Is 1.67 the Right Index for Your Prescription?

You are standing at the counter, holding a frame that looks great, and your optician is asking if you want standard plastic or high-index. Your prescription sits somewhere between -3.00 and -8.00. This is the awkward middle ground of optics. Standard CR-39 or 1.56 material will result in edges that look like thick, white rings. Go too high, to 1.74, and you are paying a premium for thickness reduction you barely need, often sacrificing clarity for minimal gain.

The 1.67 index hits a specific sweet spot. It is not the thinnest option on the market, but it is frequently the most logical engineering choice for moderate-to-strong prescriptions. At Spec Savvy, we see this index resolve the majority of "thick lens" complaints from clients who are not quite in the extreme range but are tired of the aesthetic and physical weight of lower indices.

Where 1.67 Fits in the Material Spectrum

Index refers to how much a material bends light. Higher index means the lens can bend light more efficiently, allowing the material to be thinner for the same optical power. But this physics comes with tradeoffs in cost, weight, and optical clarity.

Here is the practical breakdown for prescription ranges:

• Under -3.00: Stick to 1.56 or 1.60. The thickness difference is negligible, and you save money.
• -3.00 to -5.00: 1.67 is the upgrade. A 1.60 lens will still have noticeable edge thickness in larger frames. 1.67 cuts that bulk down significantly.
• -5.00 to -8.00: This is 1.67’s home turf. It offers a dramatic reduction in thickness and weight compared to standard options. While 1.74 is thinner, the price jump is steep, and 1.67 maintains better tensile strength.
• Above -8.00: Consider 1.74. At this level, the weight savings of 1.74 become critical for comfort, and the thickness reduction is substantial enough to justify the cost.

Most people buying 1.67 lenses are either in that mid-range prescription or are shopping on a budget and want to avoid the "coke bottle" effect without paying for ultra-high-index materials.

The Clarity Tradeoff: Abbe Value Matters

Thickness is not the only metric that matters. Optical clarity, measured by the Abbe value, is where 1.67 shines compared to its thinner sibling, 1.74.

Abbe value measures chromatic aberration, or how much colors split at the edges of your vision. A higher number is better. Standard CR-39 has an Abbe value of 58. Polycarbonate is around 30. 1.67 high-index material typically sits in the mid-30s to low-40s range, depending on the specific design. 1.74 drops into the 30s.

Why does this matter? If you have a -6.00 prescription and choose 1.74, you might notice slight color fringing when looking off-center, especially in bright light or against high-contrast backgrounds. With 1.67, that fringing is less pronounced. For many wearers, this clarity difference is more noticeable day-to-day than the millimeter of thickness saved by going to 1.74. If you are sensitive to visual artifacts, 1.67 is often the safer bet.

Weight and Comfort in Real-World Frames

A heavy lens on a lightweight frame is a recipe for discomfort. The lens pulls the glasses down your nose, creating pressure points on the bridge and behind your ears. 1.67 material is significantly lighter than CR-39 and standard plastic. For a -4.00 prescription in a large, round frame, switching from 1.56 to 1.67 can reduce the edge thickness by up to 30%.

This reduction changes the center of gravity of the glasses. They sit more securely. You stop pushing them up your nose every ten minutes. This is particularly relevant for progressive lens wearers. Progressive designs already require careful fitting; adding excessive weight and thickness to the edges can make the adaptation period longer and more frustrating. A 1.67 progressive lens offers a smoother transition zone and less peripheral distortion than a thicker, lower-index alternative.

Coatings and Durability: Don't Skimp Here

High-index materials are softer than glass or standard plastic. They scratch more easily if not properly treated. This is not a flaw in the material; it is a property of the resin. To make a 1.67 lens last, you need a robust coating package.

We recommend pairing 1.67 lenses with a high-quality anti-reflective (AR) coating. Because the lens is thinner, it can sometimes reflect more light if untreated, creating a "ghost" image on the inside of the lens. A good AR coating eliminates this. Additionally, a hard coat is non-negotiable. Without it, a 1.67 lens will haze up within a year of normal cleaning.

Blue light filters are optional. They are useful if you spend significant time on screens, but they add a yellow tint. If you are already paying for the premium index, ensure you are getting a clear, high-definition finish rather than a heavily tinted one unless you specifically need the filter.

Frame Selection Makes or Breaks the Look

You can buy the thinnest lens in the world, but if you put it in the wrong frame, it will still look thick. Frame size and shape dictate how much lens material you need.

For a -5.00 prescription, avoid large, round frames. The diameter is too wide, forcing the lens to be thicker at the edges to maintain the optical center. Choose smaller, rectangular frames. They reduce the required lens diameter, which naturally reduces edge thickness. Metal frames with a thin rim can also show the edge of the lens more prominently than a thick acetate frame, which can hide the bevel. If you have a strong prescription and love oversized frames, 1.67 is the minimum index you should consider. Anything lower will look bulky.

When to Skip 1.67

Do not buy 1.67 if your prescription is under -2.00. You are paying for material benefits you cannot use. The thickness reduction will be imperceptible, and you will lose money.

Similarly, if you are in a very small, rimless frame, the edge thickness is less of a visual issue because there is no rim to highlight it. However, weight still matters. If you are sensitive to weight, 1.67 is still better than 1.56. If you are not, save the cash.

For prescriptions above -8.00, 1.67 is a compromise. It will work, but 1.74 will provide a noticeably lighter and thinner lens. If you are in that range and have the budget, the comfort upgrade is worth it.

Choosing the right index is about balancing your optical needs with your budget and frame choice. 1.67 is the workhorse for the majority of moderate-to-high prescriptions in Calgary, AB, offering a reliable blend of thinness, clarity, and cost. If you are unsure whether your specific prescription and frame combination warrants the upgrade, Contact Us for a quick evaluation.