Know Your Lens Index

25th September 2020

What is lens index? 

Lens index is an important and very noticeable characteristic of the glasses you buy, so it’s always handy to understand how the index number of your glasses lenses will effect how you glasses look, feel, and function!

The index number of glasses lenses is a term used to describe the thickness of the lens, a higher index lens, typically around 1.74 will result in a lens that is approximately 45% thinner. Comparing this to a lower index lens of around 1.5, which is usually seen in plastic lenses, where the lens thickness is only reduced very slightly or not at all.

If you are long sighted you will require convex prescription lenses, these are thicker in the centre and thinner at the edges. Whereas if you are short sighted, you require concave lenses which are thinner in the centre and thicker at the edges. When it comes to prescription strength, the same goes for both types of lens; the stronger the prescription, the thicker the lens will be.

Due to the increase in lens thickness, strong prescription lenses can become uncomfortable to wear. For this reason, lens index becomes increasingly important as the strength of a prescription increases, it enables you to keep the lens of your glasses thinner, lighter, more comfortable, and also more attractive.

Most modern and fashionable frames are made of metal or plastic, with the frames themselves thinner than the lens of the glasses. Or the frames may even be rimless, this means the lens can play a massive role in the overall aesthetics of the glasses. In most cases a thicker lens will detract from the appearance of your glasses as it will be highly visible and stand out against the glasses frame.

So, What Do Glasses Lenses Actually Do?

Wearing glasses often becomes second nature when we need them, but we don’t usually stop and think how they are working and correcting your vision. Glasses play a corrective role with your vision, fixing refractive errors in your sight, the strength of this correction is measured in dioptres (D). The value of dioptres is either positive (for long-sightedness) or negative (for short-sightedness), the more negative or positive the number, the higher the strength. It is normal for eyes to have up to a 5% difference in vision, however some people can experience up to a 20% difference in vision between their eyes!

Glasses lenses work to refract (bend) light in order to correctly focus the light entering your eye. Higher strength lenses refract light more than lower strength lenses, but in classical cases use more material in the lens to achieve this. This is where high-index lenses start playing a vital role in making glasses more comfortable and attractive while remaining fully effective. High index lenses have the ability to refract light more efficiently than lower index frames, needing far less material in the lens.

Its Not Just a Lens Thing

A perfect addition to high-index lenses is the use of anti-reflective coating. All lens materials reflect a portion of light, this light can sometimes cause distraction, even more so in low light. About 8% of light is reflected by conventional plastic or glass lenses, this is increased by around 50% when lenses have a higher index! However, to combat this an anti-reflective coating can be added to your glasses which virtually eliminates all lens reflection, resulting in 99.5% of light being transmitted by the lens, giving you optimum vision in all scenarios. Here at Glasses2You we offer an anti-reflective coating from our G2Y silver pack upwards, covering all of our high index lenses! Our scratch resistant coating will also keep your lenses looking pristine, giving you crisp vision, in style. Take a look at our lens thickness guide for more information on what we can offer you at Glasses2You!

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