The most important component of an eye exam is the measurement of the actual quality of the central vision. We measure this by having you read an eye chart (the Snellen chart) and tweaking the vision by having you look through the phoropter (the machine with all the lenses inside used for checking your prescription). Some patient’s vision is so bad that we can’t measure and document vision using an eye chart. In these cases we measure other things, like the ability to count fingers and see hand movement. If the vision is really bad, we measure the detection of light. Most of these measurements require feedback from our patients, which makes visual acuity hard to check in young children or non-verbal patients. In these cases we estimate vision by seeing if the eye tracks objects and blinks to bright lights.