Lens thickness, honestly
How thin do kids' lenses actually need to be?
High-index lenses are sold on a percentage. Once you do the arithmetic for a child-sized frame, that percentage turns into a fraction of a millimetre.
Buddy's quick answer
Most children never need a high-index upgrade. Polycarbonate โ the material paediatric eye organisations recommend for kids anyway โ is already a high-index lens at 1.586, so paying to go up to 1.67 typically buys about two tenths of a millimetre in a child's frame. The bigger lever is the frame itself: edge thickness rises with the square of lens width, so a properly fitted smaller frame takes off more than any material upgrade, and it costs nothing.
The claim
Where โup to 30% thinnerโ comes from
Lens thickness is governed by a piece of school geometry called the sagitta โ how deep the curve of a lens surface is. For a given prescription and lens size, the depth of that curve is proportional to 1 รท (n โ 1), where n is the refractive index. Bend light more efficiently, and you need less curve to do it.
Run that formula and the reduction in curve depth between any two materials is fixed โ it doesn't change with prescription or frame size:
| Upgrading from CR-39 (1.498) toโฆ | Reduction in curve depth |
|---|---|
| Trivex (1.53) | 6% |
| Polycarbonate (1.586) | 15% |
| 1.60 | 17% |
| 1.67 | 26% |
| 1.74 | 33% |
So โ1.67 is up to 25% thinnerโ isn't a lie. It's just describing the curve, not the lens. Every lens also carries a minimum centre thickness that no index reduces โ and that fixed part is a large share of a child's lens, because children's lenses are small. The finished edge always shrinks by less than the headline number.
Worked through
What it looks like on a real child's lens
Take a fairly typical case: โ4.00 D in a 44 mm lens, the sort of width you see in a frame fitted to a primary-school face, with the centre held at 1.5 mm. The image at the top of this page shows these three drawn to true scale.
| Material | Edge thickness | Saved vs CR-39 |
|---|---|---|
| CR-39 plastic (1.498) | 3.44 mm | โ |
| Polycarbonate (1.586) | 3.15 mm | 0.29 mm |
| 1.67 high-index | 2.94 mm | 0.50 mm |
Half a millimetre, for the biggest realistic upgrade. And that comparison flatters the upgrade, because it starts from CR-39 โ a material your child probably shouldn't be wearing anyway. AAPOS recommends polycarbonate for children, and polycarbonate is already a high-index material. Measured from where your child should actually be starting, the upgrade to 1.67 buys 0.21 mm. That is roughly two sheets of paper.
Scale it up and the picture changes, which is the honest other half of this. At โ6.00 D in a 50 mm lens, moving from CR-39 to 1.67 saves closer to a full millimetre โ visible, and worth paying for. The rule isn't โhigh index is a con.โ It's that the benefit grows with prescription strength and lens size, and a young child usually has neither.
The bigger lever
The frame does more than the material
Here is the part almost nobody sells you, because there's no margin in it. In the sagitta formula, thickness scales with the square of the lens diameter. Halve the width and you quarter the curve depth. Frame size is not a minor factor โ it is the dominant one.
Same โ4.00 D prescription, same cheap CR-39 lens, three frame widths: a 52 mm lens comes out at 4.21 mm, a 48 mm at 3.81 mm, and a 44 mm at 3.44 mm. Dropping 8 mm of frame took 0.77 mm off the edge โ nearly twice what switching from CR-39 to polycarbonate would have achieved in the wider frame.
A published example from an optical lab makes the point even harder with a strong plus prescription. A +6.00 lens in a 58-eye frame comes out 13.1 mm thick in CR-39; the same prescription in a 48-eye frame is 6.6 mm. The cheap lens in the small frame beats the expensive 1.67 lens in the big frame, which lands at 8.2 mm.
Two smaller levers ride along with frame size. Shape matters, because a round lens has a smaller effective diameter than a square or aviator shape of the same nominal eye size. And centration matters: when the frame is much wider than your child's pupillary distance, the lens has to be decentred, which forces the lab to cut from a larger blank and thickens one side.
The trade-off
What you give up going thinner
Higher-index materials generally have lower Abbe values, which means more chromatic aberration โ the faint colour fringing you can see at the edges of a strong lens when you look off-axis. The relationship is arithmetic: fringing equals the lens power multiplied by how far off-centre you're looking, divided by the Abbe value. Most wearers don't notice it until it exceeds about 0.12 prism dioptres.
| Material | Abbe value | Weight (specific gravity) |
|---|---|---|
| CR-39 (1.498) | 58 | 1.32โ1.34 |
| Trivex (1.53) | 43โ46 | 1.11 |
| Polycarbonate (1.586) | ~30 | 1.20 |
| 1.60 (MR-8) | ~41 | ~1.30 |
| 1.67 (MR-7 / MR-10) | ~31 | ~1.35โ1.37 |
| 1.74 (MR-174) | ~32 | ~1.47 |
Two things in that table are worth a second look. First, polycarbonate has the worst Abbe value of the lot โ worse than 1.67 and 1.74. So โavoid high index because of chromatic aberrationโ is not a coherent argument for putting a child in polycarbonate; the real argument for polycarbonate is impact resistance, full stop. Second, higher index does not mean lighter. A 1.67 lens is denser than polycarbonate, and 1.74 is denser still. Thinner and lighter are different purchases, and Trivex is the lightest material on the market at an index of only 1.53.
