Myopia, hyperopia, and astigmatism are considered "refractive" errors that require corrective lenses to properly bend (or refract) light rays entering the eye. It is the proper refraction of light onto the retina that achieves "normal" eyesight

Myopia (Nearsightedness) means that near objects are clearer than distance objects. This occurs when your eye is too long in relation to the curvature of your cornea. Light rays entering the eye come to a focus in front of the retina which produces a blurred image.

Hyperopia (Farsightedness) means that distance objects are clearer than near objects. This occurs when your eye is too short in relation to the curvature of your cornea. Light rays entering the eye come to a focus behind the retina and this also will cause a blurred image.

Astigmatism occurs when your cornea is oval and not round in shape. Usually the cornea is shaped more like a baseball but when there is astigmatism the cornea is shaped more like a football. The result is unequal bending of light rays entering the eye. This produces two different focus points causing distortion.

Refractive visual problems are measured in units called diopters. The more nearsighted or farsighted you are the higher your prescription is in diopters.

Mild Myopia and Hyperopia	Less than 3.00 Diopters
Moderate Myopia and Hyperopia	3.10 to 6:00 Diopters
Severe Myopia and Hyperopia	6.10 to 9.00 Diopters
Extreme Myopia and Hyperopia	More than 9.10 Diopters

Mild Astigmatism	Less than 1.0 Diopters
Moderate Astigmatism	1.10 to 2.00 Diopters
Severe Astigmatism	2.10 to 3.00 Diopters
Extreme Astigmatism	More than 3.10 Diopters