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. But why? This is one way eye doctors test the refractive error of your eye, and it's known as a retinoscopy exam. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is one test your optometrist can employ to see if you need vision correction.
In short, what we are doing during a retinoscopy exam is checking how accurately your eye can focus. We do this looking for what we call 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. We use the light to determine your focal length, or in other words, to determine the precise 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 aren't focusing correctly, we hold up a number of prescription lenses in front of your eye to determine which one corrects your vision. And that is exactly how we calculate the prescription your glasses or contact lenses need to be.
The eye doctor will perform your exam in a darkened room. To make your eyes easier to examine, you'll generally be told to look at something behind the doctor. Not having to read any eye charts means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of those who may struggle with speech, like young children and the elderly.