This is due to the fact that FP4 features a double layer emulsion. It combines a low sensitive fine grain layer and a highly sensitive layer with larger grains. This kind of film was already known in the 1920s and 1930s (e.g.
This is due to FP4's dual-layer emulsion, which includes a low-sensitivity fine-grain layer and a highly sensitive layer with larger grains. Such film had been known since the 1920s and 1930s (e.g.