There may be some tests that you have seen during an eye exam and asked yourself what they measure. Having a bright light shined into your eye could be an example. This test is a retinoscopy examination, and it's a basic way to assess the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a test your optometrist can employ to determine whether you need vision correction.
Essentially, what we are looking for during the retinoscopy exam is checking how well your eye can focus. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. This is called the red reflex. We use the light to measure your focal length, or in layman's terms, it will calculate the precise angle of refraction of light off your retina. And this is what tells us how well your eye is able to focus. If it becomes obvious that you can't focus properly, we hold different prescription lenses in front of your eye to determine which one rectifies your vision. The lens power that works is the prescription you require to correct your vision with glasses or contact lenses.
These exams are generally conducted in a dark or dimmed room. You will usually be told to focus on something behind the doctor. This makes eyes easier to examine. Unlike other eye exams, you won't be asked to read any charts. This means that a retinoscopy exam is also a very useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.