What Is Lens Index?
Index measures how efficiently a lens material bends light. Higher index means the lens can be thinner while doing the same job. Standard plastic (CR-39) has an index of 1.50. High index starts at 1.61 and goes up to 1.74.
Every step up in index shaves about 15-20% off lens thickness. Someone with a -6.00 prescription could go from 6mm edge thickness in standard lenses to about 3.5mm in 1.67 high index. That is the difference between lenses that fit flush in the frame and lenses that stick out.
Index Comparison Table
| Index | Best For | Thickness vs Standard | Cost at Fytoo |
|---|---|---|---|
| 1.50 (Standard) | Mild Rx (-2.00 to +2.00) | Baseline | Free |
| 1.61 | -3.00 to -5.00 | ~20% thinner | $15-30 |
| 1.67 | -5.00 to -8.00 | ~35% thinner | $30-50 |
| 1.74 | -8.00 and stronger | ~45% thinner | $50-80 |
When to Upgrade
If your prescription is under -3.00, the thickness difference is barely visible. Save your money. Between -4.00 and -6.00, 1.61 or 1.67 is worth it if you care about how the glasses look from the side. Above -6.00, 1.67 or 1.74 is almost necessary to keep the lenses inside the frame.
Frame choice matters too. Thin metal frames expose lens edges. Thick acetate frames hide them. If you wear bold plastic frames, you can get away with a lower index. If you want rimless or thin titanium, go higher index.
Weight and Optical Quality
Beyond thickness, high index materials also affect lens weight. Standard CR-39 plastic weighs about 1.32 grams per cubic centimeter. Polycarbonate (1.59 index) drops to 1.20 g/cm³. Trivex (1.53 index) weighs just 1.11 g/cm³ - about 16% lighter than standard. For a -7.00 prescription, switching from CR-39 to 1.67 high index cuts total lens weight by roughly 35%. One trade-off: higher index materials have lower Abbe values, meaning more chromatic aberration. 1.74 index has an Abbe value around 33, compared to 58 for CR-39. Most people do not notice the difference, but if you are sensitive to color fringing at lens edges, Trivex (Abbe ~45) offers a better balance of thinness and optical clarity.
Real Thickness Numbers
Concrete thickness numbers help. For a -6.00 prescription in a 50mm lens width frame: standard 1.50 CR-39 plastic produces an edge thickness around 6.2mm. 1.61 high index drops that to roughly 4.8mm. 1.67 brings it to 3.8mm. 1.74 gets it down to 3.2mm. The jump from standard to 1.61 saves about 1.4mm - noticeable but not dramatic. Going from 1.61 to 1.67 saves another 1.0mm. Going from 1.67 to 1.74 saves only 0.6mm, which is why the price premium for 1.74 is hard to justify below -8.00. Frame material also matters: a thick acetate frame hides 2-3mm of edge thickness, while a thin titanium rim exposes nearly all of it. If you have a -5.00 prescription and want thin metal frames, 1.67 is the practical floor.
Go Thinner With High Index Lenses
High index lenses from $15 extra at Fytoo.
Shop High Index at Fytoo - From $15 Extra →Affiliate link · We earn a commission at no extra cost to you
FAQ
Is 1.74 high index worth it?
Only if your prescription is stronger than -8.00. The difference between 1.67 and 1.74 is small - maybe 10% thinner. Below -8.00, the extra cost is hard to justify. Above -8.00, every bit of thickness reduction helps with weight and appearance.
Do high index lenses have more glare?
Yes, slightly. Higher index materials reflect more light off the lens surface. That is why anti-reflective coating matters more with high index lenses - it cuts the reflections. Fytoo includes AR coating standard, so this is handled.
Can I get high index with progressives?
Yes. If you need both strong correction and multiple distances, high index progressives are the best combination. They cost more but the lens will be thin across all three zones.