We have studied the dynamic propagation of an optical beam from selfdefocusing to selffoc using in photovoltaic photorefractive crystals LiNbO3:Fe in the framework of a t wodimensional band transport model. The results show that dark irradiation pla ys an important part in the transition process. The transition occurs only when th e normalized dark irradiation is very small. We discuss also the influence of py roelectric effect on the transition process.