Simple myopia, hypermetropia and astigmatism may be aided by spectacles and contact lenses. However these may be inconvenient or intolerable by some. Refractive correction surgery can correct such and provide clear unaided vision, hence free the dependence on glasses or contact lenses.
The axial length of the eyeball is elongated or the cornea curvature is steep. As a result, light rays are focused in front of the retina causing a blurred distance vision.
The axial length of the eyeball is short or the cornea curvature is flat. As a result, light rays are focused behind the retina causing a blurred vision for near. In severe cases or in patients reaching presbyopic middle age, vision may be blurred for distance vision additionally.
When the corneal curvature varies in different direction, light rays cannot be focused sharply onto the retina and the image will become blurred and distorted. Astigmatism may coexist with myopia or hypermetropia.