Design, not just material
Aspheric, and what โfreeformโ really means
Spherical
One constant curve across the surface. Simple, cheap, and perfectly adequate at low powers โ which covers most children's first prescriptions.
Aspheric
The curve flattens gradually towards the edge. That cancels the off-axis blur a flat lens would otherwise produce, and the flatter profile trims centre thickness in plus lenses and edge thickness in minus lenses. It also reduces how magnified a long-sighted child's eyes look to other people.
Freeform
Not a shape but a manufacturing method โ a computer-controlled lathe cutting the surface point by point instead of pressing it from a stock mould, roughly ten times more precisely. It's what makes a lens designed for one exact prescription possible, rather than one averaged across a power range.
The practical version: aspheric is worth asking about in a moderate-to-strong prescription, where it does real work on both thickness and peripheral clarity. Freeform matters most for progressive lenses, which very few children wear. For a mild single-vision prescription in a small frame, a conventional spherical lens is not a compromise โ it is simply the right lens.
Buddy-picked
Small, round frames โ the free thickness upgrade
Round shapes and a properly fitted eye size do more for lens thickness than anything on the lens menu. These three are sized by age band.
Deciding
So when is high index actually worth it?
There is no official threshold โ professional bodies don't publish one, and the figures you'll see quoted (above ยฑ2.00 D, above ยฑ3.00 D) come from optometry education sites rather than from any standard. So work from the geometry instead.
In a 44 mm child's frame, going from CR-39 to 1.67 saves about 0.25 mm at โ2.00 D, 0.50 mm at โ4.00 D, and close to a millimetre by the time you reach โ6.00 D in a larger frame. Somewhere in the middle of that range the difference stops being invisible. Below it, you are paying for something nobody will ever see.
And one caution specific to children: impact resistance comes first. Industry guidance recommends a minimum 1.5 mm centre thickness in polycarbonate for children and 2 mm for active ones. If a thinner lens means moving away from the material a child's eye doctor recommended, that is not an upgrade.
Parent questions
The stuff you actually want to know
Does my child need high-index lenses?
Usually not. Polycarbonate, the material paediatric eye organisations recommend for children, already has an index of 1.586, so a further upgrade to 1.67 typically saves around 0.2 mm of edge thickness in a child-sized frame. The benefit becomes worth paying for at stronger prescriptions and larger lens sizes, which most young children don't have.
Are high-index lenses really 30 to 45 percent thinner?
No. Those figures describe the depth of the lens curve, not the finished lens, and the commonly quoted numbers are inflated even for that. Curve depth falls by about 17 percent going to 1.60, 26 percent to 1.67 and 33 percent to 1.74 versus standard plastic. Because every lens keeps a fixed minimum centre thickness, the finished edge always shrinks by less.
Does frame size affect lens thickness?
Enormously. Edge thickness rises with the square of lens width, so a smaller frame is the single most effective way to reduce it. Going from a 52 mm lens to a 44 mm lens at minus four dioptres removes about 0.77 mm, roughly twice what upgrading from CR-39 to polycarbonate achieves in the wider frame.
Are high-index lenses lighter?
Not necessarily. Higher-index materials are generally denser, so a 1.67 lens weighs more per unit volume than polycarbonate and 1.74 more again. The thinner shape can offset that, but if lightness is the goal, Trivex is the lightest lens material available even though its index is only 1.53.
What is Abbe value and should I care?
Abbe value measures how much a material splits light into colours. A lower value means more colour fringing at the edge of a strong lens when looking off-centre. Most people don't notice it below about 0.12 prism dioptres of fringing. Worth noting: polycarbonate has the lowest Abbe value of the common materials at around 30, so this is not a reason to avoid high index specifically.
What does aspheric mean, and does my child need it?
An aspheric surface flattens gradually towards the edge, which cancels off-axis blur and reduces thickness and lens bulge. It earns its cost in moderate-to-strong prescriptions. For a mild single-vision prescription in a small frame, a conventional spherical lens is the correct choice rather than a compromise.
Keep exploring
Helpful next reads
Start with what โimpact resistantโ actually means, because material safety comes before thickness. Then frame and lens tips for strong prescriptions, why glasses keep sliding down, and the best kids' glasses by age. If the prescription is changing fast, see why eyesight gets worse and when to replace their glasses.
Where this comes from
Sources
OptiCampus โ Ophthalmic Lens Design ยท OptiCampus โ Chromatic Aberration ยท Laramy-K โ Methods for Estimating Lens Thickness ยท iCare Labs โ Optics 101: Lens Thickness ยท Mitsui Chemicals โ MR series lineup ยท AAO EyeWiki โ Lens Material Properties ยท AAPOS โ Glasses Fitting for Children ยท AAO โ Eyeglasses: How to Choose Glasses ยท HOYA โ Free Form vs. Conventional
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