Depth of Field (DOF) is the focus area on the front or back of the focal plane, typically a subject or person. 



The most important factor to control DOF is Aperture. The size of the DOF is inversely proportional to the aperture size. The more you open the Aperture, the more the DOF is affected. That is, at low aperture values such as f/1.8 or f/2.8, the DOF becomes small. The aperture values such as f/16 or f/22, the greater the DOF. In photography, the shallow, selective DOF separates the object in the foreground from the background and forms "Bokeh" spots, which are simply called "background blur". DOF is easily observed in Live Viewer.