re you ready to try bifocal contact lenses, but don't know what your options are? Here are some common lens designs and fitting strategies, with information on who is a good candidate for each one.

In the aspheric multifocal contact lens shown here, both distance and near prescriptions are placed in the central visual area, on or close to the pupil. When gazing at something far away, your eyes learn to ignore near objects; when focusing near, your eyes learn to ignore far-off objects.

Another example is the concentric lens (also called annular lens), in which the near and far prescriptions are arrayed in a bulls-eye pattern. The near prescription may be in the middle, with distance correction in the outer ring, or vice-versa. The placement of the prescriptions depends on your particular lifestyle and activities. It also depends on your pupil size -- large pupils work better with near vision in the center, while small pupils benefit from having far vision in the center.

If you need an intermediate prescription for viewing 18 to 24 inches away (such as when you work at a computer), an extra ring for this may be placed between the other two, creating a multifocal contact lens. The widths of the rings may vary, to emphasize a particular vision need. Often the intermediate and far prescriptions are blended together at their edges, for a smooth transition from one to the next that is similar to wearing progressive eyeglass lenses.

These designs work well for people who are just beginning to experience presbyopia, since their near prescriptions are usually not very strong (there are exceptions, however). They are also better if you have small to medium diameter pupils, if your lower lids are positioned low, or if you have drooping eyelids (ptosis), though the latter may be corrected with surgery. These lenses can be made very thin, for excellent comfort and breathability.

In translating lenses, the near, far, and sometimes intermediate prescription zones are distinct, and your pupil moves from one to the other, depending on the visual need of the moment. The near prescription is usually placed at the bottom of the lens, with the distance prescription at the top -- similar to bifocal eyeglasses. But this can be reversed, if your work or hobby requires a near prescription at the top instead of the bottom.

Contact lenses sometimes rotate a little when you blink, which may shift the near vision area to the wrong place. To solve this problem, translating bifocal contact lens designs may be weighted (or "ballasted") at the bottom, to keep them right-side-up. The lenses may also be flattened (or "truncated") at the bottom, so the lower lid will support and shift the lens upward when the pupil needs to access the near vision zone.

Translating designs provide good vision and work well whether you need a weak or a strong near prescription. If your upper eyelids have begun to droop (this is caused by a weakening of the muscle and occurs as we age), they may interact with translating lenses too much, and a different design may work better. Translating lenses must be able to move freely on the eye in order to re-center after a blink; if your eyes are habitually dry, this could interfere with movement and create blurriness. Translating lenses can accommodate large pupils, though not as easily as smaller pupils.

Monovision: An Alternative to Bifocals
In a monovision fit, usually one contact lens has a distance correction and the other has a near correction. Your eyes learn to choose the particular lens needed for a given situation. Modified monovision may include a single vision lens in one eye and a bifocal lens in the other eye. Or there can be two bifocal lenses, but with different near vision powers.

All of this may sound complicated, but a good eyecare practitioner who is experienced at fitting contact lenses will help you choose the right design and adapt to it with ease.

Read more:

• What a bifocal contact wearer says about her lenses
   
• Order a free brochure about contact lenses for people over 40