Bifocals are a type of glasses that have two distinct optical powers. Bifocals are most commonly prescribed to people who require a pair of glasses where they can read and see distance clearly and are not disturbed by the lines in the lenses where the two optical powers meet.
History of bifocals
Probably invented by that profuse inventor Benjamin Franklin, many ideas have been made into practical inventions by more than one person at the same time, and eyeglasses with bifocal (twin) lenses may have been such a case.
Evidence exists, however, that, Benjamin Franklin certainly was amongst the first to wear bifocal spectacles, and letters he wrote indicate that he invented them independently, even though he may not have been the first.
Original bifocals eye wear was designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses in the upper section of the spectacle lens.
Up until the beginning of the 20th century two different lenses, one for distance and one for reading, were cut in half and combined together in the rim of the glasses frame. This type of construction led to a number of early complications and these types of glasses were quite fragile.
After a while a method for fusing the two sections of the lenses together was developed and today most bifocals are created by moulding a reading segment into a main distance lens and are available with the reading segments in a variety of shapes and sizes.
The most popular of these, and the one we offer, is the D-segment, 28 mm wide. While the D-segment bifocal offers superior optics, an increasing number of people opt for progressive lenses – see our section on varifocals.
It is interesting to note that nature also uses the 'bifocal lens'. Uniquely in Thermonectus Marmoratus (The Diving Beetle) it was discovered that during its stage as a larva, it was found to have two retinas with two distinct focal planes that are substantially separated, in the manner of bifocals to switch their vision from up close to distance. This would make it easy to capture prey, mostly mosquito larvae.
So far this is the first recorded use of bifocal technology in the natural world. (watch this space).
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