Enter your prescription and see all six common materials with real numbers: how thick they end up, how much they weigh, how much optical quality you give up for the thinner ones — and what each step up buys you for the extra money. The verdict recommends the cheapest index that does the job, not the thinnest one on the shelf.
| Material | Cost | Index | Edge | Center | Weight | Abbe | vs CR-39 |
|---|
| Your sphere (typical frame) | Buy | Cost band | Edge at that index | Thinner than CR-39 | The next step up buys |
|---|---|---|---|---|---|
| −2.00 | CR-39 | $ | 3.4 mm | — | 1.60 buys 0.4 mm |
| −4.00 | 1.60 | $$ | 4.6 mm | 0.8 mm | 1.67 buys 0.4 mm |
| −6.00 | 1.67 | $$$ | 5.6 mm | 1.7 mm | 1.74 buys 0.4 mm |
| −8.00 | 1.67 | $$$ | 7.0 mm | 2.3 mm | 1.74 buys 0.6 mm |
| −10.00 | 1.67 | $$$ | 8.5 mm | 3.0 mm | 1.74 buys 0.7 mm |
| −12.00 | 1.67 | $$$ | 9.9 mm | 3.8 mm | 1.74 buys 0.9 mm |
| Material | Cost | Thinnest? | Optics (Abbe) | Impact | Scratch | Choose it when |
|---|---|---|---|---|---|---|
| CR-39 (1.50) | $ | no | 58 — best | moderate | best of the plastics | Under ±2.00 D, or when optical clarity matters more than thickness and price is tight. |
| Trivex (1.53) | $$ | 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 — and usually the best value per millimetre saved. |
| MR-7 (1.67) | $$$ | very | 32 — usable | moderate | good | −4.00 D and up, where the thickness saving is still visible. Watch the optics on strong cylinders. |
| MR-174 (1.74) | $$$$ | most | 33 — usable | more brittle | good | −8.00 D and up, or a large frame you refuse to give up. Last step of the ladder: barely thinner than 1.67 for a big price jump. Not for rimless or rough use. |
Every step up the ladder costs more and buys less. At −6.00 D in an average frame, going from 1.60 to 1.67 buys about half a millimetre, and going from 1.67 to 1.74 buys under half a millimetre again — while each step moves you a whole price band up. The first step up from CR-39 buys you visible millimetres; the last one buys fractions.
So the number that decides it is not the index, it is millimetres per dollar — and that changes with your prescription and your frame. A smaller frame is the free lever: going from a 58 mm eyesize to 48 mm takes about 3 mm off the edge, more than every index upgrade on this page added together. Try it above — set the frame size, then flip the sensitivity selector from best value to thinnest, cost no object and watch which row gets recommended.
What we will not fake: real prices. They vary too much between markets, labs and coatings. The $ bands above are relative only — CR-39 and polycarbonate are the cheap rows, 1.74 typically costs several times a mid-index pair. Get two actual quotes for your prescription before you decide.
Usually not. In an average frame at −6.00 D the step from 1.67 to 1.74 buys under half a millimetre while moving you a full price band up. It becomes worth considering above roughly −8.00 D, in a large frame you refuse to give up, or if you set the calculator above to thinnest, cost no object. The verdict line tells you the exact millimetres you are paying for and what the next step costs.
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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