Depth of Field (DoF) refers to
the zone of acceptable sharpness in our picture. Let’s take that a bit further. When you focus your lens (any lens) at any given distance, technically speaking, only that point (and all points along that same plane – plane of focus) should be sharp.
However, in reality, some distance in front of and some distance beyond the focussed plane is acceptably sharp. This zone of acceptable sharpness is what we call the Depth of Field.
DoF depends on the focal length of the lens, the distance between the lens and the subject, and the aperture used.
Focal length of the lens: The wider the focal length, greater the DoF (aperture and focussed distance being constant). As an example, a 24mm lens will inherantly have greater DoF than a 28mm lens, which in turn will have greater DoF than a 35mm lens. Telephoto lenses have shallow DoF. As an example, a 300mm lens will have less DoF than a 200mm lens, which again will have lesser DOF when compared to a 100mm lens.
Distance between lens and subject:
Greater the distance between the lens and the subject, greater the DoF (focal length and aperture being constant).