Two thin lenses backlit on their edges: one almost flat, one deeply curved.

Measuring and fitting

Base curve

Optical glossaryMeasuring and fitting

The curvature of a lens's front surface. Must suit both the prescription and the frame's wrap.

The same prescription on a flatter and a steeper front curve. The back surface changes to keep the power; the front curve decides how the lens sits in the frame.

Illustrative · the shape of the principle, not measured data

In more depth

The base curve is the curvature of a lens's front surface, usually expressed as a number: the higher it is, the steeper the curve. The prescription then decides the back surface, so the same power can be made on a flatter or steeper front.

Flatter fronts look slimmer and sit well in flat frames; steeper fronts follow a wrapped frame and the shape of the face. Choosing the wrong one for a prescription or frame can bulge the lens out of the rim or distort the view toward the edge, which is why the base curve is confirmed for each order rather than fixed.

Related terms

Wrap angleHow much a frame curves around the face. High-wrap sports frames need designs compensated for it.Semi-finished blankA lens with a factory-molded front surface, finished by surfacing the back to the prescription. Every CLIFF Rx lens starts as one: freeform surfacing is how its back is finished.Refractive indexHow strongly a material bends light. Higher index means less material for the same power — a thinner lens.

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