You pick a frame you love, and then reality sets in. With a strong prescription, the lenses can bulge past the rim like the base of a jar. That thick edge turns a sleek frame into something heavy that slides down your nose and leaves red marks by lunchtime. High-index lenses are the fix, and they bend light better than ordinary plastic. The same vision correction then fits into far less material, so you get a thinner, lighter lens that looks right in the frame. Strong prescriptions are common, too. The share of nearsighted Americans rose from 25% in the early 1970s to roughly 42% by the early 2000s. [1]
Every lens material has a refractive index, a number showing how much it bends (refracts) light. Ordinary plastic sits at 1.50. The higher that number climbs, the more the material bends light, so you need less of it for the same power. That is the whole trick behind a high-index lens: more bending per millimeter means a thinner lens.
Prescription power is measured in diopters (D), and a bigger number means a stronger correction and a thicker standard lens. A mild -1.00 lens looks fine in almost any material. A strong -6.00 lens is another story, turning thick and heavy at the edges in basic plastic. A strong farsighted lens does the same toward its center. A higher index keeps either one flat and light.
We build lenses across four index levels, each suited to a different range of prescription strength. Here is the quick lens index chart:
CR-39, the long-standing standard for plastic lenses, offers excellent optical clarity at the lowest cost, so it suits a mild prescription well. Polycarbonate trades a little of that clarity for toughness, staying highly impact-resistant and filtering ultraviolet (UV) light on its own. That is why it is the default for kids' glasses and sports frames. Step up to 1.67, and a lens for a strong prescription slims down by nearly half. Ours arrive with polished edges, a scratch-resistant coating, and a UV coating already included. The 1.74 lens is the thinnest, flattest, and lightest one we offer. It uses an aspheric design (a subtly flatter curve) to shave the profile down even further.
The right index is the one that matches your diopter number, and the choice is simpler than the menu makes it look.
Your prescription card has the numbers you need. A large frame or a rimless style (where the lens edges are fully exposed) shows thickness more, so it pays to step up an index level. Not sure where you land? Our lens experts check every order against your prescription before it is cut, so nothing goes into production that will not sit right.
Thinner is not free, and the tradeoffs are worth knowing. High-index lenses cost more than standard plastic, because the material is harder to make. There is also a subtle optical point: very high-index materials bend each color of light by a slightly different amount. So a few wearers notice faint color fringing at the very edge of a strong lens. The aspheric design on our 1.74 lenses helps, because its flatter curve keeps your vision crisp across more of the lens.
Coatings matter as much as the material. An anti-reflective (AR) coating cuts glare and reflections, and it makes the lens look nearly invisible in photos. A scratch-resistant coating (standard on our 1.67 lenses) keeps the surface clear through daily wear. You can also add a photochromic treatment, so a single pair darkens outdoors and clears indoors. Our photochromic lenses guide walks through how that works.
A high-index lens opens up frame styles that a thick lens would overwhelm. Rimless and semi-rimless designs put the lens edge on full view, and they look clean because no chunky rim hides it. Slim metal frames and smaller shapes benefit too, since a lighter lens will not drag them down. Pair a slim designer eyeglasses frame with a 1.67 or 1.74 lens, and it will sit light all day. The same logic carries over to prescription sunglasses, where a high-index lens keeps a strong tinted pair from getting heavy.
Match the index to your prescription, and the choice gets easy. A mild prescription does fine in standard 1.50 plastic, and it saves you money. An active wearer gets real value from polycarbonate's toughness. Once your correction passes ±4.00 D, a 1.67 or 1.74 high-index lens earns its keep, turning a thick, heavy pair into glasses you forget you are wearing. Add the coatings that fit your day, and choose the frame you actually want. We will check the prescription and handle the lens. Ready to see how thin your next pair can be? Browse our eyeglasses collection, pick the frame first, and let the right index follow.