Bifocal glasses have come a long way since first being developed by Benjamin Franklin in 1784. The glasses originally were created to assist people who had trouble seeing both far away and up close. Rather than switch between two pairs of glasses, Franklin devised a way to have both lenses fit into one frame. Today, there are many different types of bifocal glasses available. Equal division lenses, segmented bifocals and blended lenses — also called seamless or invisible bifocals — are three main types of bifocals.
Equal division bifocal glasses feature lenses cut in two distinct halves, with the magnification part on the lower half and the long-distance lens on the upper half. The initial version of the equal division lenses, called the executive cut, had a visible horizontal line directly across the lens. The executive cut is the lens that most closely resembles the original Franklin bifocal glasses.
The ultex is another version of equal division bifocals. The separation in the ultex lens is marked by a line in the shape of an upside-down U, which creates a wide, round segment on the bottom half of the lens. Between the two types of equal division lenses, the ultex offers more peripheral vision than the executive cut.
Segmented bifocals have only one segment of the lens devoted to magnification, which is fused into the principal lens. The segments can come in many shapes, such as 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-segment is the most popular of the segmented bifocal glasses because it is the easiest for the eye to adapt to.
Blended lenses are also called no-line bifocals or progressive lenses. The lens hides the line showing the division between the two lenses. Rather than have two distinct lens powers, the whole lens is a gradient of increasing lens strength. The full-power strength, or reading strength, is at the bottom of the lens. The gradient allows for a more fluid viewing experience, without image jumps caused by the noticeable lines in the other types of bifocals, but the gradient in power can result in a distorted perspective that can prove to be unacceptable to some.