A refraction test is the part of an eye exam that shows what strength prescription a patient needs. A person who does not have 20/20 vision has a refractive error, which means light does not bend properly when passing through the eye. An accurate prescription corrects this.
During a refraction test, the doctor shines light into the patient's eyes and measures the amount of light reflected by the retina. Next, he places the patient in front of a machine called a phoroptor. This machine has holes for the patient to look through and read a row of letters placed 20 feet away. The doctor inserts a different strength lens in front of each hole until he finds the one that correct the patient's vision to 20/20. This is the strength of eyeglass lenses prescribed for the patient.