In this paper, a new quantity, normalized nonlinear error (NNE) is proposed for the first time to our knowledge for the estimation of the gray scale fidelity of images retrieved from volume holograms stored in photorefractive Fe∶LiNbO3 crystals. Based on this, the dependence of gray scale fidelity on recording object-reference beam ratios is experimentally investigated, and the results agree well with theoretical predictions. The dynamic evolution of the normalized nonlinear error during hologram recording is also studied. The results show that a satisfactory gray scale fidelity can be obtained as long as the recording beam ratio is carefully chosen, even if the recording time is short, hence the diffraction efficiency is quite low.