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. The refraction test will tell your doctor what prescription lens you should use in order to have 20/20 vision.