Len Index: What Does Lens Index Mean and Which Is the Best For You?

Lens Index is the measurement of the thickness of an eyeglass lens. The higher the index number, the thinner and lighter the lens. A lens index is calculated by dividing the lens’ refractive index by its prescribed power. The most common lens indexes are 1.50, 1.57, 1.59, 1.60 and 1.67.

Intro

Lens index is the measurement of the thickness of a lens. The higher the lens index, the thinner and lighter the lenses. High-index lenses are made from materials that bend light more efficiently, allowing your prescription to be ground into a thinner lens.

Depending on the visual condition you need to correct, a contact lens prescription should indicate which lens index you need to achieve the best vision correction. Lens index is calculated by dividing the lens’ refractive index by its prescribed power.

A good rule of thumb is to consider high-index lenses if you require a strong vision prescription (-4.00 or above). These lenses provide the thinnest, lightest and most cosmetically appealing eyewear available today. 

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When Lens Index Matters

The most common lens indexes are 1.50, 1.57, 1.59, 1.60 and 1.67, which is the thinnest and lightest of them all.

However, if your prescription is powerful, you might need a stronger lens index, such as 1.74, or else polycarbonate lenses (1.59). You may have a higher prescription if the refractive error causing vision problems is severe.

Lens index matters when choosing eyeglasses. You may want thinner and lighter or thick lenses, which can make your glasses more comfortable to wear and can improve your cosmetic appearance by reducing the so-called “bulge factor.”

Here’s a quick primer on the types of index materials, including how it affects your glasses and sunglasses.

Types of Lens Index Materials

There are different lens materials, each with advantages and drawbacks. The best lens material for you depends on your prescription and the type of frames you prefer (or select).

1.5 Standard Index

The 1.5 standard index lens is the most popular lens material and is available in many variations. They are the thickest lens, and they boast superior clarity with a single conventional vision.

Standard index lenses are for people with low SPH (spherical) correction prescriptions of +/- 2.25 and less, or a CYL (cylinder) correction of +/- 1.50 and less. They are also a way to save money on prescription lenses because they are standard lens and carry no upgrade costs.

1.56 Mid-Index

1.56 Mid-Index lenses are thinner and lighter than standard 1.5 lenses. They are about 15 percent thinner than standard 1.5 plastic lenses, and eyewear professionals recommended them for full-rim eyeglass frames and glasses for people who enjoy an active lifestyle.

1.57 Thin and Light Index

1.57 thin and light index lenses are thinner and lighter than regular lenses, making them ideal for people who need a moderately strong prescription. Most suitable to those with prescription power ranging from +/- 5.00 and +/- 4.00, and a CYL correction of +/- 3.00 and less.

1.59 Polycarbonate

Polycarbonate lenses are made from thinner, lighter and stronger materials than standard plastic lenses. They are highly flexible, allowing lenses to be made in various shapes and sizes.

The high flexibility of this lens material makes it more comfortable to wear than other lens materials for people with strong prescriptions.

1.59 Polycarbonate lenses are also more impact-resistant than regular plastic or glass lenses, making them perfect for children and people in an environment where eye injuries are possible. The lenses also provide 100 percent UV protection and superior scratch resistance.

You can choose lens coatings with them, including scratch-resistant or anti-reflective coating, to make up for any possible color differences or add further protection. 

1.61 High Index

This lens is a great mid-range option. It has a slightly higher refractive index than 1.59, but it still is much thinner and lighter than standard plastic lenses. Eyeglass professionals recommended them for people who look modern with a prescription of +/- 4.00 to +/- 6.75 and  CYL correction between +/- 2.25 to +/- 3.00.

1.67 Ultra High Index

Ultra-High index lenses are made of a dense material, which allows them to block 100 percent of UV rays and resist scratches. They are perfect for thin eyewear frames or those with a strong prescription of SPH correction of +/- 7.00 and +/- 9.00, and CYL correction of +/- 3.25 and +/- 4.00. 

