Bifocal glasses, since first being created by Benjamin Franklin in 1784, have developed a lot until today. They were originally invented to help those who had difficulties seeing both far away and up close. Franklin worked out a way to have both lenses fit into one frame rather than switch between two pairs of glasses. Today, we have many different kinds of bifocal glasses available in the market. Equal division lenses, segmented bifocals and blended lenses (also called seamless or invisible bifocals) are three main types of bifocal glasses. Here we will give you an idea of each of them.
There are two distinct parts in equal division bifocal eyeglasses, with the magnification part on the lower and the long-distance lens on the upper. The original version of the equal division lenses is called the executive cut, which had a visible horizontal line across the lens. The executive cut is the lens that is most similar to Franklin bifocal glasses.
In segmented bifocal glasses, there is only one segment of the lens devoted to magnification. The segments can be of many shapes — a rectangle, a circle or a D-shape (which looks like a capital letter D on its side, with the curve of the letter along the bottom of the lens). The D type is the most popular type because it is easier for the eye to adapt to.
Blended lenses, also called no-line bifocals or progressive lenses, hide the line which shows the division between two lenses. The whole lens is an embodiment of continuously increasing lens strength rather than have two separate lens powers. The gradient allows for a more fluent visual experience, without having image jumps caused by the clear border in other kinds of bifocal glasses. However, it can also result in a distorted perspective that is proved to be unacceptable to some people.
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