Bifocal lenses provide presbyopes with clear vision for two different ranges, usually for distance and near vision.
The power of the reading portion bifocals is obtained by moulding a segment with a smaller radius onto the bottom area of the front surface. With a segment in the lower area of the lens, a bifocal displays two different dioptic powers. Bifocals are therefore characterised by an edge at the top of the segment displaying a line across the lens.
The invention of bifocal lenses is attributable to the North American statesman Benjamin Franklin (Franklin lens). In 1785 he presented a pair of bifocal spectacles in which two half lenses were mounted on each side of the frame. The top lens was used for distance vision, and the bottom one for reading. |  Return to previous page |