Guide · lenses
Thin lenses: what the index numbers mean and what you pay for
Over about plus or minus three, the standard lens gets thick at the edges or the middle. Thinner materials fix that, at a price — and the frame you choose does half the job for nothing.
What “index” is
How strongly a lens material bends light. A higher index bends light more, so the same prescription needs less material and the lens comes out thinner and lighter. Standard plastic is 1.5. Then 1.6, 1.67 and 1.74. Each step is thinner, and costs more.
Which one your prescription needs
| Prescription | Sensible choice | Why |
|---|---|---|
| Up to about ±2 | Standard 1.5 | Thin enough already; paying more buys nothing |
| ±2 to ±4 | 1.6 | Noticeably thinner and lighter; the best value step |
| ±4 to ±6 | 1.67 | Keeps a small frame looking normal |
| Over ±6 | 1.74 | The thinnest made; worth it in a rimless or a small frame |
These are our defaults, and we bend them for the frame: a big frame needs a thinner lens than a small one at the same prescription, because the edge is further from the centre.
The free half
Frame size. Minus lenses are thick at the edge; a small frame cuts the thick part off. Plus lenses are thick in the middle; a frame that sits close to the face hides it. Frame shape. Round or oval over angular for high minus. Rim. A thicker plastic rim hides an edge that a rimless shows. Choosing the frame with your prescription in mind is worth as much as a step in index, and costs nothing — which is what the eyewear consultation is for.
When not to pay
Under about ±2, a thinner material makes very little visible difference, and we’ll say so rather than suggest it.
The short version
- Index = how much the material bends light; higher = thinner
- Up to ±2 standard; 1.6 for ±2–4; 1.67 for ±4–6; 1.74 above
- A small, rounder, rimmed frame does half the thinning for free
- Under ±2, a thinner lens makes little visible difference