The visual acuity test determines how small the letters on a Snellen Wall Chart or a card held 20 feet (6 meters) away can be read. Special charts are used while testing at distances less than 20 feet (6 meters). Some Snellen charts are video monitors that display letters or images. The top number corresponds to the length (in meters) at which the test chart was presented (usually 6m), and the bottom number corresponds to the position on the Snellen Wall Chart of the smallest line read by the patient. Typically, you will be asked to sit or stand 20 feet away from the eye chart during a vision screening.