1940s. These lightweight alternatives to glass lenses were call CR-39 plastic lenses. Plastic eyeglass lenses are about half the weight of glass lenses. They are relatively inexpensive and have excellent optical qualities. They also are more impact-resistant than glass lenses.

Polycarbonate lenses

The first lightweight polycarbonate lenses for safety glasses were introduced in the 1970s. Since then, polycarbonate lenses have surged in popularity. They are lighter than regular plastic eyeglass lenses and significantly more impact resistant. Polycarbonate is a great lens material for children's eyeglasses, safety glasses and sports eyewear.

Trivex lenses

Trivex, a new lightweight and impact-resistant eyeglass lens material, was introduced in 2001. Trivex lenses are a good alternative to polycarbonate lenses. They are lightweight and have slightly different optical and impact resistance qualities. [Read more about Trivex vs. polycarbonate lenses.]

High-index plastic lenses

In the past 20 years, several different types high-index plastic lenses have been introduced. These lenses are significantly thinner and lighter than regular plastic lenses. They have a higher index of refraction and are available in an aspheric lens design (see below for more about these).

Here are popular eyeglass lens materials, arranged in order of refractive index and lens thickness (pretty good indicators of cost). Except for the crown glass, these are all plastic materials.

Here are details about important features of lenses for your glasses:

Index of refraction

The index of refraction (or refractive index) of an eyeglass lens material is an indicator of how efficiently the material refracts (bends) light. This efficiency is related to how fast light travels through the material.

Specifically, the refractive index of a lens material is the ratio of the speed of light in a vacuum, divided by the speed of light in the lens material. For example, the index of refraction of CR-39 plastic is 1.498. This means light travels roughly 50% slower through CR-39 plastic than it does through a vacuum.

The higher the refractive index of a material, the slower light moves through it, which results in greater bending (or focusing) of the light rays. So, the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index.

In other words, for a given eyeglass lens power, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index.

The refractive index of the most popular eyeglass lens materials used today ranges from 1.498 (CR-39 plastic) to 1.74 (a type of high-index plastic). For the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest and a 1.74 high-index plastic lens will be the thinnest.

Abbe value

The Abbe value of a lens material determines how much chromatic aberration the lens produces. This is an optical error that causes colored halos around lights.

Lens materials with a low Abbe value can cause noticeable and bothersome chromatic aberration.

Chromatic aberration is most noticeable when looking through the periphery of eyeglass lenses. It is least noticeable when looking directly through the central optical zone of the lenses.

Abbe values of eyeglass lens materials range from a high of 59 (regular glass) to a low of 30 (polycarbonate). The lower the Abbe number, the more likely the lens material is to cause chromatic aberration.

Abbe number is named after the German physicist Ernst Abbe (1840-1905), who defined this useful measure of optical quality.

SEE ALSO: How To Clean Glasses — Without Scratching Your Lenses!

Aspheric design

Glasses lenses with an aspheric design have a slimmer, more attractive profile than regular lenses.

In aspheric lenses, the curvature of the lens changes gradually from its center to its edge. This enables the use flatter curves when fabricating eyeglass lenses.

Because aspheric lenses are flatter than regular lenses, they cause less unwanted magnification. This makes the wearer's eyes look more natural in size and appearance. In some cases, aspheric lens designs also improve the wearer's peripheral vision.

Most high-index plastic lenses are aspheric to optimize both appearance and optical performance. With polycarbonate and CR-39 lenses, an aspheric design is optional and available for an added cost.

Center and edge thickness

In the United States, the U.S. Food and Drug Administration (FDA) has issued guidelines for impact resistance of eyeglass lenses. For this reason, there's a limit to how thin an optical laboratory can grind your lenses.

In lenses for the correction of myopia, the thinnest part of the lens is the optical center, located at or near the middle of the lens. The thinnest part of eyeglass lenses that correct farsightedness is at the edge of the lenses.

Because of their superior impact resistance, polycarbonate and Trivex lenses can have a center thickness of just 1.0 mm and still pass the FDA impact-resistance standard. Myopia-correcting lenses made of other materials usually have to be thicker in the center to pass the standard.

The size and shape of your eyeglass frames also will affect the thickness of your lenses. This is especially noticeable if you have a strong eyeglass prescription.

Choosing a smaller, well-centered frame can significantly reduce the thickness and weight of your eyeglasses lenses, regardless of the lens material you choose.

For the most comfortable, durable and best-looking glasses, the following lens treatments should be considered essential:

Anti-scratch coating

All lightweight eyeglass lens materials have surfaces that are significantly softer and more prone to scratches and abrasions than glass lenses. Plastic, polycarbonate, Trivex and high-index plastic lenses all require a factory-applied anti-scratch coating for adequate lens durability.

Most of today's modern anti-scratch coatings (also called scratch coats or hard coats) can make lightweight eyeglass lenses nearly as scratch-resistant as glass lenses. But if you're hard on your glasses or you're buying eyeglasses for your kids, ask about adding an anti-scratch warranty to your eyeglass lens purchase.

Anti-reflective coating

An anti-reflective (AR) coating makes all eyeglass lenses better. AR coatings eliminate reflections in lenses that reduce contrast and clarity, especially at night. They also make your lenses nearly invisible, so you can make better eye contact and you and others aren't distracted by reflections in your lenses. AR-coated lenses are also much less likely to have glare spots in photographs.

Anti-reflective coating is especially important if you choose high-index lenses. This is because the higher the refractive index of a lens material, the more light the lenses reflect. Some high-index lenses reflect up to 50% more light than CR-39 lenses, causing significantly more glare. AR coating eliminates these annoying reflections.

UV-blocking treatment

Too much exposure to the sun's harmful ultraviolet (UV) radiation can harm your eyes. Over time, this can cause age-related eye problems including cataracts and macular degeneration.

For this reason, people should protect their eyes from UV beginning in early childhood.

Polycarbonate and nearly all high-index plastic lenses provide 100% UV protection. If you choose CR-39 plastic lenses, these lenses need a special coating applied to block all UV rays.

Photochromic treatment

This treatment enables your glasses lenses to darken automatically when exposed to the sun's UV rays. The tint then quickly disappears when you go indoors. Photochromic lenses are available in nearly all lens materials and designs.

If you're not going to wear sunglasses outdoors, make sure your eyeglass lenses block 100 percent of UV rays. Some lens materials don't without an added coating.

Depending on the type of lenses and lens treatments you choose and the lens design you need, your eyeglass lenses can easily cost more than the frames you choose — even if you choose the latest designer frames.

So, how much will your glasses cost? That's hard to say.

The amount you pay for your next pair of glasses will depend on many factors, including your visual needs, your fashion desires and whether you have vision insurance that covers a portion of the cost of your eyewear.

If you choose designer frames and aspheric, high-index progressive lenses with AR coating, the cost can be high. It's not unusual for the cost of eyeglasses of this type to exceed $800 or more.

But if you're buying your child's first pair of eyeglasses with polycarbonate lenses for mild myopia, the cost will be much lower. These will probably be closer to $200 (including a scratch-resistant warranty).

To get the best value, it's essential to understand the features and benefits of all eyeglasses lenses and treatments. Choose wisely with the help of a reputable optician or eyewear retailer.

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