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 a refractive error. That can 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 helps your doctor determine a lens prescription that will give you the sharpest,
most comfortable vision. The assessment can also determine that you don’t need corrective lenses.
Your doctor may use a computerized refractor to 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 masklike device that contains wheels of different lenses (phoropter). Afterwards, he asks you to judge which combination of lenses gives you the sharpest vision.