Myopia (Near-sightedness): is the condition of the eye in which a person can see well at near distances, but cannot see well in the distance. This is typically due to the combination of a steep cornea and a longer than average eye length.
Hyperopia (Far-sightedness): is the condition in which a person can see well at far distances, but cannot see well at near distances. This is typically due to the combination of a flat cornea and a shorter than average eye length.
Astigmatism: is a condition which affects vision at all distances. It is caused by an irregularly shaped cornea which causes light to be focused at two different points on the retina yielding a blurry image.
Presbyopia: is a condition found in most adults over 40 years of age in which focusing ability is gradually reduced. The focusing system loses its efficiency as the lens inside the eye becomes progressively thicker and harder to move, and the ciliary muscles which move it become weaker with time.