Glasses lenses: How to choose the best for your eyes

The lenses you choose for your spectacles — even more than frames — often will determine how happy you are with your eyewear.

And buying spectacle lenses is not an easy task. In fact, in a recent issue, Consumer Reports magazine said, "There are so many choices for lenses and coatings, it's easy to be confused about what's worth buying."

This buying guide will help you cut through the hype about different types of spectacle lenses and help you choose lenses and coatings that offer the best features and value for your needs.

Why choosing the right spectacle lenses is so important

When buying spectacles, the frame you choose is important to both your appearance and your comfort when wearing glasses. But the spectacle lenses you choose influence four factors: appearance, comfort, vision and safety.

A common mistake people often make when buying spectacles is not spending enough time considering their choices of spectacle lens materials, designs and coatings.

This article gives you the basics you need to know to buy spectacles lenses wisely.

The following information applies to all prescription lenses for glasses — whether you need single vision lenses to correct short sightedness, long sightedness, and/or astigmatism, or you need progressive lenses, bifocals or other multifocal lenses to also correct presbyopia.

SEE RELATED: The latest technology in single-vision lenses

Spectacle lens materials - features and benefits

Glass lenses

In the early days of vision correction, all spectacle lenses were made of glass.

Although glass lenses offer exceptional optics, they are heavy and can break easily, potentially causing serious harm to the eye or even loss of an eye. For these reasons, glass lenses are no longer widely used for spectacles.

Plastic lenses

In 1947, the Armorlite Lens Company in California introduced the first lightweight plastic spectacle lenses. The lenses were made of a plastic polymer called CR-39, an abbreviation for "Columbia Resin 39," because it was the 39th formulation of a thermal-cured plastic developed by PPG Industries in the early 1940s.

Because of its light weight (about half the weight of glass), low cost and excellent optical qualities, CR-39 plastic remains a popular material for spectacle lenses even today.

Polycarbonate lenses

In the early 1970s, Gentex Corporation introduced the first polycarbonate lenses for safety glasses. Later that decade and in the 1980s, polycarbonate lenses became increasingly popular and remain so today.

Originally developed for helmet visors for the US Air Force, for "bulletproof glass" for banks and other safety applications, polycarbonate is lighter and significantly more impact-resistant than CR-39 plastic, making it a preferred material for children's eyewear, safety glasses and sports eyewear.

A newer lightweight spectacle lens material with similar impact-resistant properties as polycarbonate is called Trivex (PPG Industries), which was introduced for eyewear in 2001. A potential visual advantage of Trivex is its higher Abbe value (see below).

High-index plastic lenses

In the past 20 years, in response to the demand for thinner, lighter spectacles, a number of lens manufacturers have introduced high-index plastic lenses. These lenses are thinner and lighter than CR-39 plastic lenses because they have a higher index of refraction (see below) and may also have a lower specific gravity.

Here are popular spectacle 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.

Index of refraction

The index of refraction (or refractive index) of a spectacle lens material is a number that is a relative measure of how efficiently the material refracts (bends) light, which depends on 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, which mean light travels roughly 50 percent 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 (refracting) 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 spectacle 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 current spectacle lens materials ranges from 1.498 (CR-39 plastic) to 1.74 (a specific variety of high-index plastic). So for the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest lens available, and a 1.74 high-index plastic lens will be the thinnest.

Abbe value

The Abbe value (or Abbe number) of a lens material is an objective measure of how widely the lens disperses different wavelengths of light as light passes through it. Lens materials with a low Abbe value have high dispersion, which can cause noticeable chromatic aberration — an optical error visible as coloured halos around objects, especially lights.

When present, chromatic aberration is most noticeable when looking through the periphery of spectacle lenses. It is least noticeable when looking directly through the central optical zone of the lenses.

Abbe values of spectacle lens materials range from a high of 59 (crown 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

In addition to choosing a lens material that has a high index of refraction, another way to give your lenses a slimmer, more attractive profile is to choose an aspheric design.

Aspheric designs — where the lens curvature changes gradually from the centre of the lens to its edge — enable lens manufacturers to use flatter curves when fabricating spectacle lenses, without degrading the optical performance of the lenses.

