When the shape of your eye interferes with light passing through the eye such that it cannot focus correctly on the retina (back of the eye). There are three types of refractive error: myopia, hyperopia, and astigmatism.

Refractive error is what laser eye surgery is all about. Laser eye surgery does not treat the cause of refractive error, but it aims to compensate for the effect of it by reshaping the cornea (front surface of the eye) such that it can bend the light to focus correctly on the cornea.

Refractive error can be measured at least three ways: With a refraction examination; with an automated refractor; and with wavefront aberrometry. For purposes of determining one's prescription, however, it is generally done with a manifest refraction, that is, the "Better 1, or better 2?" type of subjective test.

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