A 1.5 index lens is the standard plastic lens used in most eyeglasses. The number 1.5 refers to the lens material’s refractive index, which measures how efficiently it bends light. Higher numbers bend light more in less material, resulting in thinner lenses. At 1.5, you’re getting the baseline: the most affordable, optically clear option that works well for mild to moderate prescriptions.
What Refractive Index Actually Means
Every lens material bends light differently. The refractive index is a number that describes how much a material slows and redirects light passing through it. A higher refractive index means the material bends light more sharply, so less material is needed to achieve the same corrective power. That’s why higher-index lenses (1.61, 1.67, 1.74) are thinner than a 1.5 lens at the same prescription strength.
The 1.5 index lens is made from a plastic called CR-39, which has been the default eyeglass lens material for decades. Its actual refractive index is 1.499, rounded to 1.5. When an optician or online retailer lists “standard lenses” or “basic lenses,” they almost always mean CR-39 at this index.
Who Should Choose 1.5 Index Lenses
These lenses are best suited for mild to moderate prescriptions: a sphere (SPH) value between -2.75 and +2.75, and a cylinder (CYL) value between -2.00 and +2.00. Within that range, a 1.5 index lens will be thin enough to look and feel comfortable in most frames.
If your prescription falls outside that range, the lenses start getting noticeably thick at the edges (for nearsightedness) or in the center (for farsightedness). At a -4.00 prescription in a medium-sized frame, for example, the edge thickness of a 1.5 index lens can reach around 10mm, compared to roughly 7mm in a 1.67 index lens. That difference matters both cosmetically and in terms of weight on your face.
The Optical Clarity Advantage
One of the biggest selling points of 1.5 index lenses is something most people never hear about: the Abbe value. This is a measure of how cleanly a lens transmits light without splitting it into rainbow-like color fringes around objects, especially at the edges of your vision. CR-39 has an Abbe value of 58, which is excellent. The higher the Abbe value, the less color distortion you see.
Higher-index materials sacrifice this clarity. As the refractive index goes up, the Abbe value drops, meaning you’re more likely to notice slight color fringing around high-contrast edges like dark text on a white screen. For people with lower prescriptions, this tradeoff isn’t worth it. You’d be paying more for thinner lenses while getting slightly worse optical quality.
Durability and Frame Fit
CR-39 is a lightweight, scratch-resistant plastic, but it’s not impact-resistant. These lenses have a higher likelihood of cracking or chipping compared to polycarbonate or Trivex materials. That makes them a poor match for semi-rimless and rimless frames, where the lens edges are exposed and need to withstand drilling or grooving. They’re also not recommended for children’s glasses or anyone with an active lifestyle where eyewear might take a hit.
For full-rim frames used in everyday settings like office work, driving, or reading, 1.5 index lenses hold up fine. Adding a scratch-resistant coating extends their lifespan significantly.
UV Protection Requires a Coating
Unlike polycarbonate and most high-index plastics, CR-39 does not naturally block UV radiation. If you plan to wear your glasses outdoors, you’ll need a UV coating added to the lenses. Most retailers include this at little or no extra cost, but it’s worth confirming, especially with budget online sellers. Without the coating, a clear 1.5 index lens offers no meaningful sun protection for your eyes.
Cost Compared to Higher-Index Lenses
The 1.5 index lens is almost always the least expensive option. Many vision insurance plans cover standard CR-39 lenses with no out-of-pocket cost, while upgrading to high-index materials can add $100 to $200 or more depending on the retailer and the index you choose. Online optical shops tend to compress this price gap, and some even price all index options the same, so it pays to compare.
If your prescription is within the recommended range, there’s little practical reason to spend more on a higher index. You’d be paying for a marginal reduction in thickness that most people wouldn’t notice, while giving up some optical clarity. Save the upgrade for when your prescription genuinely needs it.