Visual acuity is a measure of our ability to discriminate the
finest detail: either by discriminating two parallel lines apart (Vernier
acuity) or two tiny spots or stars apart (point discrimination).
The primate and human visual system is capable of acuity of 1 min of
arc or 60 cycles/degree of visual angle. One degree of visual angle is
thought to cover approximately 280-300 um of retinal distance.
The area of the retina where the cone photoreceptors are most closely
packed in a more or less hexagonal array, is, of course, the fovea. So
visual discrimination of 1 min of arc is about the center to center spacing
of the cones of the central mosaic in the foveola.