Long-sightedness (hypermetropia) and short-sightedness (myopia) are both caused by minor defects in how someone’s eyes focus.
The lens of your eye focuses what you are looking at onto the light-sensitive membrane called the retina at the back of your eye. The retina converts the patterns of light into neural signals that your brain can then interpret. If you are long-sighted (like me) then when light goes through you lens into your eye it is focused behind the retina, not on it. This means that your eyes need to try to refocus but this is difficult and can cause headaches or other pains. That’s where glasses or contact lenses help because they alter the focus of light before it goes into the eye, helping your eyes to see in focus. I wear glasses pretty much all the time because otherwise I find it very hard to read, look at the computer or see details of things that are a little far away and I can get headaches if I try.
In contrast, if you are short-sighted then when the light goes through the lens it focuses in front of the retina. This means that short-sighted people can usually see thing near them but not far away.
People with perfect sight simply have the light focused properly on the retina.
Karen has explained beautifully 😉 all I can add is this short video, where you can see exactly what she means and the differences in the focus point according to each condition! http://www.youtube.com/watch?v=8bbfoC4ar_A