Refraction of the eye is the bending of light as it passes through the cornea and lens of the eye to focus on the retina. A refraction test is a part of a comprehensive eye exam that measures a persons prescription for eyeglasses or contact lenses. The test tells the eye doctor what prescription lens a person needs to see properly and can diagnose refractive errors such as nearsightedness, farsightedness, astigmatism, and presbyopia. Refractive errors occur when the shape of the eye keeps light from focusing correctly on the retina, causing blurry vision. The refraction test is usually given as part of a routine eye examination and is simple and painless. The results of the test are used to determine the powers of the lenses needed to make a person see clearly. While a refraction is essential for a comprehensive eye exam, the charge for a refraction is only covered by some insurances but not all. People with a refractive error should have an eye exam every 1 to 2 years, or when their vision changes.