There may be various tests that you have seen during an eye exam and asked yourself what they are for. Having a bright light shined into your eyes could be an example. This test is known as a retinoscopy examination, and if you have problems with accurate vision, this is a basic way the optometrist could assess it. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one way your optometrist can determine whether you need eyeglasses.
In short, what we are looking for during the retinoscopy exam is checking how accurately your eye focuses. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. This is known as the red reflex. We use the light to measure your focal length, or in layman's terms, it will measure the angle of refraction of light off your retina which lets us know how well your eye is able to focus. If it becomes obvious that you are not focusing well, that's when we use a set of lenses. We hold a number of lenses with varying prescriptions in front of your eye to determine which one will correct the refractive error. The lens power that works is the prescription you will need to fix your vision with glasses or contact lenses.
These exams are conducted in a darkened room. To make your eyes easier to examine, you'll usually be instructed to look at an object behind the doctor. The exam doesn't include eye charts, which means that a retinoscopy exam is also a really useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.