Hydrophobic / easy-clean layer

Optical glossary/Coatings and filters

A slick top layer that makes water bead and smudges wipe away rather than smear.

In more depth

The outermost layer of a modern coated lens is extremely thin and very low in surface energy, so water and skin oils cannot spread across it. Water gathers into tight beads that roll off, and fingerprints sit on top of the surface rather than soaking into it.

The same slickness makes the lens easier to clean and slightly harder to hold, which is why labs use a temporary grip pad when shaping these lenses. The layer wears slowly with use; harsh solvents, hot water and household cleaners shorten its life.