Recently, integral imaging attracted a lot of researchers as one powerful candidate for the three-dimensional display. However the limitation on the image depth of integral imaging is considered as the major obstacle for the practical use. Previously, a number of researches reported on the analysis of such limitation based on the diffraction of light. But there exists the severe mismatch between the experimental results and the simulation results that appear in the previous researches. In this paper, we propose a new assumption that exactly predicts the experimental results. Based on that new assumption, we propose the quantitative method to evaluate the image depth of integral imaging. We also propose the elemental image correction scheme that removes the distortion of the integrated image located out of the central depth plane.