Kinds of lens
Bifocal
A lens with two distinct zones separated by a visible line, and no intermediate correction between them.
Illustrative · the shape of the principle, not measured data
In more depth
A bifocal carries two powers: distance in the main lens and reading in a segment set into the lower part, divided by a visible edge. Moving the eyes across that edge makes the image jump, because the power changes all at once.
There is no power between the two zones, so the middle distance — a screen, a dashboard, a face across a table — falls into a gap. That gap, and the line that announces the lens, are why most people starting on multifocals today choose a progressive.
Related terms
Progressive lensA lens whose power changes gradually from distance at the top to reading at the bottom, with no visible line.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).Single vision lensOne power across the whole lens, for one viewing distance.