Short Sight (Myopia)
Unable to see clearly distant objects and yet close objects are well focussed.
Either too much focussing power by cornea or too long an axial length of the eye (or combination of both) resulting in the image being focussed in front of the retina.
The placement of a negative concave lens (spectacle or contact lens) in front of the eye to diverge the light rays before being focused onto the retina.
Onset & Progression
Because we are normally long sighted at birth the development of the eye is to become less long sighted or more short-sighted. Those who are at birth less long sighted will therefore become short sighted throughout their early development. Some will show a gradual increase but others especially from the Orient will show rapid increases. Studies have shown that the early correction of myopia will reduce the eventual increase.