During some eye exams, you might see the eye doctor tell you to look straight in front of you while directing a strong light into your eye. But why? Firstly, this test is a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. By examining the reflection of light off your retina, the optometrist can assess if you are nearsighted, farsighted or have astigmatism. This is how they may also get a pretty good reading on the prescription you would need to correct your vision.
How well your eyes are able to focus during the exam is the main thing we look for. 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 which tells us how well your eye is able to focus. And if it's apparent that you aren't focusing properly, that's where the lenses come in. We hold up a variety of prescription lenses in front of the eye to see which one will correct the refractive error. That lens power is the prescription you will need to fix your sight with glasses or contact lenses.
All this happens in a dark room. You will usually be asked to focus on something ahead, just behind the doctor. Because a patient isn't instructed to read eye charts during a retinoscopy exam, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.