Sometimes, especially when performing an eye exam on a small child the eye doctor will shine a beam of light in the eyes. So what does this do? This is one way we determine the refractive error of your eye, and it's called retinoscopy. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is a test your eye doctor can use to determine if you need vision correction.
How well your eyes are able to focus under the circumstance we create during the exam is the most important thing we look for. We begin the exam by looking for what we call your red reflex. The retinoscope aims a beam of light into your eye, and a reddish light reflects through your pupil and off your retina. The angle at which the retinoscope's light reflects off your retina, also called your focal length, is exactly what lets us know how well your eye can focus. If it becomes obvious that you can't focus properly, we hold a number of lenses with varying prescriptions in front of the eye to see which one rectifies your vision.
These exams are generally conducted in a darkened room. To make your eyes easier to examine, you'll usually be instructed to focus on something behind the doctor. Unlike other eye exams, your doctor won't ask you to read letters off charts. This means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of the speech-impaired, or young children.