1.67 vs 1.74 lenses
The two high-index materials an optician will offer you, compared at actual size. Every figure here is calculated from the sagitta formula rather than quoted from a brochure, and the drawings are to scale with no vertical exaggeration.
The short answer. Below about five dioptres, no — the step buys 0.26 mm at −3.00, which nobody can see. Above eight it is the single biggest thing left that still helps, and at −10.00 it is worth 0.71 mm. Between the two, spend the money on a smaller frame first.
The question this page answers: is the thinnest lens worth the money, or is 1.67 close enough?
The difference, at actual size
A 52 mm frame, drawn to scale with no vertical exaggeration. Each pair shares one scale, so the comparison is fair — two lenses drawn in boxes sized to their own contents look identical however different they are. The red bracket marks the edge thickness.
What the index number actually does
The refractive index of a lens material is how much it bends light compared with a vacuum. CR-39, the plastic everything else is measured against, sits at 1.50. A 1.74 lens bends light more steeply, so it needs less curvature to achieve the same correction — and less curvature means less glass between the front surface and the back.
That relationship is not a rule of thumb, it is arithmetic. The sag of a lens surface — how far it departs from flat across a given diameter — follows
s = d²F ÷ 8000(n − 1)
where d is the lens diameter in millimetres, F the power in dioptres, and n the refractive index. Everything about the 1.67-versus-1.74 question falls out of the n − 1 on the bottom. Going from 1.67 to 1.74 changes that term from 0.67 to 0.74, which is an improvement of about ten per cent. Going from CR-39's 1.50 to 1.67 changes it from 0.50 to 0.67 — an improvement of thirty-four per cent, more than three times as much.
This is the whole answer in one line. The first step into high index does three times as much as the second one. Opticians sell them as two rungs on the same ladder; arithmetically the first rung is three times the height of the second, and it usually costs less.
The saving, at every prescription strength
Below is what the step buys in a 52–18 frame on a 63 mm pupillary distance — an unremarkable combination, chosen because it is close to the middle of what people actually wear. Both columns are temporal edge thickness, the part of a lens visible from the side and the only part anyone ever notices.
| Prescription | 1.67 | 1.74 | Saving | Weight 1.67 | Weight 1.74 |
|---|---|---|---|---|---|
| −2.00 | 2.9 mm | 2.7 mm | 0.17 mm | 6.5 g | 6.7 g |
| −3.00 | 3.4 mm | 3.1 mm | 0.26 mm | 7.0 g | 7.0 g |
| −4.00 | 3.9 mm | 3.5 mm | 0.35 mm | 7.5 g | 7.4 g |
| −5.00 | 4.5 mm | 4.1 mm | 0.41 mm | 8.4 g | 8.3 g |
| −6.00 | 5.2 mm | 4.7 mm | 0.47 mm | 9.3 g | 9.2 g |
| −8.00 | 6.5 mm | 5.9 mm | 0.60 mm | 11.2 g | 11.1 g |
| −10.00 | 7.8 mm | 7.1 mm | 0.71 mm | 13.1 g | 12.9 g |
| −12.00 | 9.1 mm | 8.3 mm | 0.83 mm | 15.0 g | 14.7 g |
Two things in that table are worth sitting with. The first is how little the step buys at the powers most people have: at −2.00 and −3.00 and −4.00 and −5.00 and −6.00 it is under half a millimetre, which is a fifth the thickness of a credit card and nothing you would ever notice on a face. The step only reaches a full millimetre at very high powers.
The second is the weight columns, which surprise nearly everybody. 1.74 material has a density of 1.46 g/cm³ against 1.67's 1.35 — it is heavier per unit volume. The lens is thinner and the material is denser, and the two effects largely cancel. People buy 1.74 expecting lighter glasses and get, at most, marginally lighter ones. If weight is the real complaint, the material to ask about is Trivex at 1.11 g/cm³, which is lighter than everything else here by a wide margin and is a different conversation entirely.
What you give up
Optical quality, but not where you expect
Every lens material disperses light slightly — splitting white light into its colours, the way a prism does. The measure of this is the Abbe number, and lower is worse. It shows up as a faint coloured fringe around high-contrast edges when you look through the outer part of the lens rather than straight through the middle.
CR-39 scores 58, which is as good as it gets in common use. 1.67 scores 32 and 1.74 scores 33. Those last two are effectively the same number, and both are a substantial drop from CR-39. So the honest version of the warning is not "1.74 has worse optics than 1.67" — it does not, measurably — it is "high index has worse optics than standard plastic, and you crossed that line at 1.60."
Whether you notice depends mostly on your prescription. A strong prescription puts more of your field of view through the peripheral part of the lens, where dispersion is visible, so the people most likely to be sold 1.74 are also the people most likely to see its drawbacks. In practice, most wearers adapt within a fortnight and stop noticing.
Reflections
Denser materials reflect more light at the surface. A CR-39 lens reflects about four per cent of incident light; 1.74 reflects closer to seven. That is visible as surface glare to other people and as reduced contrast to you, particularly at night.
This is fixable and should be treated as compulsory rather than optional. An anti-reflective coating brings surface reflection below one per cent on any index. If a shop quotes you 1.74 without anti-reflective coating, that quote is incomplete rather than cheap — you will be buying the thinnest lens available and then looking through a mirror.
