During your eye exam, your eye doctor might shine a beam of light into your eye, and hold various lenses in front of it. But why? This test is known as a retinoscopy examination, and if you struggle with accurate vision, this is a preliminary way the optometrist could assess it. By looking at the way light reflects off your retina, the optometrist can assess whether you are nearsighted, farsighted or have astigmatism, and can also measure the prescription you would need to correct your vision.
The main thing your doctor is looking for during this exam is how well your eyes can focus. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. We call this the red reflex. This process measures your focal length, or in layman's terms, it will calculate the precise angle at which light refracts off your retina. And this is what tells us how well your eye is able to focus. And if we notice that you are not focusing correctly, that's when we use a set of lenses. We hold up a few lenses with varying prescriptions in front of the eye to determine which one fixes the error. This is exactly how we calculate what prescription your glasses or contact lenses need to be.
These exams are performed in a dark room. The patient will usually be told to look at an object behind the doctor. This makes eyes easier to examine. Because a retinoscopy exam doesn't involve any eye charts, it means that it's also a really great way to determine an accurate prescription for children or patients who have difficulty with speech.