Geometric optics teach us to expresses that a lens is only able to form a sharp image of an object at a given focus point. Objects nearer or further as to distance will image as unsharp. However, as a matter of common observation, objects before and behind the distance focused on likely appear sharply focused. The reason is, there exists a span before and behind the point of focus that reproduces acceptably sharp. This span is called “depth-of-field".
The lens handles each point of the subject individually. By point, we are talking about a spot so tiny it has no dimension. The job of the lens is to project an image of this point on the image plane. Because of residual uncorrected aberrations and inadequate focusing, the image of points is never reproduced as points. Instead, all points reproduce as circles. These are called circles of confusion because they have scalloped boundaries and are juxtaposed alongside adjacent circles of confusion.
It is the size of these circles that is the determining factor as to whether we pronounce an image as tack sharp or blurred. If the circles are tiny and below our ability to resolve them as circles, we pronounce the image “sharp”. If these image points are seen as circles and not dimensionless points, we pronounce the image unsharp. For the average person with good vision, viewing a photograph in good light, these circles of confusion must be no larger than 2/100 inches when viewed from 20 inches ( ½½ mm). when viewed from 20 inches
This is the stuff of “depth-of-field”.