The zone of acceptable sharpness within a photo that will appear in focus. In every picture there is a certain area of your image in front of, and behind the subject that will appear in focus.The lower your f-number, the smaller your depth of field. Likewise, the higher your f-number, the larger your depth of field. For example, using a setting of f/2.8 will produce a very shallow depth of field while f/11 will produce a deeper DoF.