Refractive errors
Normally, your eye can focus an image exactly on the retina:
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Nearsightedness and farsightedness occur when the focusing is not perfect.
Myopia
When nearsightedness (myopia) is present, a person is able to see near objects well and has difficulty seeing objects that are far away. Light rays become focused in front of the retina. This is caused by an eyeball that is too long, or a lens system that has too much power to focus. Nearsightedness is corrected with a concave lens. This lens causes the light to diverge slightly before it reaches the eye, as seen here:
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Hyperopia
When farsightedness (hyperopia) is present, a person is able to see distant objects well and has difficulty seeing objects that are near. Light rays become focused behind the retina. This is caused by an eyeball that is too short, or by a lens system that has too little focusing power. This is corrected with a convex lens, as seen here:
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Astigmatism
Astigmatism is an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness.
Presbyopia
As we grow older, the lens becomes less elastic. It loses its ability to change shape. This is called presbyopia and is more noticeable when we try to see things that are close up, because the ciliary body must contract to make the lens thicker. The loss of elasticity prevents the lens from becoming thicker. As a result, we lose the ability to focus on close objects.
At first, people begin holding things farther away in order to see them in focus. This usually becomes noticeable when we reach our mid-forties. Eventually, the lens is unable to move and becomes more or less permanently focused at a fixed distance (which is different for each person).
To correct this, bifocals are required. Bifocals are a combination of a lower lens for close vision (reading) and an upper lens for distance vision.