TUESDAY, Feb. 17 (HealthDayNews) -- Everyone will suffer from cataracts if they live long enough, eye doctors say.

This clouding of the eye's lens is a natural byproduct of aging. By age 80, more than half of all Americans either have a cataract or have had cataract surgery, the National Institutes of Health estimates.

The good news is that cataract sufferers can enjoy better sight than ever before, thanks to new, so-called multifocal lens implants that come closer to mimicking the sight provided by the human eye.

"The natural lens you have in your eye is constantly changing its focus," says Dr. Janine Smith, deputy clinical director at the National Eye Institute. "If you look at something up close, your natural lens will change its shape to bring it into focus. At the same time, objects that are at a distance are no longer in focus. That's something that happens automatically."

Older lens implants provide clear vision for people only at one specific distance, and will not allow people to shift focus to things near to them or far away. "That's why many people still need to wear reading glasses after they have cataract surgery," Smith says.

If cataract patients wanted both short- and long-distance vision, they had to opt for "monovision" lenses. That's when a short-distance lens is placed in one eye and a long-distance lens in the other. "Some people can handle that and some can't," Smith says.

But the new multifocal lenses let people focus at different distances without the strain caused by monovision. They're still not as good as a human eye, but they allow people to read, thread a needle or perform other fine motor tasks without glasses, eye experts say.

The multifocal lenses are formed with concentric circles, like a practice target, and each circle represents a different focus distance, Smith says. People with the implants learn to adjust their vision automatically by shifting their concentration to the appropriate circle.

"It's not like bifocal lenses where you have to shift your eyes," she says. "It's all happening within the eye itself, so your eye isn't actually moving."

The surgery is the same as with single-focus lenses, although rehabilitation is different because patients must learn how to process the visual information provided by the multifocal lenses.

The lenses aren't for everyone, and do have some drawbacks, says Dr. Kerry Solomon, director of the Magill Laser Center at the Medical University of South Carolina's Storm Eye Institute.

Some people complain of poor nighttime vision caused by glare off of the lenses' concentric rings, Solomon says. Street lights can seem to have a ring around them, blurring vision.

"Someone who is a driver by trade or an airline pilot or who needs superb night vision is not a person I would recommend for these lenses," Solomon says.

The lens also must be perfectly centered by the surgeon, otherwise the patient might suffer from glare, halos or other optical phenomena, Smith says.

The size of the pupil also is important. At night, your pupil gets larger and can allow more concentric sight zones to be exposed. If the ophthalmologist has misjudged pupil size, vision could be affected.

Age-related cataracts develop in two ways. In most cases, protein contained in the lens begins to clump up, clouding the lens and reducing the light that reaches the retina. But for some people, the clear lens slowly acquires a brownish shade, tinting their vision and making them colorblind, according to the National Eye Institute.

While multifocal lenses have been available for several years, they're gaining in popularity, Solomon says.

"More and more physicians are becoming accustomed to them, and more and more patients are asking for them," he says.

And ophthalmologists expect the lenses to get better as demand increases. "We have a very demanding American public and a significantly growing elderly population," Smith says. "This significantly improves our ability to rehabilitate vision after cataract surgery."

Solomon points to one innovation he currently is studying -- a refined multifocal lens that provides the same benefits but with better reading and night vision and a shorter acclimation period for new users.

"I expect by 2005 patients will have the option to choose between the two," he says.

More information

To learn more about multifocal lenses, visit Emory University. For more information on cataracts, check with the National Eye Institute.