Lens Index and Abbe Number: Why Thinner Glasses Aren't Always Better
Two numbers on every lens spec sheet
When you compare lens options you are usually shown one number: the index — 1.50, 1.60, 1.67, 1.74 — presented as "thinner" the higher it goes. There is a second number that almost never gets mentioned, and it governs how cleanly you actually see: the Abbe number. The two move in opposite directions, and knowing that changes which lens is right for you.
What refractive index actually is
The refractive index of a material is the ratio of the speed of light in a vacuum to its speed inside that material. A higher index means light travels more slowly through the lens, so the surface has to bend light more sharply to produce the same amount of optical power.
That is the whole mechanism behind "thinner" lenses. For a given prescription, the curvature required scales roughly as 1/(index − 1). Moving from 1.50 to 1.67 reduces the needed curve by roughly a quarter, and that reduction is what shows up as a thinner lens at the edge. The effect is small for low powers and obvious for high ones — which is why index becomes a conversation worth having above roughly ±3.00 D, and barely matters at ±1.00 D.
What the Abbe number measures
The Abbe number (also called constringence, or the V-number) describes dispersion: how much the refractive index changes from one wavelength of light to another. A high Abbe number means the material bends red and blue light by nearly the same amount. A low one means it splits them apart.
That splitting is chromatic aberration. In glasses you notice it as colour fringes on high-contrast edges — the white edge of a road sign, a lamp against a dark background, letters on a page — and mostly off-axis, so it is worst when you look away from the optical centre. It also contributes to the vague "everything looks slightly smeared" complaint some wearers have with strong minus lenses.
The trade-off, in numbers
These are typical published figures for the common materials. Manufacturers vary by a point or two, so read them as representative rather than exact.
- CR-39 (standard plastic) — index 1.498, Abbe 58. The optical reference point: best colour fidelity, thickest edges, least impact-resistant.
- Trivex — index 1.53, Abbe 45. Modest thinning, still good colour, and impact resistance close to polycarbonate.
- Polycarbonate — index 1.586, Abbe 30. Very impact-resistant — the material behind most safety eyewear — but noticeably more colour fringing.
- 1.60 mid-index — index 1.60, Abbe 42. The usual sweet spot: meaningful thinning without giving up much colour fidelity.
- 1.67 high-index — index 1.67, Abbe 32. Thinner again, but colour performance drops to polycarbonate territory.
- 1.74 high-index — index 1.74, Abbe around 33. The thinnest routinely available, the most expensive, and no better on colour than 1.67.
Read the second number as carefully as the first. Every step up in index buys thickness and pays for it in colour fringing. That trade is real, and it is not on the marketing sheet.
The myth that a higher index is a better lens
The claim: "Higher index is a better lens."
It is not a better lens; it is a different compromise. Three reasons the highest index is often the wrong purchase:
- At low powers the gain is negligible. At −1.00 D there is barely any edge thickness to remove, so you pay a high-index price for a difference you cannot see.
- You pay for it in colour. Someone who moves from 1.50 to 1.67 to shave a millimetre off the edge can end up with colour fringing they never had. For some wearers that is a worse lens than the one they started with.
- Frame and centring usually matter more. How much lens sits outside the frame, and how far the optical centre ends up from the pupil, move edge thickness more than one index step does. A smaller frame with correct centring routinely beats a higher index in a large frame.
A related myth: that polycarbonate is a premium choice. It is an impact choice — excellent where that matters, and optically the weakest of the common materials.
Matching the material to the job
- Low prescriptions, general wear: 1.50 or 1.60. Prioritise colour fidelity.
- Moderate to high minus: 1.60 as the default; 1.67 if edge thickness genuinely bothers you.
- Very high powers: 1.67 or 1.74, chosen together with a smaller frame. That combination does more than index alone.
- Impact risk — sport, children, workshop: polycarbonate or Trivex, where the Abbe penalty is the price of surviving an impact. For rated safety eyewear the relevant standards are ANSI Z87.1 (US) and EN 166 (EU); the marking on the eyewear, not the index, is what tells you it is rated.
- Plus powers and readers: centre thickness drives the appearance more than edge thickness, so high index helps less than people expect.
What to ask for
Two questions get you a better answer than "what is the thinnest you have":
- "Which index do I actually need for this prescription and this frame?" — the honest answer depends on both.
- "What Abbe number does that material have?" — if it is in the low 30s and you are sensitive to colour fringing, try 1.60 first.
It is also worth asking how much of the thickness difference comes from the frame you chose, because that is often where the millimetres actually are.
You can put numbers on this rather than guessing: our lens thickness calculator estimates edge or centre thickness for a given prescription, lens size and index — and shows how much a higher index actually saves.
When to involve an optician or optometrist
This is a materials decision, not a medical one — but bring in a professional when the prescription is strong or asymmetric, when a prism correction is involved, when the two eyes differ a lot (anisometropia), or when you have struggled to adapt to a previous pair. A qualified dispenser can work out the actual edge thickness for a specific frame and prescription rather than guessing from the index. Persistent colour fringing, doubling, or headaches after new glasses are worth a return visit rather than something to live with.
The short version: index controls thickness, Abbe controls colour, and they pull against each other. The right lens is the one that matches your prescription and how you use it — for most people that is 1.60, not the most expensive pair on the list.