This is not a sales page. We run the numbers on the upgrade that costs the most and delivers the least, in a typical frame — then show the thing that moves your lens thickness far more than any index change.
In a 52 mm eyesize frame with a 63 mm PD, 1.74 lenses are 0.36 mm thinner at the edge than 1.67 at −5.00 D, and 0.75 mm thinner at −10.00 D. Even at −14.00 D the difference stays under a millimetre.
Meanwhile, changing the frame from a 48 mm eyesize to a 58 mm one adds about 3.1 mm at −6.00 D — roughly seven times the 1.67→1.74 step. Frame choice and PD centring dominate; the index upgrade is the tie-breaker, not the lever.
Optical quality is a non-issue between these two: Abbe 32 for 1.67 and 33 for 1.74. You are paying purely for millimetres.
| Material | Index | Edge | Center | Weight | Abbe | vs 1.67 |
|---|
| Frame eyesize | Calculation Ø | 1.67 edge | vs A=48 |
|---|
| What you change | Effect on edge thickness | What it costs |
|---|---|---|
| Frame eyesize 48 → 58 mm (at −6.00 D) | +3.1 mm | usually nothing — both are normal frame sizes |
| PD centred in the frame rather than 8 mm off | up to several mm | nothing — just pick the right size |
| 1.67 → 1.74 (at −6.00 D) | −0.43 mm | a significant premium |
| 1.60 → 1.67 (at −6.00 D) | −0.56 mm | a moderate premium |
| CR-39 → 1.60 (at −6.00 D) | −1.2 mm | a small premium — the best value step |
Only if the price difference means little to you and you are chasing the last half millimetre at a high prescription — or your frame is large and you refuse to change it. As a rule of thumb, at −8.00 D and above, with a frame bigger than about 54 mm eyesize, 1.74 behaves like a genuine upgrade. Below −4.00 D it is hard to justify at all: the difference is a few tenths of a millimetre.
No — that is the one surprise in this comparison. The usual story "higher index, worse optics" holds from CR-39 (Abbe 58) down to polycarbonate (Abbe 30), but between 1.67 and 1.74 the Abbe values are 32 and 33: a wash. The trade for 1.74 is price and slightly greater brittleness, not clarity.
They are more prone to chipping and cracking than polycarbonate or Trivex, which is why labs are cautious about fitting them in rimless frames or for rough everyday use. In a normal full-rim frame and a case, they are fine — but 1.74 is the wrong choice for a rimless mount or for a child.
A bigger jump than 1.67 vs 1.74 in thickness terms — roughly 0.56 mm at −6.00 D, 0.97 mm at −10.00 D — and 1.60 keeps a much better Abbe value (42 vs 32). For prescriptions between about ±2.00 and ±5.00 D, stepping from 1.60 up to 1.67 is often the better buy than stepping from 1.67 up to 1.74.
Availability is good in single-vision, but the material comes in a limited range of semi-finished blanks, so some combinations — very large frames, unusual base curves, some progressive designs — are not made in 1.74. If a lab offers you 1.67 instead, that may be why.
It uses the standard paraxial sagitta model with Vogel's rule for the base curve, assuming 1.0 mm minimum edge thickness. Real labs may pick a different base curve for 1.74 than for 1.67, which shifts the result by a few tenths of a millimetre either way, and the model becomes unreliable above roughly ±14 D. Treat the numbers as a comparison between options — which is exactly what you are deciding between — not as a fabrication order.
Lens Thickness Calculator & Visualizer
The full material comparison with a to-scale cross-section of every lens.
All indexes compared, with optical quality
CR-39 through 1.74 with Abbe values, weights and a verdict for your prescription.
Frame Size & Fit Calculator
Find the eyesize that fits your face before you optimise the index.
Not medical advice. This page performs optical geometry on the numbers you enter. It does not measure, diagnose or correct vision. Follow your optometrist's or optician's advice for anything clinical.
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