In digital holographic microscopy, shot noise is an intrinsic part of the recording process with a digital camera. We present a study based on simulations describing how shot noise influences the quality of the reconstructed phase images under different beams intensities configurations. A model for image quality estimation based on the decision statistical theory proposed by Wagner and Brown (R.F. Wagner and D.G. Brown, "Unified SNR Analysis of Medical Imaging-Systems," Phys. Med. Biol. 30, 489-518 (1985)) will be presented. A simulation will depict the variations of the reconstructed phase images SNR for different distributions of the total intensity between the reference and the object beam.