There may be a few tests that you have experienced at an eye exam and questioned what they measure. Having beams of light shined into your eye could be an example. This is one way eye doctors test the refractive error of your eye, and it's called retinoscopy. By just examining the reflection of light off your retina, your optometrist can determine if you are nearsighted, farsighted or have astigmatism, and 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. We do this looking for what's known as the red reflex. The retinoscope aims light into your eye, and a reddish light reflects through your pupil and off your retina. We use the light to measure your focal length, or in other words, to measure the precise angle of refraction of light off your retina. And this is what tells us how well your eye is able to focus. If it becomes clear that you can't focus properly, we hold several prescription lenses in front of the eye to determine which one will correct the refractive error.
Your eye doctor will perform your exam in a dark room. To make your eyes easier to examine, you'll generally be asked to focus on something behind the doctor. The exam doesn't include charts to be read, which means that a retinoscopy exam is also a very good way to accurately determine the prescriptions of the speech-impaired, or young children.