There may be various tests that you have experienced during an eye exam and asked yourself what they are for. Having beams of light shined into your eyes may be one of them. Such as test is used to help determine the refractive error of your eye, and it's known as a retinoscopy exam. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one test your eye doctor can employ to see if you need vision correction.
The main thing your doctor is looking for during this exam is how well your eyes can focus. We begin the exam by looking for what we call the red reflex. The retinoscope sends a beam of 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 layman's terms, it will measure the angle at which light refracts off your retina. And this is what tells us how well your eye focuses. If it becomes obvious that you can't focus properly, that's where the lenses come in. We hold up a few lenses with varying prescriptions in front of your eye to see which one will correct the refractive error. This is exactly how we calculate what prescription your glasses or contact lenses need to be.
The eye doctor will perform your exam in a dark or dimmed room. The patient will usually be told to focus on an object behind the doctor. Unlike other eye exams, you won't be asked to read any 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.