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 what does this do? This test is called a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a test your optometrist can employ to determine if you need vision correction.
In short, what we are doing during a retinoscopy exam is checking how accurately your eye focuses. We begin the exam by looking for what we call the 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 light reflects off your retina, also called your focal length, is exactly what tells us how well your eye can focus. If it becomes obvious that you are not focusing correctly, we hold a variety of prescription lenses in front of your eye to determine which one fixes the refractive error. This is precisely how we calculate the prescription your glasses or contact lenses need to be.
The eye doctor will perform your exam in a dark room. To make your eyes easier to examine, you'll generally be instructed to focus on something behind the doctor. The exam doesn't include eye charts, which 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.