There are various tests that you may have noticed during an eye exam and wondered what they measure. Having beams of light shined into your eye may be one of them. Such as test is used to help determine the refractive error of your eye, and it's known as a retinoscopy exam. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one way your eye doctor is able to determine if you need vision correction.
How well your eyes focus under the circumstance we create during the retinoscopy exam is the main thing we look for. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. This is called the red reflex. The retinoscope measures your focal length, or in layman's terms, it will measure the precise angle at which light refracts off your retina which lets us know how well your eye is able to focus. If it becomes clear that you can't focus well, that's when we use a set of lenses. We hold a variety of prescription lenses in front of your eye to see which one corrects your vision.
The eye doctor will run your exam in a darkened room. You will usually be told to focus on an object behind the doctor. This makes eyes easier to examine. 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 kids who might struggle with speech, or others who might be speech-impaired.