Visual acuity measures the clarity or sharpness of vision. The standard instrument for this measurement is the Snellen chart, developed in the 1860s by Dutch ophthalmologist Herman Snellen. This chart presents rows of block letters, called optotypes, that decrease in size. The Snellen chart is the primary method used to determine a patient’s distance vision and track changes over time.
What the 20/20 Measurement Means
The 20/20 fraction represents the standard benchmark for normal visual acuity. This measurement is a ratio comparing the testing distance (numerator) to the distance at which a person with normal vision could read the same line (denominator). Since the patient is tested at 20 feet, 20/20 vision means a person sees clearly at 20 feet what a person with normal vision also sees at 20 feet. This ratio establishes the expected visual standard for the general population. While 20/20 is the accepted standard, only about 35% of adults naturally achieve this level without corrective lenses.
Locating the 20/20 Line on the Snellen Chart
The 20/20 line is situated well down the Snellen chart, typically appearing as one of the smaller rows of letters. On a standard chart, this line is often the eighth row from the top. The letters on this row are the smallest the majority of the population is expected to read clearly from 20 feet away. To pass the test, a patient must correctly identify more than half of the letters on that row. The largest letter at the top of the chart represents 20/200 vision, while lines below 20/20 represent visual acuity better than the average.
Vision Worse and Better Than 20/20
Measurements deviating from 20/20 indicate a difference in visual clarity compared to the normal standard. A score of 20/40 means a person must stand at 20 feet to read a line that a 20/20 person could read from 40 feet away, indicating worse-than-average vision. Conversely, 20/15 signifies better-than-average visual acuity, meaning the person can read a line from 20 feet that the average person must move up to 15 feet to read. A score of 20/200 or worse defines legal blindness in the United States. This means the person sees at 20 feet what a normal person sees at 200 feet.