People who are short-sighted have difficulty seeing distant objects clearly. Frowning and screwing up the eyes in an effort to see better is common and Myopia sufferers may also experience headaches.
Myopia is generally inherited. It occurs when the lens system of the eye is too strong for its length, or as is more often the case, the eye is too long for its optical power. This means that most myopic eyes are in fact healthy; they are just larger than normal.
In most myopic eyes vision can be improved with spectacles or contact lenses. Corrective lenses shift the point of focus so that it falls exactly on the back of the eye, giving clear vision.