Optical acuity--the ability to see something with the maximum possible clarity and accuracy of color--is greatest only within a tiny area of less than two degrees (about the size of a thumbnail at arm's length) around the center of the gaze; that is, only when the image is "foveated"--when the light that comprises the image falls through the pupil onto the tiny area of the fovea at the opposite side of the retina. This is what happens when you look at something head on. Outside the small cone of "foveal" vision, optical acuity is fairly good in the larger cone of so-called parafoveal vision, some ten degrees around the center of the gaze, but thereafter rapidly deteriorates in peripheral vision, as when you see something out of the corner of the eye. One reason for this is found in the two different kinds of photoreceptors. One (called cones) is mainly clustered in the fovea; the other (called rods) is placed outside the fovea. Furthermore, the photoreceptors' outputs are collected in groups, called receptive fields, which are larger and hence coarser with increasing distance from the fovea. (Acuity is usually measured in terms of how closely spaced the lines in a grating can be--its "spatial frequency"--and still be distinguished as separate.)
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