Bifocals

Bifocals are eyeglasses whose lenses provide two distinct optical powers, usually for distance and near vision. The wearer shifts gaze between lens regions rather than changing glasses.

Presbyopia: Age-related loss of the eye’s ability to focus on nearby objects. Bifocals commonly correct the near-focus difficulty caused by presbyopia.
Progressive lens: A multifocal lens with a gradual power change and no visible dividing line. It provides distance and near powers without the abrupt segment boundary of bifocals.
Benjamin Franklin: American printer, inventor, diplomat, and statesman who lived from 1706 to 1790. Franklin is widely credited with inventing bifocal spectacles in the eighteenth century.
Occupational bifocal: A bifocal designed for a particular working distance or task, such as close bench work. Its segment placement and power can be tailored to a wearer’s work.
Accommodation: The eye’s adjustment of lens shape to focus images at different distances. Presbyopia limits accommodation, creating the need for a separate near-vision power.
Trifocal lens: An eyeglass lens with separate distance, intermediate, and near-vision powers. Its third viewing zone addresses intermediate distances that standard bifocals omit.
Eyeglasses: Spectacles with lenses that correct refractive errors or protect the eyes. Bifocals are a specialized form of eyeglasses.
Prism correction: Optical correction that redirects light to address binocular alignment or related visual problems. Prescriptions may combine prism with bifocal powers when both corrections are needed.
Segmented lens: An ophthalmic lens divided into distinct regions with different optical powers. A visible segment supplies the additional power in traditional bifocals.
Reading glasses: Eyeglasses designed primarily to correct near vision. Unlike bifocals, they generally do not include a separate distance-vision region.

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