The resolving power of the human eye is 0.0003 of a radian or an arc of one minute (1/60th of a degree), which corresponds to 100 microns at 25 cm (10 in). A micron is a thousandth of a millimetre, hence 100 microns is 0.003937, or less than four thousandths of an inch or 2.5 cm. The human eye can, however, detect a bright light source shining through an aperture only 3 to 4 microns across. In October 1972 the University of Stuttgart, then West Germany, reported that their student Veronica Seider (b. 1951) possessed a visual acuity 20 times better than average. She could identify people at a distance of more than a mile (1.6 km).
Comments below may relate to previous holders of this record.