What is the smallest thing the human eye can see?
A single human eye sees roughly a 140-degree field horizontally and a 90-degree field vertically. The eye’s resolution capability varies over the area of the retina and therefore with viewing angle. It is highest in the foveal, or straight-ahead, viewing region of the retina, which lies within a 2- to 4-degree radius of the optical axis of the eye, where an angular separation of about one arc minute may be detected under ideal conditions. The eye can rotate 45 degrees vertical, so the foveal region may be directed to view any part of a 90- to 95-degree section of the horizontal monocular field of vision. Solving the previous equation using an angle of 60 arc seconds or 0.01667 degree, we find that the distance at which the foveal region of the 20/20 eye may resolve lines 1 mm apart is 3,438 mm or about 11.3 feet. You can check the resolving power of your own eyes by drawing two lines spaced 1 mm apart on a piece of white paper, using black ink to maximize contrast. Hang the paper on th
