Lens index

Lens index

Reviewed by Dr. Justin Asgarpour

Lens index is all about the material your lenses are made from, their thickness, and how that affects performance. The higher the index, the thinner the lens will be – which means people who have a powerful prescription and need thicker lenses can still enjoy a light and comfortable aesthetic.

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Lens index chart

Explore high index, mid-index and low index lens options available at Clearly. We'll provide your lens index recommendation after you've picked your frames and entered your prescription. It's also important to ask your eye doctor about which is the best option for you.

Lens Index	Features*	SPH	CYL
1.5 Low Index (Standard) The thickest type of lens. Often a little more budget-friendly compared to some high index lenses. Free with purchase of Clearly frames	Anti-scratch coating Option to choose with or without anti-reflective coating. 100% UV Protection	+/- 2.50 and below	+/- 1.50 and below
1.59 Mid Index Mid index polycarbonate lenses are lighter and subtler compared to low index (standard) lenses. Added durability and shatter-proof design. Ideal for children	Up to 25% thinner than Standard. Anti-scratch coating Anti-reflective coating UV Protection	+/- 5.00 and below	+/- 3.00 and below
1.60 Mid Index Mid-index lenses are thinner, lighter, and subtler compared to low index (standard) lenses.	Up to 25% thinner than Standard. Anti-scratch coating Anti-reflective coating UV Protection	+/- 2.75 to +/- 6.00	+/- 1.75 to +/- 3.00
1.67 High Index High index lenses are thinner, lighter, and more subtle compared to low index (standard) lenses.	Up to 35% thinner than Standard. Anti-scratch coating Anti-reflective coating UV Protection	+/- 5.25 to +/- 6.00	+/- 3.25 to +/- 4.50
1.74 High Index High index lenses are thinner, lighter, and more subtle compared to low index (standard) lenses.	Up to 40% thinner than Standard. Anti-scratch coating Anti-reflective coating UV Protection	+/- 6.25 and up	+/- 4.75 to +/- 6.00

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Frequently Asked Questions

What is lens index?

Put simply, lens index refers to the material of the lens and how that affects how thick your glasses lenses are. It’s a measurement that defines how capable your lenses are of refracting light.
High-index lenses are thinner and made from higher-quality materials. They’re compatible with all prescriptions, but people with higher prescriptions will need them explicitly.
Low index lenses are the thickest type of lens material and are often more budget-friendly, compared to certain high index lenses. While they’re compatible with lower prescriptions, they aren’t always the recommended option.

How do I check my lens index?

There is no telltale sign for whether lenses are high or low index, as there is no industry required stamp, logo, or barcode. That means, if you don’t have the original specifications for your glasses, there’s no definite way of finding out.
If you want to try to guess, it's a matter of visual inspection. If your prescription isn’t very strong, high index lenses are thinner than low index lenses of an identical prescription.
However, there’s no definite indicator so, if in doubt, head to your local optician to get a professional opinion.

Do I need high index lenses?

Most prescriptions are compatible with high index lenses, but higher prescriptions are only compatible with high index lenses.
If your prescription is particularly strong (above or below +/- 6.00 SPH and +/- 3.25 CYL), you need high index lenses to accommodate your vision correction needs.
Generally speaking, people with lower prescriptions who prefer a more lightweight and subtle finish may also benefit from high index lenses. However, not all prescriptions will benefit from high index lenses. We’ll always recommend the best lens for your prescription. You can also ask your eye doctor which lens index is ideal for you.

Does lens thickness matter?

Yes, lens thickness directly controls the lens’ ability to bend and refract light. High index lenses are designed to manipulate light more efficiently, and therefore are necessary for people with a stronger prescription.
Lens thickness is also a factor in the comfort, aesthetic, and quality of vision that your glasses provide (dependent upon your prescription). If you prefer a more lightweight and subtle finish, opt for high index lenses. If your prescription doesn’t require high index lenses and you’re looking for a more budget-friendly option, consider low index lenses.

How does astigmatism affect lens thickness?

Your prescription will dictate which lens index is required to correct your vision needs.
Generally, if you have moderate or high astigmatism, your lenses will combine your SPH and CYL powers in certain areas of the lens, which results in thicker lenses. Make sure to choose high index lenses, which will provide the thinnest aesthetic.