The material also offers the best balance between cost and thickness. These lenses also work great in fashion eyewear.

High index 1.74

High index 1.74 lenses are today’s thinnest, flattest and most cosmetically elegant lens material. These lenses are more than 50 percent lighter than standard plastic lenses and 20 percent thinner than 1.56 polycarbonate lenses.

They are an excellent choice for rimless metal frames or semi-rimless frame styles, where the edges of the lenses are visible.

The lenses are recommended for people with high prescriptions between SPH correction of +/- 9.25 and more and a CYL correction between +/- 4.25 and +/- 6.00. The thinner lens profile will make the lenses appear smaller in your frames, making them more attractive and comfortable to wear all day long.

Who Needs High Index Lenses?

High index lenses are an option for anyone who needs to wear slimmer correctives. However, they are recommended for people with stronger prescriptions for nearsightedness, farsightedness, or astigmatism. They are also suggested for rimless frames as they are more secure and look more natural.

The table below shows the recommended lens index for each prescription.

Recommended Index Lens

Lens Index	Characteristics	SPH	CYL
1.74 Index Lens -35%	Thin, Comfortable Superior DurabilityHigh Impact Resistant MaterialBest for the highest prescriptions.	+/- 9.25 and above	4.25 to -6.00or+4.25 to +6.00
1.67 Index Lens-30%	Thinnest, flattest and lightest lensesBest for extra-strong prescription	+/- 7.00 and +/- 9.00	+/- 3.25 and +/- 4.00
1.6 Index Lens -25%	The thin, lightweight and a sleek designBest for a stronger prescription	+/- 4.00 and +/- 6.75,	+/- 3.00 and less
1.59 Index Lens-20%	100% UV ray protectionDurable and impact-resistant		+/- 3.00 and less
1.57 Index Lens-15%	Thinner and lighter than standard 1.50 lensesCustomizable  Best for moderate, strong prescription	+/- 5.00 and +/- 4.00	+/- 3.00
1.5 Index Lens-15%	Traditional single-vision lenses.  Prices begin at $0.	+/- 2.25	+/- 1.50

What are the Differences Between 1.57 and 1.61 Lenses?

The main difference between 1.57 lenses and 1.61 lenses are the thickness of the lenses at the edges: With 1.61 index lenses, your glasses will be ultra-thin at the edges, giving them a stylish look than with 1.57 index lenses which are thin.

A 1.61 lens is recommended for the highest prescriptions, while a 1.5 lens is best for moderate, strong prescription.

How to Determine Your Lens Index?

The best way to know what lens index is right for you is by talking with one of our experienced opticians who can help you choose a pair of glasses that work best for your lifestyle and budget needs. You will first need to have a complete eye exam to receive a suitable prescription based on your exam results.

References

1.  

   High-index materials for 193 nm immersion lithography. (May 2005). The Society of Photographic Instrumentation Engineers.

2. Types of Contact Lenses. (March 2021). American Optometric Association.

3. Refractive Errors & Refractive Surgery Preferred Practice Pattern. (November 2017). American Academy of Ophthalmology Journal.

4. How to Choose Eyeglasses for Vision Correction. (June 2020). American Academy of Ophthalmology.

5. Perspectives on impact resistance and polycarbonate lenses. (1988) International Ophthalmology Clinics

6. Optical and nanomechanical study of anti-scratch layers on polycarbonate lenses. (August 2004). ScienceDirect.

7. Development of high refractive index plastics. (July 2007). e-Polymers Journal.

8. IMPACT RESISTANCE OF HIGH INDEX HARD RESIN PRESCRIPTION LENSES.  (December 1995). Optometry and Vision Science.

9. Myopia (Nearsightedness): What It Is, Causes & Treatment. (July 2020). The Cleveland Clinic.

10. Astigmatism.  (June 2019). The National Eye Institute.