Film speed or sensitivity

The sensitivity of the film to light is prescribed by the manufacturer. It is obviously of importance in assessing the camera exposure and also needs to be known to the film processer, because it affects the processing time. Generally, film sensitivity depends on the size of the silver halide grains in the emulsion. Larger grains lead to higher sensitivity, which means that less camera exposure is necessary. An indication of sensitivity or speed is given on every film carton in the form of an ASA or DIN figure or both. The figure is simply a guide to a relative position on a scale. It does not carry any such description as ‘speed’ or ‘sensitivity’. The usuarinscription is recommended meter settings, meaning that any meter used to assess the camera exposure should be set to that figure. This is because the sensitivity of an emulsion cannot be stated in isolation. It depends on other factors, the most important of which is the processing. The ASA scale of film speeds is arithmetic. A doubling of the film-speed figure indicates a doubling of film speed or sensitivity. The DIN scale is logarithmic. A doubling of the film speed is indicated by an increase of 3 in the DIN figure. Use of the DIN scale is confined largely to Continental Europe. Most of the rest of the world uses the ASA scale.