Bifocal
A lens with two fields of vision separated by a line. Generally the top is designated for distance-vision or computer-distance and the bottom for near-vision work such as reading.
In a Bifocal lens, the two fields of vision are specifically differentiated by a visible line. The bottom reading area is 28mm wide and is positioned just below the centerline of the lens. The physical position of the bi-focal area will be affected by the physical height of the lens chosen.
The total lens height for a Bifocal lens must be 30mm or greater. We recommend a taller lens for more comfortable wear, but 30 mm is the minimum height for a bifocal lens. If the frame chosen has a lens height smaller than 30mm, a different frame must be chosen for Bifocal lenses.
Progressive
This refers to the lens design which includes three fields of vision, without lines, and is sometimes referred to as a "no-line multi-focal".
In a progressive lens, the shape of the corrected part of the lens is approximately that of a funnel or a mushroom. The top portion is for distance-vision, narrowing to the lower middle for mid-range, finally to the bottom portion for the near-vision or reading. The reading area is expected to be smaller than the distance area. People sometimes prefer a single-vision lens for extended reading or up-close work.
The lens height for a progressive lens must be 30mm or greater. We recommend a taller lens for more comfortable wear, but the minimum lens height is 30 mm. If this frame has a lens height smaller than 30mm, a different frame must be chosen for progressive lenses.