Bifokal

Bifokal, often translated as bifocal, is a term used in optics to describe a type of eyeglass lens that combines two distinct focal powers in a single lens. Typically, one portion corrects distance vision while a lower segment corrects near vision for reading. The term is used in several languages to refer to lenses designed to address presbyopia.

The concept dates to the 18th century, with Benjamin Franklin commonly associated with the modern bifocal design, which joined two differently powered lenses in one frame. Early bifocals were ground or cemented together and had a visible line separating the powers. Modern production can use fused or molded lenses, preserving the distinct near segment, or, in some cases, a seamless progressive design that provides multiple focal powers without a visible line (though those are usually described as progressive lenses rather than bifocals).

Design and variants include the traditional lined bifocal, featuring a horizontal segment that is clearly visible in the lower portion of the lens. Other styles include larger or curved segments sometimes referred to as executive or D-segment bifocals. Near addition strengths vary by prescription, typically ranging from about +1.50 to +3.50 diopters, depending on the user’s needs.

Use and considerations: Bifokals are commonly prescribed for presbyopia, the age-related decline in near focusing ability. Users may experience a perceptual jump when shifting gaze from distance to near, and some adaptation is required. Alternatives include trifocal and progressive lenses, contact lenses, or surgical options.