What does this mean for black and white photography? As stated, the salt grain of black and white film decides the proper speed of a particular film as well as the graininess of the final print. This alone is probably the most important aspect in selecting a black and white film to use. To make it easy on the photographer the film speed is represented as an ISO number (formerly known as an ASA number). Typical film speed ratings are ISO 50, ISO 100, ISO 125 and ISO 400, however there are higher ISO films available and some in between. The higher the ISO number the faster the film becomes and therefore the grainier the film becomes.
As the film speed number doubles the faster film requires half the light to form an image. ISO 100 film needs half the light of ISO 50 (ISO 100 is twice as fast as ISO 50) and ISO 400 film needs one eighth as much light as ISO 50 (ISO 400 is eight times as fast as ISO 50).
From this point you can make a decision on the film speed you’d like to use for black and white photography. To quickly recap:
§ ISO 50 is a slow film to use when there is ample light to allow an acceptable exposure and/or the final print must have minimum graininess.
§ ISO 400 is a faster film to use when there is low light available and/or graininess does not matter to you in the remaining print.