See exactly how thick your glasses will be — before you buy. Enter your prescription, frame size and PD, and compare CR-39, Trivex, polycarbonate, 1.60, 1.67 and 1.74 high-index side by side.
| Material | Index | Edge | Center | Weight | vs CR-39 |
|---|
Every spectacle lens is a slice of two curved surfaces. The thickness at any point depends on how far the two surfaces have to travel from the optical center to the edge — that gap is the sagitta difference:
sag = r − √(r² − (d/2)²) with r = (n − 1) × 1000 / |P|
where n is the refractive index, P the surface power in dioptres and d the calculation diameter taken from your frame (eyesize + bridge − PD gives the decentration). The front curve is chosen with Vogel's rule: for a plus prescription the base curve is the spherical equivalent + 6.00 D, and for a minus prescription it is half the spherical equivalent + 6.00 D.
Because a higher index bends light with a flatter curve, the same prescription produces a smaller sagitta difference — which is why 1.74 high-index lenses can be dramatically thinner than CR-39 at high prescriptions and barely different at low ones. That crossover point is exactly what the table above shows you.
Below roughly ±2.00 D almost every index looks the same and CR-39 or Trivex is the sensible choice. Between ±2.00 and ±4.00 D a 1.60 index removes most of the noticeable edge. Above ±4.00 D the jump to 1.67 or 1.74 pays for itself visually — set your prescription above and compare the "vs CR-39" column.
Yes, but the gap is much smaller than the price gap. At −6.00 D the difference is typically under a millimetre; at −10.00 D it becomes two to three millimetres. Enter both prescriptions and compare — that is the only honest answer.
Usually yes, for two reasons: less material volume from the flatter curves, and a lower density material. Trivex (index 1.53, density 1.11) is often the lightest option even though it is not the thinnest. The weight column above estimates grams per lens from material density and the computed volume.
A bigger eyesize, or a PD much smaller than the frame's total width, pushes your optical center away from the lens edge, increasing the sagitta — and with it the edge thickness. Two customers with identical prescriptions can end up with lenses several millimetres apart in thickness purely because of frame choice.
It uses the standard sagitta approximation used in optical dispensing, ignoring aspherisation, prism thinning, surface tilt and the lab's own minimum thickness rules. Treat the numbers as a comparison tool, not as a fabrication order — your lab's calculation governs the finished lens.
High-Index Comparison: 1.50 vs 1.60 vs 1.67 vs 1.74
Thickness, weight and Abbe value side by side for your prescription — and where the extra money stops buying anything visible.
Frame Size & Fit Calculator
Will a 52□18 frame fit your face? Check a frame before you buy it, including what its size does to your lens thickness.
Not medical advice. This tool performs optical geometry only. It does not measure, diagnose or correct vision, and it cannot tell you which lens is suitable for your eyes. Always follow the advice of your optometrist or optician.
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