Lens Index Explained: 1.50, 1.61, 1.67 and 1.74
Lens Technology
Lens Index Explained: 1.50, 1.61, 1.67 and 1.74
What the index number on a prescription lens actually means, how it changes thickness and weight, and which one suits your power.
In this article
- What the index number measures
- Why higher index means thinner lenses
- The four common indexes compared
- Matching index to your prescription
- The trade-offs nobody mentions
- How frame choice changes the answer
- Optical frames in stock
- Frequently asked questions
What the index number measures
Refractive index is a measure of how strongly a material bends light. A lens with a higher index bends light more per millimetre of material, so it needs less material to achieve the same optical power.
The number itself is a ratio. An index of 1.50 means light travels through the material at roughly two thirds the speed it travels in a vacuum, and bends accordingly. You do not need the physics to use it. What matters is the practical consequence.
Why higher index means thinner lenses
A prescription lens works by having a curved surface. The stronger the correction, the more curvature is needed, and curvature adds thickness. In a minus lens for short sight, that thickness collects at the edge. In a plus lens for long sight, it collects at the centre.
Raising the index lets the manufacturer achieve the same bending with a gentler curve. The lens comes out flatter and thinner at the point where thickness would otherwise be most visible.
This is why index matters more as prescriptions get stronger. At low power the difference between materials is barely perceptible. At high power it is the difference between a lens that sits neatly in the frame and one that projects noticeably beyond it.
The four common indexes compared
| Index | Relative thickness | Typical suitability |
|---|---|---|
| 1.50 | Standard | Low powers, roughly up to 2.00 |
| 1.59 | Thinner, impact resistant | Sports, children, rimless |
| 1.61 | About 20% thinner | Moderate powers, 2.00 to 4.00 |
| 1.67 | About 30% thinner | Strong powers, 4.00 to 7.00 |
| 1.74 | Thinnest available | Very strong powers, above 7.00 |
The 1.59 entry is polycarbonate, which sits slightly apart from the others. Its index is modest but its impact resistance is far higher than standard plastic, which is why it dominates sports eyewear and children's frames regardless of power.
Matching index to your prescription
The practical rule is to let the strongest number on your prescription drive the choice. Look at the sphere value, and if there is a significant cylinder value, factor that in too.
Below about 2.00, standard 1.50 is usually fine and going higher buys very little. Between 2.00 and 4.00, moving to 1.61 makes a visible difference, particularly in larger frames. Between 4.00 and 7.00, 1.67 is the sensible default. Above 7.00, 1.74 is worth it.
The trade-offs nobody mentions
Higher index is not simply better. Three things move in the wrong direction as index rises.
Chromatic aberration increases. High index materials split light into its component colours slightly more, which can show as faint colour fringing at the edges of vision. Most wearers adapt quickly, but people sensitive to it notice.
Reflectivity increases. Higher index surfaces reflect more light, which is why anti-reflective coating stops being optional and becomes close to essential at 1.67 and above.
Impact resistance does not improve with index. If durability is the priority, polycarbonate at 1.59 outperforms 1.74 comfortably.
How frame choice changes the answer
Frame size and shape change how much index you actually need, and this is routinely overlooked.
A large frame means the lens extends further from its optical centre, and edge thickness grows with that distance. The same prescription in a small round frame and a large square frame produces visibly different edge thickness.
Choosing a smaller frame, or one with a rounder shape, can achieve much of what a higher index would, sometimes both together. If lens thickness is your main concern, frame selection is the first lever, not the last.
Rimless and half-rim frames are the exception in the other direction. They hold the lens by drilling or by a tension wire, which needs enough material to grip, so very high index is not always the right call there.
Optical frames in stock
Frame size affects lens thickness as much as index does. These sit in the moderate range.
Frequently asked questions
Key takeaways
