Snellen Acuity

The long-standardized method of measuring vision sharpness using a chart with letters of different sizes. This test measures high-contrast visual acuity. Size of letters at a given distance correlates with angular resolution of the visual system. A measurement of 20/20 means that, at a viewing distance of 20 feet, the person being tested can see the same size letters as the optically “normal” eye. A measurement of 20/40 means that the person being tested can see letters at 20 feet that the optically normal person can see at 40 feet. For reference, 20/40 visual acuity is considered the minimum standard to obtain a passenger vehicle driver’s license in most states including CA. Legal blindness is defined as having best-corrected visual acuity of 20/400 or worse.