Regarding the object scattering light as a superposition of light waves emitting from a lots of point sources on object surface of which phases have random values, in the paper, wee investigate the diffracted field of digital holographic reconstructed image and defocused image. The definition of depth of focal (DOF) is also given based on the feature that the reconstructed image is displayed in pixels. According to the characteristic that the physical dimensions of pixel are associated with computation method, focal depth expressions are deduced when image is reconstructed with different algorithms. Finally, experimental verification is given.