The simplest eyeglass lens, a single vision lens distributes the focus evenly over its surface. It implements one prescription and corrects only that vision problem. This can work well, but unfortunately is insufficient to help someone with multiple sight disorders. For instance, a person with myopia can develop presbyopia as they age, having trouble focusing on both long- and short-range objects.
Bifocals were created to rectify the multiple vision problem, affixing a smaller lens of a different prescription to the bottom part of a larger lens in a pair of glasses. They have also been in existence for a long time, and come in several shapes: half cut (the oldest), round, rectangular, and half-moon or 'D-shaped' (the latter being the most popular). While these types of lenses allow the wearer to use the upper, less convex part of the lens for long-distance viewing and the lower, more convex part for close-up tasks such as reading, they involve a problematic 'jump' from one focus to another.
Trifocal lenses operate on the same principle as bifocals, but add a third segment for accommodating an intermediate level of magnification. This additional section is located just above the short-term 'reading' prescription, usually as a thin strip forming the top part of the D-shaped or rectangular lens attached to the main lens. The intermediate level of vision helps to create more of a transition and eases the 'jump' effect found in bifocals, and is perfect for those who need to focus on mid-range objects such as a computer screen. There is still the problem, though, of the visible outlines of all these sub-lenses obscuring vision and creating disorientation on the part of the wearer.
The newest type of corrective lens, progressive lenses, aim to solve many of the issues associated with more traditional bifocals and trifocals. Instead of involving multiple lenses fused into one, a progressive lens smoothly transitions from one prescription to another within the shape of the lens itself. This allows the eye to transition more naturally between focuses on a gradient, instead of having to constantly readjust. In addition, many wearers perceive progressive lenses to be more youthful looking, as bifocals and the presbyopia they often indicate are generally associated with aging. However, progressive lenses also have disadvantages. They can cause initial discomfort as the wearer adjusts to the unfamiliar changes in perspective and peripheral vision, and tend to have a higher cost and more onerous fitting process than more conventional types of lenses. Still, for many people the benefits far outweigh the drawbacks.Want to learn more about bifocal glasses and single vision lenses? Sara Roberts writes for Just Eyewear, an online optical store, where you can find more helpful information on every eyeglasses-related topic.