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, high farsightedness 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.

Cataract 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.

Glaucoma refers to a category of eye disorders often associated with a dangerous buildup of internal eye pressure (intraocular pressure or IOP), which can damage the eye's optic nerve that transmits visual information to the brain.

With untreated or uncontrolled glaucoma, you might eventually notice decreased ability to see at the edges of your vision (peripheral vision). Progressive eye damage could then lead to blindness.

In fact, glaucoma creates at least some vision loss in more than half of the approximately 2.5 million Americans estimated to have the eye disease and is the second leading cause of blindness.

A build-up of fluid within the eye can increase pressure, which in turn damages the optic nerve. The loss of visual field often occurs gradually and slowly, and may be recognised by the sufferer only when at an advanced stage. This loss of vision can never be recovered, so prompt diagnosis is essential. Treatment is via simple eye drops.

Glaucoma Symptoms
Glaucoma often is called the "silent thief of sight," because most types typically cause no pain and produce no symptoms until noticeable vision loss occurs.

For this reason, glaucoma often progresses undetected until the optic nerve already has been irreversibly damaged, with varying degrees of permanent vision loss.

But with acute angle-closure glaucoma, symptoms that occur suddenly can include blurry vision, halos around lights, intense eye pain, nausea and vomiting. If you have these symptoms, make sure you see an eye care practitioner or visit the emergency room immediately so steps can be taken to prevent permanent vision loss.

In older people, the macula – the centre of the retina which is used for detailed vision – thins and occasionally bleeds. This can lead to distortion of, or even the loss of, central vision. The sufferer may also have trouble discerning colours. Peripheral vision remains unaffected, but central vision loss is serious, so early diagnosis and treatment is vital.

If you're experiencing vision problems, learning about the inner workings of your eye can help you understand what might be happening. Proper vision care is essential for these and other eye conditions, so have your eyes examined annually.*

Watch these illustrative, detailed videos to learn about common vision problems such as:

* Myopia (Often called being nearsighted)
* Hyperopia (Often called being farsighted)
* Presbyopia (The worsening of vision with regard to age is usually due to presbyopia, in which your lenses begin to thicken and lose their ability to bend and focus properly.)
* Astigmatism (An irregular curvature of the cornea. Astigmatism can be corrected easily with eyeglasses, contact lenses, or surgery.)

"What is a stigmatism?" is a question that eye doctors hear every day, but actually it's spelled "astigmatism." Although astigmatism is the most common vision problem, most people don't know what it is.
Astigmatism may accompany farsightedness or nearsightedness. Usually it is caused by an irregularly shaped cornea (called corneal astigmatism). But sometimes lenticular astigmatism results from an irregularly shaped lens, which is located behind the cornea.

Either kind of astigmatism can usually be corrected with eyeglasses, contact lenses, or refractive surgery.

Astigmatism Symptoms and Signs
You may not notice small amounts of astigmatism at all or have just slightly blurred vision. But sometimes uncorrected astigmatism can give you headaches or eye strain and distort or blur your vision at all distances.
Because not only adults can be astigmatic, you need to make sure you schedule an eye exam for your child.