When ordering eyeglass lenses, numbers such as 1.50, 1.56, 1.60, 1.67 and 1.74 appear often.
They are often presented as a simple thinner-is-better ladder, but final lens thickness is not decided by this number alone.
These numbers primarily identify the refractive index of the lens material.
01 · REFRACTIVE INDEX
Refractive index relates to how much light bends in the material
Light changes direction when it enters or leaves a lens material.
A higher-index material can produce the same prescription with a smaller difference in surface curvature, which can allow a flatter and thinner form. This does not mean that a higher index is superior in every optical or mechanical property.
Refractive index is one optical property of the material and describes how light propagation changes at the air-lens boundary. A higher index can achieve a given power with less surface curvature, which can allow a flatter lens form. Final thickness, however, is still set by both surface designs, lens diameter and minimum-thickness requirements rather than index alone.
02 · 1.50 & 1.56
These are relatively lower-index options
Standard plastic around index 1.50 has a long history in ophthalmic lenses, and 1.56 materials are also used in many conventional products.
As prescription strength rises, thickness can increase quickly depending on frame size and lens shape.
1.50 and 1.56 can look like simple quality tiers, but they are better understood as index labels that may represent different material families and products. PPG CR-39® is a classic example around 1.50, while 1.56 products can use different organic materials depending on the manufacturer. The useful question is not whether 1.56 is automatically “better,” but whether it creates a meaningful thickness or performance difference for the prescription and frame.
03 · 1.60 & 1.67
These are commonly compared for moderate to higher prescriptions
Indices 1.60 and 1.67 are often considered when reducing thickness matters.
Abbe value, density and impact properties can also change with material, so index should not be chosen on thinness alone.
Indices such as 1.60 and 1.67 are commonly used to reduce lens form in moderate and stronger prescriptions. As index changes, Abbe value, density, impact behavior and processing characteristics can also change, and not all products move in the same direction. A material therefore needs to be evaluated as a package of optical and mechanical properties rather than as one step on an index ladder.
04 · 1.74
A very high index can be useful
Index 1.74 can be useful for reducing thickness at stronger prescriptions.
At lower powers or in small frames, moving from 1.67 to 1.74 may produce only a modest visible difference.
A very high index such as 1.74 can become more valuable as prescription and frame geometry create a real thickness problem. With a lower prescription or small frame, moving from one high-index level to another may produce only a modest visible difference. Lenses also require practical center or edge thickness for manufacturing and mounting, so thickness cannot be reduced without limit.
05 · WHOLE DESIGN
Index alone does not determine thickness
Prescription, frame size and shape, pupil position, required blank diameter, lens design, base curve and minimum center or edge thickness all contribute to the final result.
A realistic thickness estimate combines sphere and cylinder, frame size and shape, optical-center location, decentration, required lens diameter, base curve, design and the manufacturer’s minimum-thickness rules. In strong myopia, reducing the farthest distance from the optical center to the frame edge can sometimes matter more than moving up one index step. This is why prescription and frame geometry should be optimized before the material index is finalized.
SOLUTION GUIDE
Solution guide
Compare whether a high index is actually necessary.
Consider frame size together with index.
Consider frame size and eye position as well as index.
Compare Abbe value and lens design too.
GLANOS LENS CHECK
Do not select a lens by one number
A better sequence is prescription → frame → eye position → lens size → material index → lens design. In this lens series, index, material, design and coating are not treated as independent quality grades; each is evaluated in the context of prescription, frame geometry and actual wearing conditions.
CAUTION
A higher index number does not automatically mean a better lens
Choose a material appropriate to the prescription and frame conditions. Material chemistry, design rules, minimum thickness and coating architecture vary by manufacturer and product, so a value or feature from one lens should not be generalized to all spectacle lenses.
GLANOS EYEWEAR
There is a reason glasses feel uncomfortable.
refractive index · 1.50 lens · 1.60 lens · 1.67 lens · 1.74 lens · eyeglass lenses