Light waves are bent as they pass through your cornea and lens. If light rays don't focus perfectly on the back of your eye, you have "refractive error." Having refractive error may mean you need some form of correction, such as glasses, contact lenses or refractive surgery, to see as clearly as possible. Assessment of your refractive error or refraction helps your doctor determine a lens prescription that will give you the sharpest, most comfortable vision. Refraction assessment may also determine that you don't need corrective lenses.
Your doctor may use a computerized refractor to measure the curve of the surface of your eyes and estimate your prescription for glasses or contact lenses. Or he or she may use a technique called retinoscopy. In this procedure, the doctor shines a light into your eye and measures the refractive error by evaluating the movement of the light reflected by your retina back through your pupil.
Your eye doctor usually fine-tunes this refraction assessment by having you look through a mask-like device that contains wheels of different powers of lenses (phoropter). You'll be asked to judge which combination of lenses gives you the sharpest vision. By repeating this step several times, your doctor finds the lenses that give you the sharpest visual acuity.