If you are over 40 and have difficulty seeing close up, you probably have a common age-related condition called presbyopia which is when the eye’s natural lens loses the ability to focus on close objects. Presbyopia is a natural process as the eye ages and affects the majority of people from age 40 and upward. Individuals with presbyopia may need to hold reading material away from their eyes to see clearly. Luckily, reading glasses, bifocal or multi-focal (progressive) lenses can help.
Fortunately for those who don’t like the look, feel or inconvenience of reading glasses, there is another option. Bifocal and multi-focal lenses are also available in contact lenses in both soft and Rigid Gas Permeable (RGP) varieties.
The Difference Between Bifocal and Multi-focal Lenses
Just as the name indicates, bifocal lenses are divided into two distinct segments for different vision powers, one for distance vision and the second for near vision. This enables you to clearly switch your focus from near to far as needed, but your vision will not necessarily be clear in between. The term multi-focal lenses can refer to any lenses with multiple powers including bifocals, trifocals or progressive lenses. Progressive addition (PAL) multi-focal lenses have a range of powers that enable you to constantly adjust your focus to see clearly from up close to far and in-between.
Multi-focal contact lenses are generally designed in one of two ways- simultaneous vision lenses or alternating vision lenses.
Simultaneous vision lenses
The most popular version of multi-focal contact lenses, simultaneous vision lenses present the distance and near vision zones of the lens at the same time. Typically after a short adjustment period your eyes learn to utilize the segment of the lens that they need to focus on the desired object and essentially ignore the other.
They come in two designs:
- Concentric ring design: In the most basic form these are bifocal lenses that are comprised of a central circular area of one power with a ring around of the alternate power, similar to a bulls-eye. In this design the power of the rings (either near or distance vision) is interchangeable. For intermediate viewing (18-24 inches away) extra rings can be added to create a trifocal or multi-focal lens. The width of each ring is variable depending on the power that is needed most and the edges of the rings can be blended for a smooth transition of focus, similar to progressive eyeglass lenses.
- Aspheric design: These multi-focal lenses attempt to provide a natural vision experience by blending many lens powers across the surface and center of the lens. In this design both distance and near vision power are located in the central visual area and your eyes will adapt to focus on the area needed to view what you are looking at.
Translating or Alternating Vision lenses
Similar to bifocal eyeglass lenses, these contacts are divided into distinct areas or zones and your pupil will move to the desired zone depending on your vision needs. Typically the top of the lens, which is what you look through when looking straight ahead, is for distance vision, and the bottom area (what you look through when you look down) is for near vision. However, this can be reversed according to unique vision needs.
Since contact lenses move on your eye, translating lenses are held in place by a ballast which is an area that is thicker than the rest of the lens or by truncating or flattening the bottom to stay in line by the lower lid. These lenses are only available in rigid gas permeable lens material.
An Alternative Option to Multi-focal Contact Lenses: Monovision
Monovision is another contact lens alternative for presbyopia particularly if you are having difficulty adapting to multi-focal lenses. Monovision involves having (usually) the dominant eye for distance vision and your non-dominant eye for near vision. This can be done with a single vision lens in each eye, a multi-focal lens in each eye or one of each.