A refraction test is usually given as part of a routine eye examination. It may also be called a vision test. This test tells your eye doctor exactly what prescription you need in your glasses or contact lenses.
Normally, a value of 20/20 is considered to be optimum, or perfect vision. Individuals who have 20/20 vision are able to read letters that are 3/8 of an inch tall from 20 feet away.
If you don’t have 20/20 vision, you have what is called a refractive error. A refractive error means that the light is not bending properly when it passes through the lens of your eye.
Causes of refractive errors
The eye's ability to refract or focus light sharply on the retina is primarily based on three eye anatomy features
Eye length. If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina
Curvature of the cornea. If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism.
Changes related to lens.