Bifocal and Multifocal Contact Lenses

Bifocal contact lenses are designed to give good vision to people who have a condition called presbyopia. The key sign that you're developing presbyopia is that you need to hold reading material, like a menu or newspaper, farther from your eyes to see it clearly.

Bifocal contact lenses are available in both soft and rigid gas permeable (GP) materials, and some can be worn on a disposable basis. That means you can have the convenience of throwing the lenses out at specified intervals (even daily, in some cases) and replacing them with fresh, new lenses. Also, one bifocal brand is available in silicone hydrogel material.

Bifocal contacts lenses have two prescriptions in the same lens. Multifocal contact lenses have a range of powers (similar to progressive spectacle lenses) in each lens. "Multifocal" is also used as a catch-all term for all lenses with more than one power, including bifocals.

Bifocal and multifocal contact lenses work in several different ways, depending on the design of the lens. The designs fall into two basic groups:

1. "Alternating vision" (translating) lenses are so named because your pupil alternates between the two powers, as your gaze shifts upward or downward.
    
2. Simultaneous vision lenses require your eye to be looking through both distance and near powers at the same time. Although this might sound unworkable, your visual system learns to select the correct power choice depending on how close or far you're trying to see. Simultaneous vision lenses come in two types:

• Concentric ring designs
• Aspheric designs

Alternating or translating bifocals work much like bifocal eyeglasses. They have two power segments, with an obvious line of separation between the distance correction on top and the near correction below. Your pupil looks through either one or the other, depending on whether you're looking far or near.

With bifocal eyeglasses, this mechanism works because the lenses stay in place even as your eye moves. That can happen with contact lenses, too. Since most alternating bifocals are GP lenses, they are smaller in diameter than soft lenses, and they ride on your eye above your lower eyelid. Therefore, when your gaze shifts downward, the lens stays in place, allowing you to see through the lower, near-correction part of the lens.

This type of bifocal contact lens features a prescription in the center and one or more rings of power surrounding it. If there are multiple rings, they alternate between the near and distance prescription. Typically at least two rings are within your pupil area, but this varies as your pupil expands and contracts due to varying light.

Concentric ring bifocal contact lenses can be made of either soft or rigid (GP) material. The locations of the powers will vary:

• GP bifocals usually have the distance power in the center (called center-distance).
• Soft bifocal contact lenses usually have the near power in the center (center-near).
• Some soft multifocal designs are center-near on your dominant eye but center-distance on your non-dominant eye.
   

These multifocal contact lens designs work more like progressive eyeglass lenses, where the different prescriptive powers are blended across the lens. Unlike eyeglasses, however, aspheric contact lenses are simultaneous vision lenses, so your visual system must learn to select the proper prescription for the moment.

This is the only type of multifocal contact lens that can be described as "progressive." It's also concentric, like the concentric ring designs, and it has become the most popular type of multifocal contact lens.

Bifocal contact lenses have been around for many years, but until recently they weren't very popular. Older bifocal designs didn't satisfy many people, leading to frustration among wearers and prescribers alike.

Today, new technology has produced more successful designs, as well as a greater variety of designs. So if one design doesn't work for you, another might. Your doctor may also try these related techniques:

• Monovision involves using single-vision lenses to put your near prescription on one eye and your distance prescription on the other [read more about monovision].
• Modified monovision uses a single-vision lens on one eye and a multifocal lens on the other.

Two factors that your eye care practitioner will consider in choosing a bifocal contact lens are pupil size and your "add," or near prescription.

There are no hard and fast rules. But in general, low adds are better suited to an aspheric multifocal. Alternating vision, accomplished with a translating bifocal, is a better choice for high adds. Too large a pupil can be problematic for an aspheric multifocal lens.

You may need to try different bifocal contact lens designs before finding the one that's right for you. Most practitioners have free trial lenses to help you through the process. Keep in mind that although trial lenses are free, your doctor will still charge you for the fitting process, which can be more time-consuming than a regular lens fitting. See your eye doctor to determine if bifocal contact lenses make sense for your needs and to identify the best lens for you.