Lens index numbers appear on most prescription paperwork, but they’re rarely explained clearly at the point of sale, leaving most people to just nod along with a number they don’t actually understand — which is a shame, since it’s one of the more directly useful specifications to grasp.
What the index number actually measures
The index number refers to a lens material’s refractive index — essentially, how efficiently it bends light. Standard plastic lenses sit around 1.50 index. Higher numbers like 1.60, 1.67, and 1.74 indicate progressively higher-index materials that bend light more efficiently, meaning they need less physical material to achieve the same optical correction.
Why higher index means thinner lenses
Because higher-index materials correct vision more efficiently per unit of thickness, the same prescription strength produces a noticeably thinner, flatter, and lighter lens as the index number increases. This matters most for stronger prescriptions, where standard 1.50 index lenses would otherwise become thick and heavy at the edges — a higher index lens achieves the same correction in a much slimmer, more comfortable form.
The trade-offs of higher index
Higher-index lenses generally cost more than standard index, reflecting more complex manufacturing. Some higher-index materials also have slightly more chromatic aberration — a minor tendency to split light into faint colour fringing at the edges of vision — though quality coatings largely offset this in most modern lenses. For a mild prescription, the benefit of jumping to a very high index is often minimal, since standard index already produces a reasonably thin lens at low strengths.
Choosing the right index for you
Generally, the stronger the prescription, the more a higher index is worth the added cost, since the thickness and weight savings become more noticeable. For milder prescriptions, standard or mid-range index usually already looks and feels fine, making the extra cost of a very high index harder to justify.
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The right frame changes everything. Finding yours is what we’re here for. Trend Optic is an independent eyewear boutique in Middle Park, Melbourne, where every appointment is one-on-one and unhurried. Book a private 45-minute fitting, or simply walk in. We don’t do eye tests in Melbourne, so bring a current prescription from your own optometrist.