Diffraction is the loss of sharpness that comes from looking through a small opening. Those who have shot a rifle with a peep sight have seen this effect; the view through the center of the sight is sharp but the edges are blurred. A balance must be struck between an f stop small enough to give a sufficient depth of field but not one that induces diffraction. A lens will usually performs best at the mid point of its f stop range. I usually don't go over f 16, and it is better to stay around f 8.