A Snellen eye chart assesses distance vision, according to WebMD. It consists of several rows of letters printed on a white card. The letters on the top row are the largest; the letters become smaller on each succeeding line.
During a Snellen eye test, the patient is positioned 20 feet away from the chart. He is then asked to read from chart. If the patient is unable to read the largest letter, he moves incrementally closer to the chart until he is able to read the top line. Test results are expressed as fractions, with the numerator referring to the distance (feet) of the patient from the chart and the denominator being the distance individuals with normal vision are able to read the chart, explains the National Center for Biotechnology Information.