The index on eyeglass lenses refers to how efficiently the lens material bends light. A higher index number means the material can achieve the same vision correction in a thinner, lighter lens. Standard plastic lenses have an index of 1.50, and values range up to 1.74 for the thinnest options available.
How Lens Index Works
The number itself comes from physics. The refractive index measures the speed of light through a material compared to its speed in a vacuum. Light travels at roughly 300,000 km/s in empty space but slows down when it passes through glass or plastic. The more a material slows light, the more powerfully it bends that light, and the higher its refractive index.
For eyeglasses, this matters because a material that bends light more efficiently doesn’t need as much curvature or thickness to correct your vision. That’s why a 1.74 index lens can be noticeably thinner than a 1.50 index lens at the same prescription strength. The lens material is simply doing more work per millimeter.
Common Index Options
When you’re choosing lenses, you’ll typically see these index values:
- 1.50 (standard plastic, also called CR-39): The baseline. Works well for mild to moderate prescriptions and offers excellent optical clarity, with an Abbe value of 58 (a measure of how little color distortion you see through the lens).
- 1.53 to 1.56 (mid-index plastic): A modest step up in thinness, often available as an affordable upgrade.
- 1.59 (polycarbonate): A popular choice because it’s impact-resistant and often covered by insurance. However, it has a lower Abbe value of around 30, meaning slightly more color fringing at the edges of your vision.
- 1.60 (high-index plastic): Noticeably thinner than standard lenses, with an Abbe value of about 36.
- 1.67 (high-index plastic): Designed for stronger prescriptions, with an Abbe value around 32.
- 1.74 (ultra high-index plastic): The thinnest plastic lens available. Best suited for very strong prescriptions.
Trivex is another material worth knowing about. It sits at an index of about 1.53, so it’s not especially thin, but it’s lighter than polycarbonate, more impact-resistant in testing, and delivers better optical clarity. PPG, the company behind Trivex, published data showing its lenses sustained less surface damage under high-impact forces than polycarbonate and held up better against chemical exposure. For children’s glasses, safety eyewear, or sports frames, Trivex and polycarbonate are the most common choices because both meet high-impact safety standards.
Which Index You Actually Need
Your prescription strength is the main factor. If your prescription is below about 2.00 diopters (the number on your Rx, ignoring the plus or minus sign), standard 1.50 index lenses will be perfectly fine. They’ll be thin enough that you won’t notice edge thickness, and they offer the best optical clarity for the price.
Once your prescription climbs above 2.00 diopters, higher-index lenses start to make a real difference. At prescriptions of -5.00 or stronger, the edges of standard lenses become thick and heavy, which is both a comfort issue and a cosmetic one. That’s where 1.67 or 1.74 lenses pay off.
Frame size also plays a role. Larger frames expose more of the lens edge, making thickness more visible. If you prefer oversized frames and have a moderate-to-strong prescription, bumping up one index level can keep the lenses from looking bulky.
The Tradeoff: Thinner Lenses, More Color Fringing
There’s an inverse relationship between lens index and optical clarity. As the index goes up, the Abbe value tends to go down. A lower Abbe value means more chromatic aberration, which shows up as slight color fringes around high-contrast edges, especially in your peripheral vision.
Standard CR-39 plastic (1.50) and crown glass (1.52) both have Abbe values around 58, which is excellent. Polycarbonate and 1.67 plastic lenses sit around 30 to 32. At 1.74, some materials drop even lower. Most people don’t notice this effect in everyday life, but if you’re sensitive to visual quality or spend long hours at a screen, it’s worth considering. You may not need the absolute thinnest lens if a slightly thicker option gives you cleaner vision.
How Much Thinner and Lighter Are They?
The difference depends on where you’re starting and how strong your prescription is. Going from polycarbonate (1.59) to 1.67 high-index lenses produces roughly a 12% reduction in thickness and weight. That can translate to less than 1mm of actual difference, which may not feel dramatic in the hand.
Jumping from 1.59 to 1.74, however, gives you about a 25% reduction in both weight and thickness. At a prescription of -5.00 or stronger, that’s a difference you can see and feel. The lenses sit flatter in the frame, the edges don’t protrude as much, and the glasses are more comfortable to wear all day.
For mild prescriptions, the thickness savings from going high-index are minimal because the lenses were already thin. That’s why opticians generally don’t recommend 1.74 lenses for someone with a -1.50 prescription. You’d be paying more for an improvement you can barely detect.
Cost Differences
Higher-index lenses cost more to manufacture, and the price increase can be significant. As a rough benchmark, upgrading from polycarbonate to 1.74 high-index might add around $200 out of pocket on top of what insurance covers. The exact amount varies widely depending on the retailer, coatings, and whether you’re buying online or in-office.
Insurance plans frequently cover standard plastic or polycarbonate lenses at no additional cost but treat high-index options as an upgrade. If you’re deciding whether the upgrade is worth it, the prescription threshold matters most. Below -3.00 or so, most people are happy with polycarbonate or 1.60. Between -3.00 and -6.00, 1.67 offers a solid balance of thinness and cost. Above -6.00, 1.74 lenses make the biggest visible difference and are generally worth the investment for both comfort and appearance.
Anti-Reflective Coating Matters More at High Index
High-index materials reflect more light off their surfaces than standard plastic. This means more glare and visible reflections on your lenses, which can be distracting and make your eyes harder to see behind the glasses. Anti-reflective coating becomes especially important at 1.67 and above. Many high-index lens packages include it automatically, but if it’s offered as a separate add-on, it’s worth selecting. Without it, you lose some of the cosmetic benefit you paid for with the thinner lens.