During your eye exam, you might see the eye doctor direct a beam of light into your eye, and hold various lenses in front of it. But why? Such as test is used to help test the refractive error of your eye, and it's called a retinoscopy exam. By looking at the way light reflects off your retina, the optometrist can decide if you are nearsighted, farsighted or have astigmatism. This is how they can also measure the prescription you would need to correct your vision.
Essentially, what we are doing during a retinoscopy exam is checking how accurately your eye focuses. We begin the exam by looking for what's known as the red reflex. The retinoscope aims light into your eye, and a red or orange light reflects through your pupil and off your retina. The retinoscope measures your focal length, or in layman's terms, it will calculate 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 are not focusing properly, we hold several lenses with varying prescriptions in front of your eye to see which one will correct the refractive error. And that is precisely how we find out what prescription your glasses or contact lenses need to be.
These exams are generally conducted in a dark room. You will usually be instructed to focus on something ahead, just behind the doctor. Unlike eye examinations you may have had, your doctor won't ask you to read letters off charts. This means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of the speech-impaired, or young children.