During an eye examination, you may have had a doctor instruct you to look straight in front of you while shining a light into your eye. But what does this do? Firstly, this test is called a retinoscopy examination, and it's a basic way to measure the refractive error of your eye. By merely looking at the reflection of light off your retina, the eye doctor can assess whether you are nearsighted, farsighted or have astigmatism. This is how they may also measure the prescription you would need to correct your vision.
The most important thing an eye doctor is looking for during this exam is how accurately 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. This process measures your focal length, or in other words, to calculate the angle of refraction of light off your retina. And this is what tells us how well your eye focuses. And if it's apparent that you aren't focusing properly, we hold a number of lenses with varying prescriptions in front of the eye to determine which one will correct the refractive error. The lens power that works is the prescription you require to correct your vision with glasses or contact lenses.
All this happens in a dark room. You will usually be told to focus on something behind the doctor. Unlike other eye exams, 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.