During your eye exam, you might see the eye doctor tell you to look ahead while directing a strong light into your eye. But what does this do? Firstly, this test is called a retinoscopy examination, and if you struggle with accurate vision, this is one way the optometrist might assess it. By examining the reflection of light off your retina, the eye care professional can assess if you are nearsighted, farsighted or have astigmatism. This is how they can also measure the prescription you would need to correct your vision.
The main thing an eye doctor is checking for during this exam is how well your eyes can focus. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. We call this the red reflex. 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 clear that you can't focus properly, that's when we use a set of lenses. We hold different lenses with varying prescriptions in front of the eye to determine which one rectifies the error. That lens power is the prescription you will need to fix your sight with glasses or contact lenses.
All this happens in a dark or dimmed room. To make your eyes easier to examine, you'll usually be asked to look at an object behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it means that it's also a really great way to determine the prescriptions of children or patients who have difficulty with speech.