There are some tests that you may have noticed at an eye exam and asked yourself how they work. Having a bright light shined into your eye could be one of them. This is one way eye doctors determine the refractive error of your eye, and it's known as a retinoscopy exam. By merely looking at the reflection of light off your retina, the eye care professional can assess whether 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.
How well your eyes focus under the circumstance we create during the exam is really what we're looking for. We shine light into your eye because we are looking for what's known as your red reflex. The retinoscope sends a beam of light into your eye, and a reddish light reflects through your pupil and off your retina. The angle at which the light reflects off your retina, also called your focal length, is precisely what lets us know how well your eye can focus. And if we notice that you aren't focusing well, we hold up different lenses with varying prescriptions in front of your eye to determine which one rectifies the error. This is precisely how we find out what prescription your glasses or contact lenses need to be.
These exams are conducted in a dark room. To make your eyes easier to examine, you'll generally be told to look at something behind the doctor. Unlike other eye exams, you won't be asked to read letters off charts. This means that a retinoscopy exam is also a really useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.