Bifocal sunglasses, with two different prescriptions in the same lenses, is to correct myopia, hyperopia and/or astigmatism while blocking sun’s harmful rays. Bifocal sunglasses are great for reading maps, menus, books and newspapers easily, tying on your lure while fishing, marking the scorecard during a round of golf, or anything where you want to see up close while still protecting your eyes from the sun. When you look up, the top half lets you automatically see in the distance without constantly changing back and forth from one pair to the next.
Today, there are many different types of bifocal sunglasses available. Equal division lenses, segmented bifocals and blended lenses also called seamless or invisible bifocals, are three main types of bifocals. Benjamin Franklin is generally credited with invention of bifocals. Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes. Bifocal lenses are available with the reading segments in a variety of shapes and widths including bifocal sunglasses options. The most popular reading bifocal is the flat-top known as straight-top or D segment, 28 millimeter wide.
Bifocal sunglasses can cause headaches and even dizziness in some users. Though most people who require bifocals have difficulty focusing on objects both up close and far away, they have always been designed with the corrective lens for close range focusing at the bottom and the corrective lens for distance vision at the top. While this design once made sense, because close range focusing was generally required for reading and writing, activities traditionally preformed while looking down, more and more people find it awkward to use such lenses with a computer. Since the computer monitor is directly in front of the user, the up close corrective lens seems out of place on some glasses.
