subject: Bifocals- what are they and how do they work? [print this page] Bifocals are very similar to varifocal lenses in that they have different sections- one for improving near-sightedness, one for improving far-sightedness. However, bifocals do not have a corridor like varifocals, instead, they have two separate sections with no graduation or variation in between.
Benjamin Franklin, who, like so many others, was fed up of constantly switching between his reading glasses and his distance-viewing glasses, invented bifocals. His solution was to cut his glasses lenses in half and fix the distance-viewing portion of the lens to the top of his glasses frames, and the reading portion of the lens to the bottom of his frames.
The basic idea behind the bifocal hasn't changed since Franklin's day, but with improvements in technology and understanding of the eye and vision correction, different types of bifocals have been developed to suit individual needs. The four most common types are round segment, semi-circular, small area and half-cut vision correction lenses. This means that the total area of the lens used for near or distance viewing differs to suit individual needs and lifestyles e.g. someone who spends more time reading or using a computer will have a larger reading vision improving section to their lenses than someone who works outdoors.
Bifocals are a great solution to having to have two pairs of glasses with you at all times. They also eliminate the nuisance of having to constantly change between one pair of glasses and the other, just so that you can see well.
