The Real Difference Between CR-39 and Mid-Index 1.56

Most patients assume that paying more for lenses automatically buys them better vision. That is a misconception. If you have a low-to-moderate prescription, upgrading to a high-index lens often solves a problem you do not actually have. The standard plastic lens, known as CR-39 or index 1.50, has been the industry baseline for decades. It offers excellent optical clarity and is inexpensive. But it is also thick and heavy. Enter the mid-index 1.56 lens. This material sits in the sweet spot for a specific group of people: those with prescriptions between -1.00 and -3.00, or +1.00 to +2.50.

At Spec Savvy, we see this scenario daily. A patient brings in a strong frame with thin rims. They want the aesthetic of the frame but have a prescription that would make CR-39 look bulky. Switching to 1.56 index reduces the edge thickness by roughly 20-25% compared to standard plastic. It does not require the complex, expensive digital surfacing needed for 1.67 or 1.74 lenses. The result is a lens that looks modern, fits the frame, and costs significantly less than premium high-index options.

Who Actually Needs 1.56 Index?

Let’s be honest about the numbers. If your prescription is under -1.00 or +1.00, stick with CR-39. The thickness difference is negligible, and you will save money. If you are over -4.00, 1.56 is likely too thick for a comfortable, slim look. You need to move up to 1.60, 1.67, or polycarbonate. The 1.56 index is designed for the middle ground.

Consider a patient with a -2.50 prescription. In a large, round frame, a CR-39 lens might have an edge thickness of 4.5mm. A 1.56 lens drops that to roughly 3.5mm. Visually, that is a major improvement. It eliminates the "coke bottle" effect where the lens edge is visibly protruding from the frame front. For moderate prescriptions, this material provides the best balance of weight reduction and cost efficiency. It is not about chasing the thinnest possible lens; it is about matching the lens material to the prescription strength and frame size.

Weight and Comfort: The Invisible Benefit

Weight is subjective, but it is measurable. A 1.56 lens is approximately 20-25% lighter than a CR-39 lens of the same power and diameter. For someone who wears glasses for ten hours a day, that reduction matters. It reduces the pressure on the bridge of the nose and the tension on the temples. You might not notice the difference immediately, but by 4:00 PM, when fatigue sets in, the lighter lens feels better.

This is particularly relevant for larger frame sizes. A 52mm lens in CR-39 is manageable. A 58mm lens in CR-39 for a -3.00 prescription can feel heavy. Switching to 1.56 mitigates this without requiring the density of polycarbonate. Polycarbonate is impact-resistant and thin, but it has a lower Abbe value, meaning it can introduce slight chromatic aberrations (color fringing) for sensitive users. 1.56 index maintains a higher Abbe value, closer to CR-39, offering clearer optics than polycarbonate while still being lighter than standard plastic.

Optical Clarity and Abbe Value

Not all thin lenses are created equal. The Abbe value measures how much the lens disperses light. A higher number means less color distortion. CR-39 has an Abbe value of 58, which is excellent. 1.56 index sits around 42-45. Polycarbonate drops to 30. High-index 1.67 can fall into the 30s as well. For most people, the difference between 42 and 58 is invisible. However, for patients with astigmatism or those who are visually sensitive, the clarity of 1.56 is often superior to polycarbonate or higher-index plastics.

If your primary concern is sharpness and you have a moderate prescription, 1.56 is a strong contender. It avoids the optical compromises of polycarbonate while providing the thinness needed for modern frame styles. We recommend this material for patients who prioritize visual fidelity alongside aesthetics.

Coatings and Durability

Material is only half the equation. The coating system determines how the lens performs in daily life. At our lab, we pair 1.56 lenses with premium anti-reflective coatings. Because 1.56 is a softer material than CR-39, it benefits significantly from a hard-coat overlay. Without a proper hard coat, 1.56 lenses can scratch more easily than CR-39. This is a critical detail often overlooked by discount retailers. They sell you the thinness but skip the durability.

We ensure every 1.56 lens receives a robust coating package. This includes anti-reflective treatment to reduce glare, blue light filtering if requested, and scratch resistance. The combination of mid-index material and high-quality coating creates a lens that is both light and resilient. If you are active or work in environments with dust and debris, this durability factor is just as important as the weight reduction.

Cost-Effectiveness: Where Your Money Goes

Premium high-index lenses can cost two to three times more than 1.56. For a moderate prescription, that price jump does not deliver a proportional benefit. You are paying for thinness you do not need and optical clarity you already have with 1.56. The savings can be redirected toward better frame quality or enhanced coatings. For example, choosing 1.56 allows a patient to afford a premium anti-reflective coating that they might have skipped if they chose a 1.67 lens. That coating improves night driving and computer use, offering a tangible benefit that a slightly thinner lens might not.

We see this trade-off constantly. Patients over-spend on index when they should be investing in coatings. 1.56 index frees up budget. It is the pragmatic choice for the majority of prescriptions. It is not the "cheap" option; it is the "smart" option for the right prescription range.

When to Skip 1.56

There are scenarios where 1.56 is the wrong choice. If you have a high astigmatism component (over -2.00 cylinder), the lens may still be thick at the edges despite the index. In these cases, moving to 1.60 or polycarbonate might be necessary to manage the thickness. Also, if you require impact resistance for sports or safety reasons, polycarbonate or Trivex is mandatory regardless of prescription strength. 1.56 is not impact-rated.

Finally, if you have a very high plus prescription (+3.00 and above), 1.56 will still be thick in the center. High-plus lenses require high-index materials to reduce the central bulk. Don’t let a salesperson push you into 1.56 if your prescription demands a higher index. Know your numbers. If your sphere is between -1.00 and -3.00, you are in the ideal zone for this material.

Fit and Frame Selection

The frame plays a huge role in how a 1.56 lens performs. Smaller, rounder frames maximize the thinness advantage. Large, rectangular frames expose more edge thickness, even with mid-index. If you choose a large frame, you might need to step up to 1.60 index to maintain a sleek profile. Conversely, a small, oval frame with a -2.00 prescription in 1.56 will look nearly identical to a 1.67 lens. We help our clients navigate this by reviewing the frame measurements before surfacing the lenses. This prevents the disappointment of receiving lenses that are thinner than CR-39 but still not slim enough for the chosen frame.

Our digital surfacing technology allows us to optimize the lens shape for the specific frame curvature. This ensures that the 1.56 lens sits correctly in the groove, avoiding stress on the lens material. Proper edging is critical for mid-index lenses to prevent chipping or stress cracking.

If you are trying to determine if 1.56 is the right fit for your prescription and frame choice, Contact Us to discuss your specific needs with our lab team.