A refractive error occurs when light rays entering the eye are not precisely focused on the retina (internal light-absorbing surface) of the eye.
The cornea and the lens help to focus light onto the retina in the eye.
Myopia occurs when light entering the eye is focused in front of the retina. This happens when the eyeball is too long or has excessive focusing power. Distant objects will appear blurred. Myopia usually progresses yearly through the pre-teen and teenage years, and should stabilise by late teens to early twenties.
Hyperopia occurs when light entering the eye is focused behind the retina. This happens when the eyeball is too short, or has insufficient focusing power. In general, this causes both near and distant objects to be blurred, but vision may be more blurred when viewing near objects.
Astigmatism occurs when the shape of the cornea (front surface of the eye) is irregular and not equally round in all directions. Instead of being regularly round in all directions (like a ping-pong ball), a cornea with astigmatism is shaped more like an egg, which is more rounded in one direction and less rounded in another direction. Light entering an eye with astigmatism is not clearly focused onto the retina, thus resulting in blurred and distorted vision.
Myopia can be hereditary and has been associated with excessive near work.
The tendency to develop astigmatism is usually inherited and a small amount of astigmatism is common. A large amount of astigmatism can be associated with thinning disorders of the cornea, such as keratoconus, that change the shape of the cornea. Injury to the cornea and corneal scars can also result in astigmatism. Very rarely, excessive and vigorous eye rubbing can induce astigmatism by altering the shape of the cornea.
Significant refractive errors are usually treated with glasses, spectacles or contact lenses. Laser and surgical vision correction can also be considered after the age of 21 to achieve spectacle independence (freedom from wearing spectacles).