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Refractive index refers to the extent of how efficiently light is bent, or refracted, as it passes through the lens. It directly impacts the thickness and weight of the lens. The index is measured against air, which therefore has an index of 1.
Essentially, as the number increases, the lens gets thinner and lighter. This means that lenses with a refractive index 1.5 will be slightly heavier and are best suited to thicker frames that can support them, compared to a 1.74 lens which is ultra-thin and lightweight. Stronger prescriptions in a 1.5 lens are likely to cause the lens to be too thick for the frame, so we suggest that if your combined SPH is greater than 3.00, you should upgrade to a slightly thinner lens. We have attached a table to guide you on which lens thickness to go for based on your prescription.
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