During an eye examination, you might see the eye doctor direct a beam of light into your eye, and hold various lenses in front of it. But why? Such as test is used to help test the refractive error of your eye, and it's called retinoscopy. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a test your eye doctor can use to see whether you need vision correction.
The most important thing your doctor is checking for during this exam is how well your eyes can focus on the light. We begin the exam by looking for what we call the red reflex. The retinoscope aims a beam of light into your eye, and a reddish light reflects through your pupil and off your retina. The degree at which the light refracts off your retina, which is what eye care professionals call your focal length, is exactly what lets us know how well your eye can focus. If it becomes clear that you can't focus properly, that's when we use a set of lenses. We hold up a few lenses with varying prescriptions in front of the eye to determine which one corrects your vision. That lens power is the prescription you require to rectify your sight with glasses or contact lenses.
Your optometrist will perform your exam in a dark room. To make your eyes easier to examine, you'll generally be told to keep your eyes fixed on an object behind the doctor. The exam doesn't include eye charts, which means that a retinoscopy exam is also a very useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.