“The circle of confusion is basically about the degree of tolerance that the human eye has before it starts to make a clear distinction between an object that is out of focus and one that is in focus. Let’s say that there you are shooting an arrangement of dinner spoons laid out on a straight line. You are using an aperture of f/4. You set your AF point on one of the spoons and click. Do all the spoons appear in focus? No. While the spoon you focused on appear tack sharp, the one immediately in front and immediately behind appear to be out of focus. Now change the aperture to f/8 and make the exact same image again. Did you notice the difference? The spoon in focus is tack sharp. But the spoon immediately in front and behind also appear to be reasonably sharp. If you set your aperture to f/11 and remake the shot all three spoons will appear perfectly sharp.””