How to understand it
Together with surface geometry, it influences how a lens provides power. It does not by itself determine weight or finished thickness.
An example
Both 1.60 and 1.67 materials can be manufactured for a −3.00 D prescription.
A common mix-up
Index is not prescription power, a visual-acuity score or a universal quality ranking.
See the idea in a full guide →
Sources & verification
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