During your eye exam, you may have had a doctor shine a beam of light into your eye, and hold various lenses in front of it. So what does this do? Such as test is used to help test the refractive error of your eye, and it's known as retinoscopy. By merely looking at the reflection of light off your retina, the optometrist can determine whether you are nearsighted, farsighted or have astigmatism. This is how they can also get a pretty good reading on the prescription you would need to correct your vision.
How well your eyes are able to focus during the exam is the most important thing we look for. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. We call this the red reflex. The angle at which the light reflects off your retina, also called your focal length, is exactly what tells us how well your eye can focus. And if we see that you can't focus properly, we hold up a few lenses with varying prescriptions in front of the eye to see which one will correct the refractive error. And that is precisely how we find out the prescription your glasses or contact lenses need to be.
Your eye doctor will run your exam in a darkened room. To make your eyes easier to examine, you'll usually be told to keep your eyes fixed on something 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.