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.

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.

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.

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.

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.