Sightedness?

What causes nearsightedness and farsightedness??Sightedness?
A person who is nearsighted either has eyes that are too long from front to back or the cornea (clear front portion of the eye) is curved too steep. This causes the image that a person is looking at to land in front of the retina (inner lining of the back of the eye), when it should land directly on it. A person who is farsighted is just the opposite. His or her eyes will be either too short from front to back or their corneas are not curved steep enough. This causes the image to land behind the retina. Glasses or contacts simply bend the light before it enters the eye so that the image lands right on the retina.
Well the above is correct but if you want to know how does somebody become near/farsighted its like this. Farsightedness is extremely common in ages past 40. Almost everybody will be farsighted at one point or another and age happens to increase your chances of being farsighted. Nearsightedness or myopia can be caused from herdity or reading small text in bad light and overstraining the eyes. It is seen in all ages. Hope I could help.
Shorty