There are obviously two types of film: colour film and black and white film. Here at Sheffield University we only have facilities for developing and printing black and white film (colour printing is a lot more complicated). The main differences in film are quality, often reflected in price, and the speed of the film. The term “film speed” is what shall be explained in this section.
All film is given an ISO (International Standards Organisation) rating. This rating is also known as the speed of the film.
Slow speed films are generally those with ISO ratings below 100 (10ISO would be very slow). Medium speed films, for general everyday use, are those with ISO ratings from 100 to 400. Fast speed films are those with ISO ratings above 400, and go all the way up to 6400ISO, which is very fast.
The terms ‘slow’ and ‘fast’ refer to the amount of light required to hit the film to produce a ‘well balanced’ negative (i.e. a negative that shows tones from black all the way through to white). A film rated as 100ISO should require the same amount of light as any other film rated 100ISO regardless of manufacturer, black and white or colour.
A 200 ISO speed film is ‘twice as fast’ (requires half as much light) as a 100 ISO speed film. A 400 ISO speed film is ‘twice as fast’ (requires half as much light) as a 200 ISO speed film, and is ‘four times as fast’ (requires a quarter as much light) as a 100 ISO speed film.
Slower films have a finer grain than faster films. The crystals that form on a slow film (e.g. 100ISO) are very small but there are a lot of them – hence a fine grain. Conversely, the crystals that form on a fast film (e.g. 800ISO) are large but there are fewer of them – hence a coarse grain. The diagram below shows the difference in grain between a slower 100ISO film and a faster 800ISO film.
Each crystal is formed when a certain amount of light hits that part of the film. Therefore, since a slow, fine grained film has lots of crystals on it, it will require more light to hit it. A faster, courser grained film, has fewer crystals and therefore needs less light to hit it. Hence a fast film can be used when there is less light available, or when you need to use a fast shutter speed (e.g. to capture sports action). If you want to take landscapes (which tend to remain quite still), you will probably want to use a slow film because of its fine grain and may want to use a tripod to keep your camera steady.
After reading the sections on shutter speed and aperture in The Camera, you should be able to deduce that you can use faster shutter speeds and smaller apertures with a fast film than you can with a slow film. Your choice of film will be determined by considering which of these aspects are important in the environment you want to take photographs in and how you want your photographs to look once printed.