Lens index guide: 1.50 to 1.74, which one do you need?

Key Points

• Lens index (refractive index) controls lens thickness and weight, not how well you see — a 1.50 and a 1.74 lens correct vision identically.
• Common indexes range from 1.50 (thickest, cheapest) to 1.74 (thinnest, priciest), with 1.60 and 1.67 covering most moderate-to-high prescriptions.
• Use your spherical equivalent (SPH + CYL ÷ 2), not SPH alone, to pick the right index.
• Go up one tier for rimless frames, significant astigmatism, or a much stronger eye.
• Higher index means higher cost and slightly more chromatic dispersion at the lens edges.
  

If you've ever picked up a new pair of glasses and noticed the lenses were noticeably thinner than your last pair, lens index is the reason why. It's one of the most confusing specs on a prescription order form, mostly because lens shops throw out numbers like 1.67 or 1.74 without explaining what they actually mean for you.

This guide breaks down what lens index is, how the different options compare, and how to use your own prescription to pick the right one.

What is lens index?

Lens index (formally known as refractive index) is a measure of how much a material bends light — the higher the number, the more it bends, and the less material is needed to correct your vision. In practical terms: a higher index lets your prescription be ground into a thinner, lighter lens.

It's worth saying clearly: index has nothing to do with how well you see. A 1.50 lens and a 1.74 lens with the same prescription correct your vision identically. The only difference is thickness, weight, and (to a smaller degree) optical clarity at the edges.

Lens index comparison chart

Here's how the common indexes stack up against each other.

Index	Material type	Best for (SPH)	Relative thickness	Price range
1.50	Standard plastic (CR-39)	±2.00 or less	Thickest	Lowest
1.56	Mid-index	Low to moderate	Moderately thick	Low
1.60	Thin	±2.00 to ±4.00	Medium-thin	Mid
1.67	High index	±4.00 to ±6.00	Thin	Mid-high
1.74	Very high index	Above ±6.00	Very thin	High

How do you pick a lens index from your prescription?

Calculate your spherical equivalent rather than relying on SPH alone — it accounts for astigmatism and gives a more accurate picture of real-world lens thickness. Here's the process:

1. Start with the SPH (sphere) value. This is your baseline. Match it against the ranges in the chart above for an initial pick.
2. Check the CYL (cylinder) value, which represents astigmatism. If it's significant (roughly beyond -2.00), your actual lens will be thicker than SPH alone suggests — consider moving up one index tier.
3. Calculate your spherical equivalent (SE), using SE = SPH + (CYL ÷ 2). This single number reflects the real-world thickness better than SPH on its own.
4. If your two eyes differ, base your index choice on whichever eye has the stronger prescription, so both lenses end up reasonably matched in thickness.
5. Adjust for frame style and budget. Rimless or half-rim frames expose more of the lens edge, so it's worth going up a tier if you've chosen one of those. If budget is tight, a slightly thicker lens in a well-chosen frame can still look completely fine.

Take a real example: a prescription reading O.D. (right eye) SPH -1.75 / CYL -0.50, and O.S. (left eye) SPH -2.25 / CYL -0.75. The spherical equivalents work out to about -2.00 for the right eye and -2.625 for the left. Since the eyes differ, you'd base your index choice on the stronger one — -2.625 — which lands just inside the ±2.00 to ±4.00 range, pointing to 1.60. If the remarks section also calls for polycarbonate (often noted for impact resistance), 1.59 becomes a reasonable alternative even though the prescription itself doesn't demand it.

Want to understand every field on your prescription, not just SPH and CYL? Read our full guide to reading an eyeglass prescription.

Which lens index matches your prescription?

Your SPH (sphere) power is the main factor — match it to one of four ranges and you'll land close to the right index. For ±2.00 or under, 1.50 or 1.56 will look fine and keep cost down. For ±2.00 to ±4.00, 1.60 is the standard sweet spot. For ±4.00 to ±6.00, 1.67 noticeably improves thickness and weight. Above ±6.00, 1.74 is usually worth it, with 1.80 or 1.90 reserved for extreme prescriptions.

If you also have significant astigmatism (a high CYL value), or you're choosing a rimless or half-rim frame, it's common to bump up one tier from what your SPH alone would suggest — the next section explains why.

What's the difference between 1.50, 1.60, 1.67, and 1.74?

Each index step trades a bit more cost for a bit less thickness, but the relationship isn't linear.

1.50 is the standard starting point, best suited for mild prescriptions of ±2.00 or under. It's the most affordable option, but the lenses are noticeably thicker and heavier — fine for low prescriptions in a full-rim frame, but less ideal if your prescription is climbing or you want a slimmer look.

1.56 is a modest step up from 1.50 in thinness without a significant price jump. It works well for low-to-moderate prescriptions where 1.50 feels a bit bulky but upgrading all the way to 1.60 isn't necessary. A good middle-ground option for budget-conscious buyers.

1.60 is the most common "thin lens" upgrade for people in the low-to-moderate range. It's noticeably thinner than 1.50 without a big price jump, which makes it the default recommendation for most first-time high-index buyers.

1.67 is where the visual difference starts to matter for most people. Anyone with a prescription in the ±4.00 to ±6.00 range will usually see and feel the difference — lighter on the face, less magnification or minification of the eyes, and a noticeably slimmer edge profile in the frame.

1.74 is a meaningful step up from 1.67 in thickness, but the price difference is often disproportionate to that improvement unless your prescription is genuinely high (above ±6.00 or so). Below that, you're often paying a premium for a difference that's hard to see with the naked eye.

What are the pros and cons of high index lenses?

The upside is thinner, lighter, more flattering lenses; the downside is a higher price and slightly more chromatic dispersion. On the plus side, less lens bulge and less magnification or minification make eyes look closer to natural, which matters most for rimless frames and stronger prescriptions.

On the downside, the price climbs with each index step, and a lower Abbe value can introduce faint color fringing at the edges of vision in strong prescriptions. Some high-index materials also scratch slightly more easily, so a scratch-resistant coating is worth adding regardless of index.

Frequently asked questions (FQA)

• What's the difference between 1.67 and 1.74 high index lenses?

The thickness difference is only really noticeable above roughly ±6.00, while the price gap is significant at every level.

• Do I really need 1.67 high index lenses?

Only if your prescription is roughly ±4.00 or higher, or you're especially sensitive to lens weight.

• What does SPH +0.25 or +0.50 mean, and is it worth getting glasses for?

It's a very mild prescription, and whether it's worth correcting usually comes down to symptoms like eye strain rather than the number itself.

• Is −2.25 legally blind?

No — legal blindness is defined by visual acuity, not diopter power, and -2.25 is fully correctable with standard lenses.

• Will high index lenses make my eyes look bigger or smaller?

They reduce the minification effect in minus lenses and the magnification effect in plus lenses, making eyes look closer to their natural size.

• What lens index do I need for my prescription?

Calculate your spherical equivalent and match it against the ranges in this guide, adjusting up a tier for rimless frames or high astigmatism.

Got Questions? We’re Here to Help. If you have any inquiries or need assistance selecting the perfect lens, please feel free to reach out to us at support@migafas.com.

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