In ptychographical imaging with X-ray or electron beam, the recorded speckle intensity pattern of far field diffraction was often saturated by the presence of strong zeroth-order which was undesirable. A beam-stop was widely used to block the area of strong central diffraction in the pattern. The image was then reconstructed by disregarding the diffraction constraints within that area where inaccuracies exists due to saturation or blocking of the beam. Though reconstructions were obtained in this way by omitting saturation regions, the rationale behind this method was not discussed in detail and its effect on the final image quality was not studied. The physics of this method was analyzed theoretically and experimentally in this study. The convolution effect between illumination and object functions was able to retrieve and bring back the information lost in the blocked or saturated area from its neighborhood. This was possible while the saturated or blocked area was not wider than the spectral width of illumination, and once it exceeds this minimum, the quality of reconstruction would degrade seriously. The theoretical model was verified by numerical simulations and experiments.