There are a few tests that you have seen during an eye exam and asked yourself how they work. Having beams of light shined into your eye may be one of them. This test is called a retinoscopy examination, which is a preliminary way to determine the refractive error of your eye. By looking at 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.
In short, what we are looking for during the retinoscopy exam is checking to see how your eye can focus. We begin the exam by looking for what's known as your red reflex. The retinoscope aims light into your eye, and a reddish light reflects through your pupil and off your retina. The degree at which the light reflects off your retina, also called your focal length, is exactly what tells us how well your eye can focus. If it becomes obvious that you aren't focusing correctly, that's when we use a set of lenses. We hold up a variety of prescription lenses in front of the eye to see which one rectifies your vision. And that is exactly how we calculate the prescription your glasses or contact lenses need to be.
The retinoscopy exam is generally conducted in a dark or dimmed room. You will usually be instructed to look at an object behind the doctor. This makes eyes easier to examine. Because a retinoscopy exam doesn't involve any eye charts, it's also a particularly useful way to determine the prescriptions of children or patients who have difficulty with speech.