Errors of Refraction

Normally, your eye can focus an image exactly on the retina:

Nearsightedness and farsightedness occur when the focusing is not perfect.

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:

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:

See How Refractive Vision Problems Work and How Corrective Lenses Work for details.