Judith Lee and Gretchyn Bailey
A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. The lens works much like a camera lens, focusing light onto the retina at the back of the eye. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away.  The lens is mostly made of water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it.
But as we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract, and over time, it may grow larger and cloud more of the lens, making it harder to see.
Researchers are gaining additional insights about what causes these specific types of proteins (crystallins) to cluster in abnormal ways to cause lens cloudiness and cataracts. One recent finding suggests that fragmented versions of these proteins bind with normal proteins, disrupting normal function.
Cataracts are classified as one of three types:
A subcapsular cataract begins at the back of the lens. People with diabetes, highfarsightedness or retinitis pigmentosa, or those taking high doses of steroids, may develop a subcapsular cataract.
A nuclear cataract is most commonly seen as it forms. This cataract forms in the nucleus, the center of the lens, and is due to natural aging changes.
A cortical cataract, which forms in the lens cortex, gradually extends its spokes from the outside of the lens to the center. Many diabetics develop cortical cataracts.
Symptoms and signs
A cataract starts out small and at first has little effect on your vision. You may notice that your vision is blurred a little, like looking through a cloudy piece of glass or viewing an impressionist painting.
A cataract may make light from the sun or a lamp seem too bright or glaring. Or you may notice when you drive at night that the oncoming headlights cause more glare than before. Colors may not appear as bright as they once did.
The type of cataract you have will affect exactly which symptoms you experience and how soon they will occur. When a nuclear cataract first develops, it can bring about a temporary improvement in your near vision, called "second sight." Unfortunately, the improved vision is short-lived and will disappear as the cataract worsens. On the other hand, a subcapsular cataract may not produce any symptoms until it's well-developed.
If you think you have a cataract, see an eye doctor for an exam to find out for sure.
Causes
No one knows for sure why the eye's lens changes as we age, forming cataracts. Researchers are gradually identifying factors that may cause cataracts - and information that may help to prevent them.
Many studies suggest that exposure to ultraviolet light is associated with cataract development, so eye care practitioners recommend wearing sunglasses and a wide-brimmed hat to reduce your exposure.
Other types of radiation may also be causes. For example, a 2005 study conducted in Iceland suggests that airline pilots have a higher risk of developing nuclear cataract than non-pilots and that the cause may be
exposure to cosmic radiation. A similar theory suggests that astronauts, too, are at risk from cosmic radiation.
Other studies suggest people with diabetes are at risk for developing a cataract.
 The same goes for users of steroids, diuretics and major tranquilizers, but more studies are needed to distinguish the effect of the disease from the consequences of the drugs themselves.
Some eye care practitioners believe that a diet high inantioxidants, such as beta-carotene (vitamin A), selenium and vitamins C and E, may forestall cataract development. Meanwhile, eating a lot of salt may increase your risk.
Other risk factors include cigarette smoke, air pollution and heavy alcohol consumption.
A small study published in 2002 found lead exposure to be a risk factor; another study in December 2004, of 795 men age 60 and older, came to a similar conclusion.
But larger studies are needed to confirm whether lead can definitely put you at risk and, if so, whether the risk is from a one-time dose at a particular time in life or from chronic exposure over years.
Treatment
When symptoms begin to appear, you may be able to improve your vision for a while using new glasses, strong bifocals, magnification, appropriate lighting or other visual aids.
An intraocular lens (IOL) is implanted in the eye in place of the clouded natural lens. Shown is Alcon's AcrySof Natural IOL; it filters out blue light, which may be harmful to eyes.
Think about surgery when your cataracts have progressed enough to seriously impair your vision and affect your daily life. Many people consider poor vision an inevitable fact of aging, but cataract surgery is a simple, relatively painless procedure to regain vision.
Cataract surgery is very successful in restoring vision. In fact, it is the most frequently performed surgery in the United States, with more than 3 million Americans undergoing cataract surgery each year. Nine out of 10 people who have cataract surgery regain very good vision, somewhere between 20/20 and 20/40.
During surgery, the surgeon will remove your clouded lens and in most cases replace it with a clear, plastic intraocular lens (IOL).
New IOLs are being developed all the time to make the surgery less complicated for surgeons and the lenses more helpful to patients. Presbyopia-correcting IOLs potentially help you see at all distances, not just one. Another new type of IOL blocks both ultraviolet and blue light rays, which research indicates may damage the retina (see illustration).
Read more on this website about what to expect if you have cataract surgery and how to deal with rare cataract surgery complications. Also, men should be aware that certain prostate drugs can cause intraoperative floppy iris syndrome (IFIS) during a cataract procedure.