An ideal eye has perfect near and distance vision. This means that the cornea (the front surface of the eye) and lens (located in the middle of the eye) focus objects clearly onto the retina (the part of the eye that processes vision).

Unfortunately, not everyone has perfect vision. Many people have one or more of the following conditions:

Nearsightedness (myopia)

Nearsightedness (myopia) occurs when your eyeball is too long or your cornea curves too much. For nearsighted people, images enter the front of the eye and focus directly in front of the retina. This results in distant objects being blurry, while near objects appear clearly.

Nearsightedness most often begins in the pre-teenage years. Heredity and, possibly, environmental factors cause nearsightedness. Eyeglasses or contact lenses improve vision by properly focusing distant objects clearly on the retina.

Farsightedness (hyperopia)

Farsightedness (hyperopia) occurs when your eyeball is too short or your cornea curves too little. For farsighted people, images enter the front of the eye and focus in back of the retina. This results in close objects being blurry, while distant objects appear clearly. This can cause eyestrain, fatigue, and headaches when doing close work. Severe farsightedness can result in blurred distance vision, too.

In mild cases, eyes may focus without corrective lenses. Other people may need corrective lenses. Age lessens the eyes' ability to focus and adapt. Corrective lenses clear vision and make reading more comfortable.

Vision screenings done in school may not detect farsightedness. Students typically identify letters on an eye chart 20 feet away, which only tests distance vision. If a child complains of eye strain or headaches when doing school work, s/he may be farsighted.

Astigmatism

Astigmatism is a common visual condition. The cornea is oval or egg-shaped. Objects cannot focus properly on the retina. This causes two focal points.

People with severe astigmatism usually have blurred or distorted vision at all distances. Those with mild astigmatism may experience eye strain, headaches, or blurred vision at certain distances.

Astigmatism may be hereditary, but environmental factors may add to the problem. Astigmatism may increase slowly over time, but usually remains relatively stable. Symptoms can include: headaches, seeing the vertical direction of objects very clearly while the horizontal direction is blurry (or the opposite), blurred vision, seeing a comet-like flare around lights.

Glasses and, usually, contact lenses can clear vision and relieve symptoms. Most people who wear glasses have some astigmatism. When you begin wearing corrective lenses, your new vision may be peculiar or distorted. For instance, doorways may appear curved or bent. It also is not unusual to feel uneasy walking during the first days or week of wearing your new prescription.

You should begin wearing your new glasses gradually. Start using them for stationary activities such as reading or watching television. As you adjust to your new sight, walking and other activities will feel normal again. If, after several weeks, you do not adapt to your new vision, your eye doctor may need to adjust your prescription.

Patients with astigmatism may need to be fit for astigmatic soft lenses or gas-permeable hard lenses. These lenses cost more than standard soft lenses because of higher manufacturing costs and the technical skills required for fitting.

Presbyopia

Presbyopia is experienced by everyone as they age. However, most people do not notice any significant problems until they are 40 to 50 years old. This is when they usually require reading or bifocal glasses for close work. In a normal eye, the lens changes shape when you look at close or distant objects. When you focus on objects at close range, eye muscles constrict and the lens thickens. During the aging process the lens becomes harder and less flexible, making it difficult to focus on close objects. Farsighted individuals may notice symptoms at an earlier age. Although this is a gradual process, it may seem sudden. As the lenses of the eyes lose their flexibility, periodic prescription changes are required, especially in the first few years of presbyopia.

Symptoms can include: objects blur at near, difficulty focusing back and forth from close and far distances, need to hold reading material far away, increased need for good light while reading.

Glasses may be simple reading glasses, bifocals, or multifocals. Contact lenses are possible in some cases.

To date, there is no way to prevent presbyopia.