A person with myopia or short sight can see clearly objects close to them, but not far away. This is caused by the shape of the eye - either the eyeball is slightly too long or the cornea (the clear covering of the front of the eye) is too steeply curved. It is corrected by spectacles or contact lenses with lenses which are 'minus' or concave in shape. If you are short sighted your prescription will have a minus lens power eg -2.50D.