Photographic emulsions based on silver halide grains and their developers are characterized by the H-D curve, relating density with exposure. The H-D curve depends on a number of parameters from both of the emulsion and of the developing process, and always has a typical shape. In this study we present a H-D curve obtained with an ultra fine emulsion, Colourholographics BB640, developed with a high contrast developer, Kodak D8, whose shape differs considerably from that of the standard curve. This process leads to very high values of density as well as to an apparent holographic overmodulation. Based on these results an image formation mechanism is proposed.