Contrary to popular belief, depth of field in a photograph does not change abruptly from being pin sharp to soft. Instead, the softness occurs as a gradual transition.
In actuality, objects both in front of and behind the focusing distance immediately begin to lose focus, even if this is not perceivable to the human eye.
Because of this, a more exact term is used -- "circle of confusion" -- to describe how much a point needs to blurred before it is perceived as being soft and unsharp. When the circle of confusion becomes obvious to our eyes, that region is deemed to be outside the depth of field.
A sharp circle of confusion is defined as one which would go unnoticed in an 8-by-10-inch print, when viewed from a regular distance of about one foot. Manufacturers assume the circle of confusion is negligible if it's no larger than 0.01 inches when enlarged. However, it should be noted that the circle of confusion will change with different print size and viewing distance combinations.