Sometimes, particularly when doing an eye exam on small children the eye doctor will shine a light in the eyes. But why? This is one way we determine the refractive error of your eye, and it's known as a retinoscopy exam. By merely looking at the reflection of light off your retina, the optometrist can decide whether you are nearsighted, farsighted or have astigmatism. This is how they may also get a pretty good reading on the prescription required to correct your vision.
How well your eyes focus during the retinoscopy exam is the main thing we look for. We shine light into your eye because we are looking for what's known as your red reflex. The retinoscope aims a beam of light into your eye, and a red or orange light reflects through your pupil and off your retina. The angle at which the light refracts off your retina, also called your focal length, is precisely what tells us how well your eye can focus. And if we notice that you are not focusing properly, we hold a variety of prescription lenses in front of the eye to determine which one rectifies your vision. And that is exactly how we find out the prescription your glasses or contact lenses need to be.
Your optometrist will perform your exam in a dark or dimmed room. To make your eyes easier to examine, you'll generally be instructed to keep your eyes fixed on an object behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it's also a particularly useful way to determine an accurate prescription for kids who might struggle with speech, or others who might be speech-impaired.