Kinds of lens
Corridor
The channel of clear vision running down the center of a progressive, connecting the distance and reading zones.
Illustrative · the shape of the principle, not measured data
In more depth
The corridor is the narrow channel down which a progressive's power climbs from distance to reading. Its length is the vertical distance over which the full addition is reached; its width is how far the eye can move sideways before the view softens.
A longer corridor changes power more gently and is usually wider, but needs a deeper frame to fit. A shorter one suits shallow frames and reaches reading sooner, at the cost of a narrower channel. A higher addition narrows any corridor, because the same length now has more power to climb.
Related terms
Progressive lensA lens whose power changes gradually from distance at the top to reading at the bottom, with no visible line.Fitting heightThe vertical distance from your pupil center to the bottom of the lens in your chosen frame. The most common cause of progressive remakes when wrong.Peripheral distortionThe 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.Addition (ADD)The extra reading power added to the lower part of a progressive or bifocal. Typically starts near +1.00 in the early forties and usually settles near +2.50 (up to +3.00 for close working distances).