The classic CR-39.
A standard organic lens with excellent optics and outstanding value. It is the benchmark against which every other option is measured.
Recommended: up to ±2.00 dioptres · avoid rimless frames
Optical Guide · Lenses
Lens thickness, explained.
The refractive index determines how thin, light and discreet your lenses will be. Four numbers — 1.5, 1.6, 1.67 and 1.74 — and a choice our technicians verify for every order.
01 · The concept
Same prescription, four thicknesses.
The refractive index measures how much the lens material bends light. The more it bends, the less material is needed for the same correction — and the thinner the lens becomes.
Comparison
Profile of a −4.00-dioptre lens
1.5baseline
1.6−20 %
1.67−35 %
1.74−50 %
Rule of thumb
The stronger your prescription, the more you will notice a higher index. With low prescriptions, however, the standard option is hard to beat: the same clarity, at a lower price. That is why more is not always better.
02 · The four indices
Choose for your prescription.
Each index has its place. The key figure is the highest sphere (SPH) value on your prescription — in absolute value, regardless of sign.
20 % thinner.
Noticeably thinner and lighter than the standard option, making it more comfortable for long wear. Its strength also makes it suitable for rimless and nylon frames.
Recommended: ±2.00 to ±4.00 dioptres
Aspheric: thin and natural.
Up to 35 % thinner than CR-39. Its aspheric geometry reduces edge thickness and distortion for more natural vision and a more refined look.
Recommended: ±4.00 to ±6.00 dioptres · ideal for rimless frames
The lightest option.
Up to 50 % thinner than the standard option: the definitive choice for strong prescriptions, where every millimetre of edge thickness shows — in weight and in the mirror.
Recommended: above ±6.00 dioptres
03 · At a glance
The full comparison.
Hover over the headings to see what each column means.
| PrescriptionRecommended range. Use the highest sphere (SPH) value on your prescription, in absolute value. If there is cylinder correction, our technicians take it into account. | ThicknessApproximate reduction in edge thickness versus standard 1.5, with the same prescription and frame. | WeightHow it feels on your bridge and ears. Less material = fewer grams. You will appreciate it after eight hours of wear. | FramesCompatibility. Rimless and nylon frames require stronger lenses: index 1.6 or higher. | |
|---|---|---|---|---|
| 1.5Standard | up to ±2.00 | baseline | Standard | Acetate and metal |
| 1.6Thin | ±2.00 – ±4.00 | −20 % | Light | All |
| 1.67Extra-thin | ±4.00 – ±6.00 | −35 % | Very light | All · ideal for rimless frames |
| 1.74Ultra-thin | > ±6.00 | −50 % | Lightest | All |
All indices support every treatment — anti-reflective coating is included as standard.
Verified by a qualified optician before glazing
04 · How to choose
Your index, in three steps.
And if you choose the wrong one, do not worry: we review it before glazing.
Find your sphere value.
Find the highest SPH (or sphere) value on your prescription, regardless of sign. Not sure what the fields mean? Our prescription guide explains them.
Apply the rule.
Up to ±2.00 → index 1.5 · ±2.00 to ±4.00 → 1.6 · ±4.00 to ±6.00 → 1.67 · above ±6.00 → 1.74. It is that simple.
We confirm it.
Before glazing, a qualified technician reviews your prescription, frame and chosen index. If another option suits you better, we will let you know — at no cost and with no obligation.
05 · Frequently asked questions
What you also ask us.
The questions about lens indices our team receives most often. If yours is not here, write to us — we reply within 24 hours.
No. With low prescriptions, a 1.74 lens differs very little from a 1.5 in thickness — but not in price. Very high indices also disperse a little more light at the edges (a lower Abbe number), something aspheric geometry compensates for but cannot eliminate entirely.
The honest rule: pay for a higher index only when your prescription benefits from it. Our technicians will tell you if you are choosing more than you need — even when it means charging you less.
In a rimless frame, the lens is drilled and bears stress directly through the material, without a rim to protect it. Standard CR-39 can chip at the mounting points; materials with index 1.6 and above are stronger and withstand drilling safely.
The same applies, to a lesser extent, to nylon frames (semi-rimless): 1.6 is the safer minimum.
No: anti-reflective, hard coating, blue-light filter and Transitions photochromic treatments are available for every index. At OpticalH, anti-reflective coating is included as standard, whichever index you choose.
Find the full details in our guide to materials and treatments.
It depends on frame size and prescription, but as a guide: with a −5.00 prescription, moving from index 1.5 to 1.67 can reduce the weight of each lens by roughly a third. On the bridge of your nose after eight hours, that difference is very noticeable.
Check the current terms on the page for the 2-for-1 prescription-lens offer — where we publish which indices and treatments are included in each campaign. If you have a question about your specific case, write to us and we will confirm it before you order.
Keep reading
Related guides.
Optical Guide
How to read your glasses prescription.
Sphere, cylinder, axis, addition and PD — every value on your prescription, explained in plain language.
Read the guide → Optical GuideMaterials and treatments.
Anti-reflective, hard coating, blue light, photochromic… the layers in a modern lens and which ones suit you.
Read the guide → ServiceRenew your lenses.
Fit new lenses to your favourite frame: configure online, upload your prescription and our opticians take care of the rest.
Configure now →
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