There may be various tests that you have experienced at an eye exam and questioned what they measure. Having a bright light shined into your eye may be one of them. Firstly, this test is called a retinoscopy examination, and it's a basic way to assess the refractive error of your eye. By merely examining the way light reflects off your retina, your eye doctor can determine if you are nearsighted, farsighted or have astigmatism, and can also get a pretty good reading on the prescription you would need to correct your vision.
Basically, what we are doing during the retinoscopy exam is checking how accurately your eye can focus. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. This is known as the red reflex. The angle at which the light reflects off your retina, also called your focal length, is the thing that tells us how well your eye can focus. And if we see that you are not focusing well, we hold a few prescription lenses in front of your eye to determine which one rectifies the error. And that is exactly how we find out what prescription your glasses or contact lenses need to be.
The eye doctor will run your exam in a darkened room. You will usually be instructed to look at something ahead, just behind the doctor. The exam doesn't include eye charts, which means that a retinoscopy exam is also a really useful tool to determine the prescriptions of the speech-impaired, or young children.