During some eye exams, you may have had a doctor shine a beam of light into your eye, and hold various lenses in front of it. But why? This test is a retinoscopy examination, and if you have problems with accurate vision, this is a basic way the eye doctor could assess it. By examining the way light reflects off your retina, the optometrist can assess whether you are nearsighted, farsighted or have astigmatism, and can also get a pretty good reading on the prescription required to correct your vision.
The most important thing your doctor is checking for during this exam is how accurately your eyes can focus on the light. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. This is known as the red reflex. The angle at which the light reflects off your retina, which is what eye care professionals call your focal length, is exactly what tells us how well your eye can focus. If it becomes clear that you aren't focusing well, we hold a variety of prescription lenses in front of the eye to determine which one fixes the refractive error. And that is precisely how we calculate the prescription your glasses or contact lenses need to be.
The optometrist will run your exam in a dark or dimmed room. The patient will usually be instructed to look at something behind the doctor. Because a retinoscopy exam doesn't require you to read eye charts, it means that it's also a really great way to determine the prescriptions of kids who might struggle with speech, or others who might be speech-impaired.