Enter your prescription and see all six common materials with real numbers: how thick they end up, how much they weigh, and how much optical quality you give up for the thinner ones. The last part is the one nobody tells you about.
| Material | Index | Edge | Center | Weight | Abbe | vs CR-39 |
|---|
| Material | Thinnest? | Optics (Abbe) | Impact | Scratch | Choose it when |
|---|---|---|---|---|---|
| CR-39 (1.50) | no | 58 — best | moderate | best of the plastics | Under ±2.00 D, or when optical clarity matters more than thickness and price is tight. |
| Trivex (1.53) | no | 45 — very good | very high | good | Kids, sports, rimless, safety eyewear — or anyone who wants both toughness and clean optics. |
| Mid-index (1.56) | slightly | 39 — usable | moderate | good | A cheap step up from CR-39 at low-to-mid powers. Little reason to pick it over 1.60. |
| Polycarbonate (1.59) | yes | 30 — lowest | highest | softest, needs coating | Safety and impact protection above all — sports, industrial, kids. Accept the optics. |
| MR-8 (1.60) | yes | 42 — fine | high (some grades pass safety standards) | good | The all-rounder. Best balance of thickness, weight and optics for most prescriptions. |
| MR-7 (1.67) | very | 32 — usable | moderate | good | −4.00 D and up, where the thickness saving becomes visible. Watch the optics on strong cylinders. |
| MR-174 (1.74) | most | 33 — usable | more brittle | good | −7.00 D and up, or a large frame you refuse to give up. Not for rimless or rough use. |
Only above roughly −7.00 D, or when a big frame forces the issue. Below that the two are typically within half a millimetre of each other at the edge while the price difference is large. Put your prescription in above — the verdict line tells you the exact millimetres you are paying for.
Not thinner — 1.67 always wins on thickness. But 1.60 has a much better Abbe value (42 vs 32), so it produces cleaner optics with less colour fringing, and it is usually noticeably cheaper. For prescriptions between ±2.00 and ±5.00 D a lot of opticians consider 1.60 the smartest buy; 1.67 starts to win when the edge thickness difference becomes visible, which is around −4.00 to −5.00 D in an average frame.
It does not change your prescription or your visual acuity in the clinical sense. What it does is increase chromatic aberration, because high-index materials disperse colour more (lower Abbe). Most wearers never notice it at moderate powers. People with high prescriptions, high cylinders, small pupils or an unusual sensitivity to colour fringing sometimes do — and they are often happier with 1.60 or Trivex despite the extra thickness.
Both are impact materials. Polycarbonate is the toughest and the cheapest, with the lowest Abbe value (30) — it can look slightly less crisp and it scratches more easily without a good coating. Trivex is nearly as tough, lighter (specific gravity 1.10 vs 1.20), and optically cleaner (Abbe 45 vs 30). If the prescription is low and the frame is rimless or the wearer is active, Trivex is usually the better buy.
At −8.00 D the thickness difference between materials becomes dramatic — CR-39 is roughly twice the edge thickness of 1.74 in a 52 mm frame. If your frame is small, 1.67 is often the sensible stop; if your frame is large, 1.74 earns its price. Run your own numbers above rather than trusting a rule.
Lens Thickness Calculator & Visualizer
The same maths, but with a to-scale cross-section of every material so you can see the difference.
Frame Size & Fit Calculator
Frame choice can change your lens thickness by millimetres — check the fit before you buy.
Not medical advice. This tool performs optical geometry and quotes published material properties. It does not measure, diagnose or correct vision. Follow your optometrist's or optician's advice for anything clinical.
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