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Bifocals contain two lens powers (Progressive multifocal lenses gradually change in power from the top half of the lens to the bottom, and thus contain many lens powers.)
Regardless of the reason you need a prescription for near-vision correction, bifocals all work in the same way. A small portion in the lower part of the lens contains the power required to correct your near vision. The rest of the lens usually is for your distance vision.
The lens segment (or "seg") devoted to near-vision correction can be one of several shapes:
Generally, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or objects within 18 inches of your eyes.
Bifocals typically are placed so the line rests at the same height as the wearer's lower eyelid.
As a bifocal wearer drops his eyes downward to read, the eyes naturally seek out the near-vision portion of the lens.
Multifocals are fitted a bit higher, with the top line of the intermediate seg placed even with the lower margin of the pupil.
A multifocal wearer looks through the intermediate zone when viewing something between 18 and 24 inches away. The eyes gravitate straight ahead, or up and over the multifocal segments, when gazing at something in the distance.
Bifocals have a visible line. If you want to wear a multifocal lens without visible lines, progressive lenses usually are your best choice.