Straight wooden slats seen through a progressive lens: straight at the center, rippling at its lower sides.

Kinds of lens

Peripheral distortion

Optical glossaryKinds of lens

The soft, swimming zones beside a progressive's corridor — a geometric consequence of continuous power change, present in every progressive ever made. Better designs manage it; none eliminate it.

A progressive lens seen face-on: distance at the top, the corridor where the power changes, reading at the bottom — and softer areas either side that a good design manages but never removes.

Illustrative · the shape of the principle, not measured data

In more depth

Beside a progressive's corridor sit areas of unwanted astigmatism, felt as blur and a gentle swim when the head moves. They are not a defect of manufacture: a surface whose power changes continuously must produce them, and the faster the power changes along the corridor, the faster the blur grows beside it — a relationship known as Minkwitz's theorem.

Designers choose where that astigmatism goes. "Hard" designs confine it to small areas with steep edges; "soft" designs spread it thinly over more of the lens for a gentler feel. Calculating the surface for the individual prescription pushes it further out. The brain learns to ignore what remains, usually within a couple of weeks.

Related terms

CorridorThe channel of clear vision running down the center of a progressive, connecting the distance and reading zones.Progressive lensA lens whose power changes gradually from distance at the top to reading at the bottom, with no visible line.Non-adaptationThe inability to adjust to a progressive design. Rare, usually fixable with a different design — and covered under the CLIFF Promise for 30 days.

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