Myopia (Nearsightedness)
Distance vision is impaired when the eye is too long in relation to the curvature of the cornea, and light focuses in front of the retina.

Hyperopia (Farsightedness)
Close vision, and to some extent distance vision, is impaired when the eye is too short in relation to the curvature of cornea, and light focuses behind the retina.

Astigmatism
When the cornea is oval, rather than round, light focuses on more than one point on the retina. Astigmatism impairs both distance and close vision.

Presbyopia
This natural loss of close vision with age is caused by hardening of the natural lens. It may be treated by changing the focus of one eye for reading, a procedure called monovision or blended vision.