Bifocals

Bifokalbrille (fcm)

Bifokalbrille (fcm)

Jan-Öjvind Swahn vid doktorspromovering 2006-06-02

Bifocals are eyeglasses with lenses that have two distinct optical powers. They are typically prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism. The term "bifocals" was coined by Benjamin Franklin, who is often credited with inventing them.

History

The invention of bifocals is attributed to Benjamin Franklin in the late 18th century. Franklin, who suffered from both myopia and presbyopia, grew tired of switching between two pairs of glasses and devised a way to combine the lenses. He had his optician cut the lenses of his distance glasses and his reading glasses in half and then mount them together in the same frame.

Design

Bifocals are designed with two distinct sections: the upper part of the lens is for distance vision, while the lower part is for near vision. The line separating the two sections is called the "seg line." There are several types of bifocal lenses, including:

• Flat-top (D-segment) bifocals: These have a distinct, flat-top segment for near vision.
• Round-segment bifocals: These have a round segment for near vision.
• Executive bifocals: These have a full-width near vision segment that extends across the entire lens.

Usage

Bifocals are commonly used by individuals over the age of 40 who have developed presbyopia, a condition where the eye's lens loses its flexibility, making it difficult to focus on close objects. Bifocals allow these individuals to see clearly at both distance and near without needing to switch between different pairs of glasses.

Advantages and Disadvantages

Advantages

• Convenience of having two prescriptions in one lens.
• Cost-effective compared to having multiple pairs of glasses.
• Immediate transition between distance and near vision.

Disadvantages

• Visible line can be cosmetically unappealing.
• Can cause image jump, where the image shifts abruptly when moving from one segment to another.
• May require an adaptation period for new users.

Alternatives

Alternatives to bifocals include progressive lenses, which provide a gradual transition between multiple focal points, and reading glasses, which are used solely for near vision tasks.

Related Pages

• Eyeglass prescription
• Progressive lenses
• Reading glasses
• Benjamin Franklin
• Presbyopia
• Myopia
• Hyperopia
• Astigmatism

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