During your eye exam, you may have had a doctor tell you to look straight in front of you while shining a beam of light into your eye. So what does this do? This is one way eye doctors test the refractive error of your eye, and it's called retinoscopy. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a way your eye doctor is able to determine whether you need eyeglasses.
How well your eyes are able to focus under the circumstance we create during the retinoscopy exam is really what we're looking for. We shine light into your eye because we are looking for what's known as 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. This process measures your focal length, or in other words, to calculate the precise angle at which light refracts off your retina which tells us how well your eye focuses. And if it's apparent that you are not focusing correctly, we hold a few lenses with varying prescriptions in front of your eye to see which one will correct the refractive error. This is precisely how we calculate the prescription your glasses or contact lenses need to be.
All this happens in a dark or dimmed room. To make your eyes easier to examine, you'll usually be told to keep your eyes fixed on an object behind the doctor. Because a patient isn't required to read eye charts during a retinoscopy exam, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.