A refractive error is a common disorder which occurs when the eye cannot clearly focus images on the retina, resulting in blurry vision. About 40% of the world population have significant refractive errors, and different eye conditions lead to different errors:
- Myopia, called nearsightedness, results in a negative refractive error
- Hyperopia, or farsightedness, results in a positive refractive error
- Astigmatism results in blurry wavy lines, and correction is described by a cylindrical power and an axis angle
Perfect eyesight means there is no significant spherical or cylindrical error. Once the error is known, corrective eyeglasses or contact lenses can be made to help the eye focus images on the retina, achieving clear vision for the wearer.