Myopia is more commonly known as nearsightedness. It is is when you cannot see objects at a distance, such as a street sign, but are fine while looking at near objects, such as a book, unless the myopia is very severe. Nearsighted people might have headaches and eyestrain, and may squint or feel tired while playing sports or driving. It runs in families and usually appears when they are a kid.
    

How a myopic eye focuses on far-away objects Obtained from Encarta Encyclopedia 2004

    If the eye has myopia, the eye is too long,  and the image falls short of the retina when the target object is far away.

How a concave lens corrects myopia Obtained from http://www.psych.ucalgary.ca

    Myopia may be corrected with lenses.  Concave lenses are used here.  They spread the light out before it reaches the convex lens in the eye, therefore letting the image focus directly on the retina.
    

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    Hyperopia is known to you probably as farsightedness. People with hyperopia will usually see far-away objects, such as the TV, crystal clear (unless the hyperopia is very severe), but cannot see close objects clearly (such as needlework). Hyperopic people may feel the same things a nearsighted person does, but in this case, they feel it at close range, not far range.

How a hyperopic eye focuses on near objects Obtained from Encarta Encyclopedia 2004

    The eye is shorter than normal in the case of hyperopia and the image focuses behind the retina if the target object is near. This means it doesn’t focus even when it reaches the retina.

How a convex lens corrects hyperopia Obtained from http://www.psych.ucalgary.ca

    
    Hyperopia can be corrected with a convex lens. A convex lens focuses the light in addition to the natural lens and makes the image focus exactly on the retina. Many people have hyperopia as a kid, but some of them lose is as the eyeball grows longer with normal growth.
    

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    Presbyopia is part of aging. When someone gets into their forties, and is caused by the hardening of the crystalline lenses in the eye. People with this will find that their arms are “getting shorter” as they have to hold reading material farther and farther away.

    Presbyopic people and hyperopic people have the same general view, but hyperopia is caused by a short eye, and presbyopia is caused by a hard lens. Presbyopia is also corrected with a convex lens. It is corrected using the same principle as hyperopia.

    Astigmatism is the most common eye disorder. Astigmatism can accompany myopia or hyperopia.  Astigmatism can be barely noticeable in small amounts. Most people don’t even aware it! However, some uncorrected astigmatism give you eyestrain or headaches, and distort or blur vision at all distances.
    
    Astigmatism is the result of an irregularly shaped cornea (the transparent part of the eye that covers the rest of it) or lens. Astigmatism is when the cornea is shaped like a football instead of like a baseball. This makes the light rays focus on two spots instead of one on the retina because the cornea has a steeper and flatter curve than normal.

    The steeper and flatter curve can too be corrected with lenses. A cylindrical curve in the glasses lens compensates for the football shape and makes it equivalent to the normal baseball-shaped curve. Both soft and hard contact lenses can be also used to correct astigmatism.  In conjunction to the one for astigmatism, the lens may contain a correction for myopia or hyperopia .

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    Strabismus, more commonly known as crossed-eyes, is when the two eyes do not look toward the same object together. One eye is normal, while the other one points in some other direction. Strabismus can be corrected with prisms. The prisms bend the light so the light goes to the correct spot.

    A cataract is when the lens of the eye or its capsule is opaque. It is the leading cause of blindness in the world. The word “cataract” means waterfall. For someone with a serious cataract, it's like seeing through a waterfall to them.

    Age-related cataracts develop in two ways. The fibers that make up the lenses in your eyes clump together to blur vision or the lens gets a yellowish/brownish tint. Symptoms of a cataract include cloudy or blurry vision, faded colors, excessive glare from light or a halo around lights, bad night vision, double vision, or frequent glasses or contact lens prescription changes.

    Early cataracts can be treated with new glasses or magnifying lenses. If these do not work, surgery will have to be used. Cataract surgery involves removing the natural lens inside the eye and replacing it with an artificial one.

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