WHAT ARE BIFOCALS AND HOW DO THEY WORK? : eBay Guides

Bifocals are glasses that have corrective lenses containing two different optical powers. They are most commonly prescribed for people with presbyopia, and who also require a distance correction for myopia, hyperopia, and/or astigmatism.

Taking advantage of that fact that when a person views something at close range, they usually look down and the reverse is true when they view at a distance, the early split 'Franklin' bifocals were designed with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Originally the lenses were simply cut in half and combined together in the rim of the frame. Later examples involved the cementing of a bifocal segment onto a larger lens, but now most bifocals are made by molding a small reading segment into the lens.

Bifocal lenses are available with the reading segments in a variety of shapes and widths. The most popular is the flat-top or D-shaped segment, 28 mm wide.