The main cause of blurred vision is refractive error.  The cornea and lens provide the refractive power of the eye.  Light refracts through the cornea to the lens, where it is focused onto retina.  Two-thirds of the optical power of the eye comes from the cornea.  Diopter power of the cornea depends on it’s radius of curvature.  Emmetropic patients do not need distance correction, and on average have a corneal power of 41 diopters.  Myopic patients, far sighted, have steeper corneas, which provide too much power to the subject.  Therefore the patient has very good near vision, but distance is blurred.  Hyperopic subjects have flatter cornea, so they need more power to see distance and near.  When the corneal curvature differs from one meridian to the perpendicular meridian, the cornea is classified as astigmatic; the difference is quantified as corneal cylinder.  Astigmatism distorts the apparent spacing of vertical or horizontal lines depending on the axis of cylinder.

 

 

In order to correct these refractive errors, prescription glasses or contacts are worn.  Although major improvements have been made in contact design, including correcting astigmatism and improving oxygen permeability, people still find wearing contacts to be burdensome.  To eliminate the need of glasses or contacts, doctors perform laser refractive surgery (e.g. LASIK).

T. Icenogle 2008