The lens index you need depends almost entirely on your prescription strength. If your prescription is between -2.00 and +2.00, standard 1.50 index lenses work fine. Between -2.50 and -4.00 (or the plus equivalent), move to 1.60. Between -4.00 and -6.00, go with 1.67. Anything beyond -6.00, you’ll want 1.74. Those are the general breakpoints, but your frame choice and pupillary distance also shift the equation.
What “Index” Actually Means
The index number refers to how efficiently a lens material bends light. A higher number means the material bends light more sharply, so it doesn’t need as much thickness to achieve the same correction. Standard plastic lenses (called CR-39) have an index of 1.50. From there, the common steps are 1.60, 1.67, and 1.74.
Compared to standard 1.50 lenses, 1.67 lenses are about 30% thinner and 1.74 lenses can be up to 50% thinner. Both high-index options are also roughly 10% to 20% lighter than standard plastic. For mild prescriptions, that difference is barely noticeable. For strong ones, it’s the difference between thick, heavy glasses and something comfortable you forget you’re wearing.
Matching Index to Your Prescription
Here’s a practical breakdown by prescription range (using the SPH value on your script, whether plus or minus):
- Up to ±2.00: Standard 1.50 index. Lenses will be thin enough in most frames without any upgrade.
- ±2.50 to ±4.00: 1.60 index. This is the sweet spot for moderate prescriptions, offering a meaningful thickness reduction without much added cost.
- ±4.00 to ±6.00: 1.67 index. At this range, standard lenses start getting noticeably thick at the edges (for minus prescriptions) or bulging at the center (for plus). The 30% thickness reduction makes a real difference.
- Beyond ±6.00: 1.74 index. This is where the investment pays off most. The thickness reduction reaches 25% to 30% compared to even 1.67 lenses at these strengths.
One important nuance: for prescriptions between -3.00 and -5.00, the actual edge thickness difference between 1.67 and 1.74 is often less than 1 millimeter. In that range, paying extra for 1.74 may not produce a visible improvement. Save the upgrade for prescriptions above -6.00, where the gap becomes meaningful.
Your Frame Choice Changes the Math
A small, round frame hides thick edges far better than a large rectangular one. If you pick a compact frame and your prescription is moderate, you can often get away with a lower index than the guidelines above suggest. The reverse is also true: a trendy oversized frame paired with a strong prescription will make thickness more obvious, pushing you toward a higher index.
Rimless and semi-rimless frames add another consideration. These styles drill into or clamp onto the lens itself, so the material needs to resist cracking. Lenses at 1.60 index tend to be the strongest option for rimless frames in the -2.50 to -4.00 range. Higher-index materials (1.67 and 1.74) work for semi-rimless styles but can be more brittle, making them a riskier pick for fully rimless designs.
Why Your PD Matters Too
Your pupillary distance, the measurement between the centers of your pupils, affects lens thickness in a way most people don’t expect. If your PD is narrow but you choose a wide frame, the lab has to shift the optical center of each lens inward. This forces them to cut from a larger, thicker lens blank. Even a 2-millimeter error in this process can increase edge thickness and introduce unwanted prismatic effects that cause eye strain. Getting an accurate PD measurement (your optician can do this, or you can measure at home with a ruler and mirror) helps ensure you’re actually getting the thin profile you’re paying for.
The Tradeoff: Thinner Lenses vs. Optical Clarity
Higher-index lenses bend light more efficiently, but they also scatter colors slightly more. This is measured by something called the Abbe value, where higher numbers mean less color fringing around objects. Standard CR-39 plastic scores 58. Trivex scores 43. A 1.67 high-index lens drops to 32, and polycarbonate sits at 30.
In practice, most people with moderate prescriptions won’t notice color fringing. But if you have a strong prescription and you’re sensitive to visual quality, or if you do a lot of nighttime driving, the lower Abbe value of 1.74 and 1.67 lenses can occasionally produce slight rainbow-like halos around high-contrast edges. It’s not dramatic for most wearers, but it’s worth knowing that the thinnest lens isn’t automatically the “best” lens optically. If your prescription is mild enough for 1.50 or 1.60, those materials will give you sharper, cleaner vision.
UV Protection by Material
Polycarbonate and high-index plastic lenses block ultraviolet light completely without any additional coating. Standard CR-39 plastic blocks about 80% of UV on its own and needs a coating to reach full protection. If you go with standard index lenses, make sure UV protection is included, either as a coating or built into the lens treatment. Most optical shops add this automatically, but it’s worth confirming.
What You’ll Actually Pay
Lens index upgrades are one of the biggest price variables in buying glasses. For a pair of single-vision 1.74 lenses with anti-reflective and scratch-resistant coatings, expect to pay roughly $80 to $150 through online retailers, $400 to $580 at national optical chains, or $430 to $700 at independent opticians. Local shops often list 1.74 as a $300 to $400 add-on to a frame purchase.
The gap between 1.67 and 1.74 pricing is typically $50 to $100. For prescriptions in the -3.00 to -5.00 range, that premium buys you less than a millimeter of thickness reduction. For prescriptions above -6.00, the same premium delivers a genuinely noticeable difference in both thickness and weight. Progressive (multifocal) lenses cost significantly more than single-vision at every index level.
If budget is tight and your prescription is moderate, 1.60 or 1.67 lenses paired with a smaller frame will get you 90% of the cosmetic benefit of 1.74 at a fraction of the price.