There may be some assessments that you have experienced at an eye exam and questioned what they are for. Having a bright light shined into your eye may be an example. This is one way eye doctors determine the refractive error of your eye, and it's called a retinoscopy exam. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a way your eye doctor can determine whether you need eyeglasses.
In short, what we are looking for during the retinoscopy exam is checking how accurately your eye can focus. We shine light into your eye because we are looking for what we call your red reflex. The retinoscope aims light into your eye, and a reddish light reflects through your pupil and off your retina. The retinoscope measures your focal length, or in layman's terms, it will measure the angle of refraction of light off your retina. And this is what tells us how well your eye is able to focus. If it's apparent that you are not focusing properly, we hold a number of prescription lenses in front of your eye to see which one fixes your vision. This is exactly how we find out what prescription your glasses or contact lenses need to be.
The optometrist will perform your exam in a dark room. You will usually be asked to look at something ahead, just behind the doctor. This makes eyes easier to examine. Unlike eye examinations you may have had, 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.