Because aspheric lenses are flatter than conventional (spherical) lens designs, they cause less unwanted magnification of the wearer's eyes, for a better appearance. In some cases, aspheric designs also improve the clarity of the wearer's peripheral vision.

Most high index plastic lenses are made with aspheric designs to optimise both the appearance and the optical performance of the lenses. With polycarbonate and CR-39 lenses, an aspheric design usually is an option that increases the cost of the lenses.

Minimum centre thickness (or edge thickness)

There are BS/EN standards for impact resistance, so there's a limit to how thin an optical laboratory can make your lenses.

In (concave) lenses for the correction of myopia, the thinnest portion of the lens is the optical centre, located at or near the middle. In (convex) lenses that correct longsightedness, the thinnest portion of the lens is at its edges.

Because of their superior impact resistance, polycarbonate and Trivex lenses that correct myopia can be fabricated to a centre thickness of just 1.0 mm and still pass the BS/EN impact-resistance standard. Myopia-correcting lenses made of other materials usually have to be thicker in the centre to pass the standard.

The size and shape of your spectacle frames also will affect the thickness of your lenses, especially if you have a strong prescription. Choosing a smaller, well-centered frame can significantly reduce the thickness and weight of your lenses, regardless of the lens material you choose.

Generally, the thinnest lenses for your prescription will be aspheric lenses made of a high-index material, worn in a small frame.

Spectacle lens treatments

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

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.

Scratch-resistant coating

All lightweight spectacle lens materials (see table) have surfaces that are significantly softer and more prone to scratches and abrasions than glass lenses. The softest spectacle lens is also the one that is the most impact-resistant: polycarbonate. But all plastic and high-index plastic lenses require a factory-applied scratch resistant coating for adequate lens durability.

Most of today's modern scratch-resistant coatings (also called hard coats) can make your spectacle lenses nearly as scratch-resistant as glass. But if you're hard on your glasses or you're buying spectacles for your kids, ask about lenses that include a warranty against scratches for a specific period of time.

Anti-reflective coating

An anti-reflective (AR) coating makes all spectacle 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, because the higher the refractive index of a lens material, the more light the lenses reflect. In fact, high-index lenses can reflect up to 50 percent more light than CR-39 lenses, causing significantly more glare, unless AR coating is applied.

UV-blocking treatment

Cumulative exposure to the sun's harmful ultraviolet (UV) radiation over a person's lifetime has been associated with age-related eye problems including cataracts and macular degeneration.

For this reason, people should protect their eyes from UV beginning in early childhood. Thankfully, polycarbonate and nearly all high-index plastic lenses have 100 percent UV protection built-in, due to absorptive characteristics of the lens material.

But if you choose CR-39 plastic lenses, be aware that these lenses need an added coating applied to provide equal UV protection afforded by other lens materials.

Photochromic treatment

This lens treatment enables spectacle lenses to darken automatically in response to the sun's UV and high-energy visible (HEV) light rays, and then quickly return to clear (or nearly clear) when indoors. Photochromic lenses are available in virtually all lens materials and designs.

Cost of spectacle lenses and frames

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

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.

Keep in mind that if you choose high-end designer frames and aspheric, high-index progressive lenses with premium anti-reflective coating, it's not unusual for the cost of your eyeglasses to exceed £500.

To get the best value, it's essential to understand the features and benefits of the products you are considering and to choose wisely with the help of a trusted optician.

FIND AN OPTICIAN OR OPTICAL SHOP NEAR YOU: Our locator lists nearby opticians and optical shops to make booking appointments easy.

When buying spectacle lenses, there's no substitute for expert advice

Buying spectacle lenses can seem daunting, but it doesn't have to be. The key is getting accurate, unbiased spectacle lens information from sources you can trust.

For greatest satisfaction with your eyewear, in addition to using this guide, follow this advice echoed by Consumer Reports: During your eye exam, ask your optician which spectacle lenses and lens treatments are best for your specific needs and eyeglass prescription.

Page published on Tuesday, 25 June 2019