There may be various tests that you have seen 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 determine the refractive error of your eye, and it's called a retinoscopy exam. By examining the reflection of light off your retina, your optometrist can decide if you are nearsighted, farsighted or have astigmatism, and can also measure the prescription you would need to correct your vision.
The most important thing your doctor is looking for during this exam is how well your eyes can focus on the light. We shine light into your eye because we are looking for what we call your red reflex. The retinoscope sends a beam of light into your eye, and a reddish light reflects through your pupil and off your retina. 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. If it becomes clear that you can't focus correctly, that's when we use a set of lenses. We hold several lenses with varying prescriptions in front of your eye to see which one fixes the error. That lens power is the prescription you will need to correct your sight with glasses or contact lenses.
All this happens in a darkened room. The patient will usually be instructed to focus on something ahead, just behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it means that it's also a really great way to determine the prescriptions of children or patients who have difficulty with speech.