Bifocals

Bifoklabrille fcm

Jan-Öjvind Swahn vid doktorspromovering 2006-06-02

Bifocals are a type of eyeglasses that contain two different optical powers. They are most commonly prescribed to people with presbyopia who also require a correction for another refractive error, such as myopia, hyperopia, or astigmatism. Bifocals allow the wearer to see clearly at two different distances without the need to switch between two pairs of glasses. The concept of bifocals is credited to Benjamin Franklin, who is believed to have invented them in the 18th century to avoid having to change his glasses while performing tasks at different distances.

Design and Function[edit | edit source]

The design of bifocals traditionally involves incorporating a small segment of lens with the required power for close work into the lower part of the main lens, which is prescribed for distance vision. The segment can be shaped in various ways, such as a half-moon, a round segment, or a narrow rectangular area, known as the "Franklin" or "Executive" style. Modern advancements have led to the development of progressive lenses, which offer a gradual change in power from the top to the bottom of the lens, eliminating the visible line found in traditional bifocals.

Types of Bifocals[edit | edit source]

Flat-top Bifocal: Also known as the D-segment bifocal, this design has a D-shaped segment for near vision on the lower half of the lens.
Round Segment Bifocal: This older style features a circular segment for near vision.
Executive Bifocal: Also known as the Franklin bifocal, this type has a horizontal line across the entire width of the lens, dividing it into distinct areas for distance and near vision.
Progressive Lenses: Although not technically bifocals, progressive lenses serve a similar purpose by providing a smooth transition between different focal lengths without a visible line.

Advantages and Disadvantages[edit | edit source]

Advantages:

Bifocals allow for clear vision at both near and far distances without the need for multiple pairs of glasses.
They can be more convenient and cost-effective than having separate eyeglasses for reading and distance vision.

Disadvantages:

The visible line on traditional bifocals can be cosmetically unappealing to some wearers.
Some users experience difficulty adjusting to bifocals, particularly in navigating stairs or looking at objects at an intermediate distance.
Progressive lenses, while eliminating the line, can cause peripheral distortion and require time to adapt to the different viewing areas.

History[edit | edit source]

The invention of bifocals is often attributed to Benjamin Franklin, who sought a solution to his own vision problems. However, there is some debate among historians about whether Franklin was the true inventor or simply an early adopter of the concept. Regardless, bifocals have undergone significant evolution since their inception, with improvements in materials, design, and manufacturing techniques.