During an eye examination, your eye doctor might shine a beam of light into your eye, and hold various lenses in front of it. But why? This is one way eye doctors determine the refractive error of your eye, and it's known as a retinoscopy exam. It sounds fascinating, but by looking at the way light reflects off your retina, the optometrist can determine if you are nearsighted, farsighted or have astigmatism, and can also get a pretty good reading on the prescription required to correct your vision.
In short, what we are looking for during the retinoscopy exam is checking to see how your eye can focus. When we use the retinoscope to shine light into your eye, a reddish orange light reflects off your retina, through your pupil. This is known as the red reflex. The retinoscope measures your focal length, or in layman's terms, it will determine the angle of refraction of light off your retina. And this is what tells us how well your eye focuses. If it becomes clear that you are not focusing correctly, we hold different lenses with varying prescriptions in front of your eye to determine which one fixes the refractive error.
Your optometrist will run your exam in a darkened room. To make your eyes easier to examine, you'll usually be asked to keep your eyes fixed on something behind the doctor. Unlike eye examinations you may have had, your doctor won't ask you to read any charts. This means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of the speech-impaired, or young children.