by Jamie Bartlett 5 min read

Strong prescription huh?

Previously, you might’ve got away with standard lenses. Those were the days…

But now your Sphere and Cyl have crept up, you’ve been told you should get some “high index” ones. So what does that actually mean?

 

“High index lenses are recommended if you have a strong prescription. Depending on the dioptric strength of your Sphere and/or Cylinder power, lenses with a high refractive index will be more efficient, weigh less and be thinner.”

 

In this article, you can learn what high index actually means and how it’ll determine the look, weight and costs of your lenses.

 

Single high index lens dispersing strong source of light across white surface

Example of a high index lens made of glass | 1.52 index

 

What are high index lenses?

Not all spectacle lenses are created equal.

Sure, they might differ in strength, but they also differ in refractive index. In basic terms, this measures how efficiently lenses can bend light.

When light enters any type of spectacle lens, it slows down and bends at an angle within the lens material. This effect is known as refraction. The more the light is bent, the higher the index of refraction.

Low index lenses are less refractive. Light is only slightly bent, thus taking longer to pass through. For strong prescriptions, your lenses have to be thicker to sufficiently refract the light which can become problematic in terms of weight and aesthetics.

High index lenses are more refractive. Light is bent more effectively, thus being quicker to pass through your lenses. Less material is required to achieve the same amount of refraction thus making your lenses thinner and lighter in weight.

Get the idea?

As a rule of thumb, lenses with an index of 1.52 or more are considered as “high index.”

At Banton Frameworks, our lenses range from;

  • 1.50 = standard
  • 1.60 = thinned
  • 1.67 = super thin
  • 1.74 = ultra thinned

 

Thick spectacle lens lying flat on a white tabe

Spectacle lens made of glass | 1.52 index

 

What lens thickness should I get?

Choosing the correct thickness of your lenses is informed by it’s refractive index. The higher the index, the thinner the lens can be.

However, choosing the correct index depends on your prescription details, your desired weight and appearance of your glasses frame as well as your budget.

To make things easier, I’ve made two handy tables which you can use to see which lens index you should be aiming for.

These are some general guidelines which can help you avoid having overly thick “milk bottle” lenses. Equally, this advice can prevent you from paying for lens thinning when it isn’t really required for your prescription.

*2020 prices

Index

Max Sphere

Max Cyl

Thinning

1.50

Less than +/- 4.00

Less than +/-2.00

None

1.60

+/-4.00

+/-4.00

Thinned

1.67

+/-7.00

+/-4.00

Super thin

1.74

+/-9.00

+/-4.00

Ultra thin

 

Thinning

Single vision

Bifocal

Varifocal

None

£28

£60

£70

Thinned

£35

£88

£94

Super thin

£95

£119

£124

Ultra thin

£132

NA

NA

 

For more accurate prices and other lens options, head to our handy lens menu here.

 

High index lens being edged inside a grey machine enclosure

"Lens edging" | How spectacle lenses are cut to shape using water and grinding wheels

 

Advantages of high-index lenses

 

Slimmer lenses

High index lenses are more efficient at letting light pass through, so they can afford to be thinner compared to standard index lenses.

This reduces their bulkiness, especially at the edges of the lens. If you have a particularly strong sphere, a high refractive index will help to prevent the edges of your lenses from protruding from your frame.

If you are hyperopic (long sighted,) this is a particularly good advantage in terms of aesthetics. It’ll reduce that “milk bottle” lenses look.

 

Lighter glasses frame

Because they’re thinner, high index lenses are also lighter.

This’ll make your glasses frame more comfortable to wear as it reduces their overall weight. We’re only talking grams here, but it can really help to reduce their pressure on your face and hopefully stop them from slipping down your nose.

Calculate your lens costs

 

High index lens being cut to shape within an automated edging machine

"Dry edging" | A method of cutting spectacle lenses to shape without using water

 

High index lenses disadvantages

 

Higher cost

Why are high index lenses so expensive?

Lenses with a high refractive index are more costly to make. This is because they take longer to thin down and waste more material from the grinding and finishing processes during manufacture.

As a rule of thumb, the higher the index of the lenses, the higher the cost.

 

Vision distortion

Despite their externally aesthetic benefits, high index lenses give a slightly poorer quality of vision compared to standard index equivalents.

Resulting from their higher refractive index and thinner lens section, they can cause some visual distortion. In optics, this is known as aberration, or Abbe for short and affects all optical lenses regardless of their refractive index.

To avoid this, accurate alignment of your pupil and the centre of your lenses is re-emphasised to reduce the chances of distortion through your lenses.

This is achieved through proper centration by horizontally and vertically aligning your eyes behind the optical centre of each lenses.

You can help to avoid unnecessary distortion by providing your pupillary distance and providing your ocular heights when ordering your lenses.

 

More brittle

Depending on your prescription, your optometrist may suggest the use of glass lenses.

Reason being? Glass has a superior refractive index to plastic, as high as 1.90 and are the thinnest lenses for high prescription.

In case you weren’t aware, most glasses lenses are now made using a variety of polymer resins which were introduced in the mid 1900’s due to the increased use of plastics.

But, if you’ve ever watched cartoons, you’ll know that glass lenses are more likely to shatter. They’re far more brittle than their modern plastic equivalents and run the risk of breaking if you drop your glasses frame.

Certainly something to consider if you’re thinking of using glass lenses.

 

More reflective

Lenses made from glass are also more reflective than plastic ones. This isn’t overly detrimental to their performance but can result in harsh reflections across the surface of your lenses.

To make your eyes more visible to people around you, an anti-glare lens coating will certainly compensate for this.

 

Close view of a high index glass lens dispersing light

 

Are high index lenses better?

To summarise, here’s some key points to remember when you’re choosing your next pair of glasses lenses.

High index lenses are;

  • Lighter
  • Thinner
  • More brittle
  • More expensive
  • Less cumbersome
  • Let light pass through quicker
  • Relevant to your prescription
  • Sometimes made of glass
  • Aren’t always required

 

Do I need high index lenses?

It’s worth remembering that high index lenses aren’t obligatory. In fact, according to your budget, you can have your lenses in any index you like

The suggested index of your lenses is merely a guideline to avoid that “milk bottle” look, especially if you have a high Sphere and/or Cyl.

Hopefully you’ve found this article useful. If you did, hit those share buttons below to pass it on to your friends or anyone you think could benefit.

 

 

Jamie Bartlett
Jamie Bartlett

Co-founder of Banton Frameworks.



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