Eyesight is one of those human advantages, like brain capacity and opposable thumbs, that stand out in nature. Having evolved over 600 million years from some of the earliest light-sensitive proteins, our primate eyes are exquisite living cameras, capable of seeing not only in color but also in three dimensions. They have the acuity it takes to distinguish a venomous snake among dark leaves or to recognize the subtlest changes in someone else's expression.

How Eyes Age
Our vision has another virtue, less remarked on, and that is durability. Human eyes are sturdy little machines capable of working for a lifetime, with a bit of help from medical science. Eyes age, it's true, as surely as the skin around them and the hair above. And they do so gradually, beginning many years before we even notice. Consider the lens: In adolescence, it is supple enough to change shape and shift focus in a nanosecond -- from the printed page in our hands to the horizon, and back to the words again. In our youth, the lens is also transparent, "a completely clear window," says Sharon Solomon, an ophthalmologist at the Wilmer Eye Institute at Johns Hopkins University, in Baltimore.

At 40, 50, and 60
But by age 40 or so, the lens can no longer change shape as easily, so we start pushing the page away to see it better. By 50, some of the proteins that make up the lens start to form clumps, producing a whitish or yellowish translucence that doctors usually discern through their scopes and that may one day become a vision-blurring cataract. In our 50s, too, the eye's natural coating of tears dries a little. People who are susceptible to glaucoma may, by age 60, experience some extra pressure inside the eye, detectable by an exam -- even if they are not yet aware of any loss of peripheral vision that, if the disease is allowed to progress, will ensue. At this point in life, the earliest indications of deterioration of the retina that could lead to macular degeneration may also appear.

Keeping Watch
We can protect our eyes to some extent by wearing sunglasses to block ultraviolet rays (to reduce the risk of eyelid skin cancer and possibly cataracts and macular degeneration) and by eating a diet rich in antioxidants that can slow cell damage (see page 160). We can avoid smoking and get the frequent exercise it takes to keep blood vessels, including those in the eyes, in good working order. Still, none of these steps can work miracles. Our job is to pay attention to what's happening by having regular eye exams and by understanding how normal aging affects eyes.

Common Conditions
Presbyopia refers to the gradual inability to see things close-up, which many of us first experience in the dim light of a restaurant. It happens when the lens becomes less flexible and loses its ability to change shape and focus on things nearby. "It's not a disorder," Solomon says. "It's like getting gray hair.

The problem is easily corrected with glasses: bifocals or progressives, which provide more than one prescription in the same lens, or reading glasses -- prescription or the generic kind sold in drugstores. People with myopia (difficulty focusing on distant objects) can try monovision, the practice of correcting nearsightedness in one eye, either with a contact lens or surgery. The corrected eye is used for distance; the other eye, for seeing things up close. Monovision impairs depth perception, however, and some people can't tolerate it.

Cataracts are an inevitability of aging. Proteins that make up the lens break down and clump together, and as the damage builds, the lens grows cloudier. All people, by the time they reach their 60s or 70s, have some translucence in their lenses, although some aren't bothered by it right away -- or ever.

People who live near the equator or spend a lot of time outdoors, with more exposure to direct sunlight, are more susceptible to cataracts, as are smokers and people with diabetes. "A cataract is like a dirty window," says Ralph Levinson, an ophthalmologist at the Jules Stein Eye Institute at the University of California-Los Angeles. "At some point, you're not going to be able to see through it." The only long-term solution is surgery to replace the natural lens with an artificial one.

Glaucoma is a disease, rather than an inevitability of aging, that causes damage to the optic nerve and progressively impairs peripheral vision, sometimes narrowing the field of vision to the point of blindness. It tends to run in families and is especially prevalent among people of African descent, who are encouraged to be tested for the disease from age 40 on (compared with age 60 for people of other backgrounds). Glaucoma is initially asymptomatic.

The most common form of glaucoma is caused by improper drainage of the fluid in front of the lens, which leads to a buildup of pressure that harms the optic nerve. The nerve damage is irreversible, but doctors can prescribe medicine that reduces pressure inside the eye and in some cases stalls progression of the disease. Or they can perform surgery to improve fluid drainage.

Macular degeneration is the leading cause of blindness in the United States for people over the age of 65. Those of European descent, with light-green or blue eyes, are especially susceptible. Smoking and obesity are also risk factors. One of the most common signs of degeneration in the macula, in the center of the retina, are yellowish deposits beneath the surface called drusen. At the early stage of the disease, known as dry M.D., patients often still have good vision. Doctors prescribe high-dose supplements of vitamins C and E, beta-carotene, and zinc to slow the progression of the disease to wet M.D. That's when the drusen lead to bleeding in the retina, which can gradually destroy central vision. Since 2006, doctors have been able to treat wet M.D. with a drug called ranibizumab (Lucentis). In 95 percent of patients, the drug can stabilize vision, and in 30 percent to 40 percent, even improve it.

Feast for the Eyes
Diet: Healthful foods cant prevent aging of the eyes. But certain nutrients can, it seems, slow deterioration and the progression of certain diseases.

Studies have shown that eating foods rich in vitamins C and E, or taking supplements of those vitamins and beta-carotene -- all antioxidants -- may slow the development of cataracts. Those same nutrients, plus the minerals zinc and copper, impede the development of macular degeneration.

Scientists have found some evidence that supplements containing lutein and zeaxanthin -- both found in the lens of the eye -- may retard cataracts. And studies suggest that eating kale, mustard greens, collard greens, and spinach (all high in lutein and zeaxanthin) may reduce the risk of developing advanced macular degeneration. This research is still at an early stage.

People who eat fish, especially ones rich in omega-3 fatty acids, appear to have a lower risk of macular degeneration.