A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.

Epidemiology

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion

Types

An eye that has no refractive error when viewing a distant object is said to have  emmetropia  or be emmetropic. An eye that has a refractive error when viewing a distant object is said to have  ametropia  or be  ametropic .

Refractive errors are frequently categorized as spherical errors and cylindrical errors:

• Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refraction error frequently have blurry vision.

  • When the optics are too powerful for the length of the eyeball (this can arise from a cornea with too much curvature or an eyeball that is too long), one has myopia.
  • When the optics are too weak for the length of the eyeball (this can arise from a cornea with not enough curvature or an eyeball that is too short), one has hyperopia.

• Cylindrical errors occur when the optical power of the eye is too powerful or too weak across one meridian. It is as if the overall lens tends towards a cylindrical shape along that meridian.
  
  The angle along which the cylinder is placed is known as the axis of the cylinder, while 90 degrees away from the axis is known as the meridian of the cylinder.
  
  People with a simple astigmatic refractive error see contours of a particular orientation as blurred, but see contours with orientations at right angles as clear. When one has a cylindrical error, one has astigmatism.