Availability
1.74 exists in a narrower range than 1.67. Photochromic versions are limited and sometimes unavailable, polarised versions more so. Some labs will not surface 1.74 below a given power because the resulting lens would be too thin to edge and mount without cracking. If you want a specific tint, coating or photochromic behaviour, check it is available in 1.74 before you commit — being told after the fact is a common and avoidable annoyance.
What to spend the money on instead
The sagitta formula has a d² in it, and d is not the frame's marked lens width — it is twice the distance from the optical centre of the lens to its furthest edge. That distance depends on the frame's width, its shape, and how far the frame's own pupillary distance differs from yours.
Because the relationship is squared, that distance is the most powerful lever available. A frame 6 mm narrower cuts the edge thickness more than a whole index step does, on nearly any prescription. Here is the same −6.00 prescription in three frame widths, all in 1.67:
| Frame | Material | Edge |
|---|---|---|
| 46 mm wide | 1.67 | 3.8 mm |
| 52 mm wide | 1.67 | 5.2 mm |
| 58 mm wide | 1.67 | 6.9 mm |
| 52 mm wide | 1.74 — the upgrade | 4.7 mm |
A 46 mm frame in 1.67 comes out thinner than a 52 mm frame in 1.74, and costs less rather than more. This is not a trick — it is the squared term doing what squared terms do. It is also the single most useful thing anyone can tell you about buying glasses on a strong prescription, and it is almost never said at the counter, because a shop makes its margin on the lens.
The other levers, in rough order of how much they help: a frame whose own pupillary distance is close to yours, so the optical centres do not have to be shifted inward; a rounder lens shape rather than a wide rectangular one, since the corner is the thickest point on any rectangular lens; and a full rim rather than rimless, which lets the lab leave the edge thinner because the rim does the holding.
When 1.74 is clearly the right answer
There is a case for it, and it is a good one above about eight dioptres. At those powers the lens is thick enough that every millimetre counts, the frame has already been made as small as the wearer will tolerate, and there is nothing else left to try. The table above shows the step buying 0.71 mm at −10.00 — a visible difference, on a lens where visible thickness is the thing that bothers people.
It is also the right answer when the frame cannot be made smaller for a reason outside your control: a rimless design the wearer has settled on, a prescription with a large cylinder that drives thickness in one meridian, or a face wide enough that a small frame does not sit correctly. In those cases the material is the only lever left, and it should be pulled.
How to decide, in order
- Find your strongest meridian. For a prescription with cylinder, that is sphere plus cylinder if the cylinder is negative, or the sphere alone if it is not — the larger absolute number of the two governs thickness.
- Under about four dioptres, stop. Standard plastic or 1.60 is fine and the difference is invisible. Spend the money on anti-reflective coating, which you will notice every day.
- Between four and eight, choose the frame first and the index second. A smaller frame in 1.67 beats a larger one in 1.74 and leaves money over.
- Above eight, take 1.67 as the floor and consider 1.74 seriously. Check the coating and photochromic options you want are available in it before agreeing.
- At every level, insist on anti-reflective coating on any high-index lens. It is not an upsell on these materials, it is a correction for a problem the material introduces.
Check it against your own numbers
Everything above is a 52 mm frame on a 63 mm pupillary distance, which is nobody in particular. The lens thickness calculator runs the same formula on your prescription and your frame, across all six materials at once, and draws the cross-sections to scale. It also tells you when the answer is that none of it is worth paying for, which is the answer more often than the trade admits.
These figures are calculated from lens geometry and the published properties of each material. Real lenses vary with the lab's minimum thicknesses, the base curve chosen, the lens shape and the glazing method, so treat them as close estimates rather than quotations. Nothing here is a substitute for an optician's advice on your own eyes.
Questions people ask
- Is 1.74 worth the extra money?
- It depends almost entirely on your prescription. In a 52 mm frame, moving from 1.67 to 1.74 saves 0.17 mm at −2.00 and 0.83 mm at −12.00. Below about five dioptres the difference is smaller than the variation between two frames, so the money buys more if you spend it on the frame instead.
- Is a 1.74 lens lighter than a 1.67?
- Usually, but by less than people expect, and not always. 1.74 material is denser — 1.46 g/cm³ against 1.35 — so it is thinner but each cubic millimetre weighs more. The two effects partly cancel. At −6.00 in this frame the 1.74 lens weighs 9.2 g against 9.3 g.
- Does 1.74 look worse optically than 1.67?
- Barely, and not in the direction most people assume. Their Abbe numbers are 32 and 33 — essentially the same, and both well below CR-39's 58. The step that costs you optical quality is the one into high index at all, not the one from 1.67 to 1.74.
- Can I get 1.74 in any frame?
- No. 1.74 is not available in every design, is rarely available in a photochromic or a polarised version, and some labs will not surface it below a certain power because the lens would be too thin to edge safely. Rimless and half-rim frames add their own restrictions.
- Which matters more, the lens index or the frame?
- The frame, for most prescriptions. Edge thickness grows with the square of the distance from the optical centre to the rim, so a frame 6 mm narrower does more for a strong minus lens than a whole index step does. A small frame in 1.67 beats a large one in 1.74 nearly every time.