Sometimes, especially when doing an eye exam on small children the optometrist will focus a light in the eyes. But why? This test is called a retinoscopy examination, and it's a basic way to measure the refractive error of your eye. By merely examining the reflection of light off your retina, your eye doctor can assess if you are nearsighted, farsighted or have astigmatism, and can also measure the prescription required to correct your vision.
The main thing your doctor is looking for during this exam is how well your eyes can focus on the light. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. This is known as the red reflex. The angle at which the retinoscope's light refracts 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 obvious that you aren't focusing correctly, that's where the lenses come in. We hold a number of prescription lenses in front of your eye to determine which one rectifies the error.
These exams are performed in a dark room. You will usually be told to look at something 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 very good way to accurately determine the prescriptions of the speech-impaired, or young children.