There are a few assessments that you may have noticed at an eye exam and questioned what they are for. Having beams of light shined into your eye may be one of them. This is one way we test the refractive error of your eye, and it's called a retinoscopy exam. By merely looking at the way light reflects off your retina, your optometrist can assess whether you are nearsighted, farsighted or have astigmatism. This is how they can also measure the prescription you would need to correct your vision.
The main thing your doctor is looking for during this exam is how accurately your eyes can focus on the light. We shine light into your eye because we are looking for what we call the red reflex. The retinoscope aims light into your eye, and a red or orange light reflects through your pupil and off your retina. We use the light to measure your focal length, or in layman's terms, it will determine the precise angle of refraction of light off your retina. And this is what tells us how well your eye is able to focus. And if we see that you are not focusing well, we hold up different prescription lenses in front of your eye to determine which one fixes your vision. And that is precisely how we calculate what prescription your glasses or contact lenses need to be.
All this happens in a dark room. To make your eyes easier to examine, you'll usually be asked to keep your eyes fixed on an object behind the doctor. Because a retinoscopy exam doesn't require you to read eye charts, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.