In order to provide a reliable basis for the photometric method in emulsion chamber experiments, investigations were made on the characteristic curves of X-ray film such as SAKURA type-N and type-RR used in emulsion chambers. Characteristics curves were obtained over a wide density region by irradiating with an electron beam from an electron synchrotron. Comparison of the observed data to theoretical predictions of the one hit model shows a significant difference at high density. A modification of the one hit model is proposed taking into account the size-frequency of sensitive grains in the emulsion and the development effect on the optical density. The curves based on the size-frequency of grains are in agreement with experiment.