How do Multifocal and Bifocal Contact Lenses Work?
Bifocal glasses have been around for many years and are easily recognizable. They are very useful for correcting two vision problems at once. Myopia, or nearsightedness, requires a majority of the lens to bend light in a way that vision is clear at medium to long range distances. Presbyopia, on the other hand, affects near vision, and is common in people over the age of 40. A second prescription is built into the lower part of the lens, and used to correct vision for close tasks, like reading and writing. Together they allow clear and focused vision at all distances.
This concept had been applied to contact lenses as well. In fact, the capabilities of multifocal contact lenses far exceed those of traditional eyeglasses. They come in a wide variety of styles and designs, with comfortable options for just about everyone. You can choose from soft lenses, rigid gas permeable lenses, or even hybrids that are rigid in the center for sharp vision, and soft on the edges for a comfortable fit. There are lenses that can be worn for weeks at a time or disposed of and replaced daily.
What's the difference?#
Bifocal lenses, by definition, provide two different focal powers in one lens. Multifocal lenses can have two or more focal powers at the same time. Sometimes the two terms are used to differentiate how many prescriptions are used in a single lens, while other times the term multifocal is used as a catch-all for all lenses with more than one power.
The larger differences lie in how the lenses are designed and function. There are two main groups of multifocal lenses. Alternating vision lenses, and simultaneous vision lenses each correct multiple vision problems at the same time, but through different methods.
Alternating Vision Lenses
These lenses work very similarly to bifocal eyeglasses, where the top portion of the lens is used for distance viewing, and the lower third for up close tasks. Most alternating vision lenses are rigid gas permeable (RGP), and are slightly smaller than common soft contact lenses. They float on the surface of your eye, and rest just above your lower eyelid. When focusing on medium to long distances, your gaze remains level, and the middle to upper part of the lens is used. As your gaze drops to read something in your hands, the contact remains stationary, and your pupil views through the lower portion of the lens. The lens can be made in a way that offers a clear, distinct line between the different powers, or there can be a gentle gradient between them that slowly transitions from one power to the other as your gaze drops.
Simultaneous Vision Lenses
Unlike alternating vision lenses, where your pupil only looks through one focal power at a time, simultaneous vision lenses allow you to see through multiple focal powers at once. This is done by arranging different focal strengths, starting from the center of the lens and moving outward towards the edges. There are two ways this is accomplished.
- Concentric Circles – Rings around the center of the lens, like a bulls eye on a target. From the center out, each ring with have a different focal strength, alternating between near and far sight prescriptions. The number of rings that align with your pupil can change, and will depend on ambient light and pupil dilation.
- Aspheric Lenses - Like progressive eyeglasses, there's no hard line between powers. Instead there is a gentle gradient as the powers blend/change between near and distant.
Simultaneous vision lenses are available in both soft and RGP varieties, but each will be made differently from one another. RGP lenses are center-distance, meaning that the very center of the lens is designed to focus clearly on distant objects, while soft lenses are center-near, which is just the opposite. In some cases, a third option is available. A center near lens is used on your dominant eye, while a center-distance lens is used on your non dominant eye. In any of these scenarios, there will be a period of adjustment and your brain and eyes learn to use the new lenses effectively, and become accustomed to focusing through multifocal contact lenses.
Multifocal lenses have been around for years, but older versions were frustrating and unpopular. New technology has developed better, more satisfying lenses, with greater variety to choose from. With all of the available options on the market today, finding the right lenses for you should be easy. Discuss with your eye care practitioner which lenses he recommends for you, and together you can weigh your options.