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SLOW FILM
Film with speeds in the ISO 25 to 50 range are considered to be slow films. ISO 64 film is sometimes also categorized as a slow speed film because its characteristics are so similar to films of lesser speed. The coating on the emulsion of slow film is often relatively thin and its silver halide grains are unusually small. Characteristically, they have extremely fine grain, show minute detail with great sharpness and will register a wide range of tones.
Bright light is needed for hand-holding the camera, otherwise the use of a tripod for long exposures will be necessary, particularly when increased depth of field is desired, necessitating very slow shutter speeds. On the other hand, some photographers will use slow films to purposefully show blur in moving objects, even in bright light. Images from slow films can generally be enlarged significantly and still show great detail.
MEDIUM-SPEED FILM
Film with speeds in the ISO 64 to 200 range are considered to be medium-speed films. This is the most popular speed of film, finding wide general use by amateurs and professionals alike. It has a good balance between light sensitivity and fine grain. Medium-speed films are more versatile and convenient to use than slow films, since shutter speeds are faster and permit more opportunity for hand-holding a camera. They still require relatively bright light (daylight or studio flash), but will perform adequately under overcast conditions, having a wider exposure latitude.
They make a good general, all-purpose film. Medium-speed film is less contrasty than slow film. Sharpness, grain and tonal rendition are usually quite good, and black and white films of this speed react well to fine-grain developing. Enlargements to 20" by 24" and sometimes larger can be satisfactorily made from properly-exposed 35 mm medium-speed film. Many films in this range produce their best results with short exposures. Note that some people categorize films that have speeds of ISO 160 and 200 as fast films.
FAST FILM
Films can be subdivided into fast films (ISO 400 and 800) and super-fast or ultra-fast films (ranging from ISO 1000 to 6400).
Fast film permits very fast shutter speeds and small apertures under normal daylight conditions, and its wide exposure latitude and relatively-good grain structure make it an extremely versatile film. This is the film you would use for action shots, in dim or uncertain lighting, for “freezing” fast-moving objects or when you require great depth of field. The quality of images from fast films is remarkably good, and many occasional photographers use ISO 400 film in 35 mm size for all of their family photos and general picture-taking.
SUPER-FAST FILM
Super-fast films (ISO 1000 to 6400) permit picture-taking in extreme low-light conditions, and in fact some are termed “surveillance” films because of their ability to capture images surreptitiously in dim light without flash. They do not show detail as well as slower films. Images are characteristically quite grainy - particularly evident in enlargements of 8" by 10" or greater from 35 mm film - and lack contrast, but are surprisingly good considering the trade-offs. Photographers who wish to have the effect of strong graininess in their images can achieve it with this film.
Since much of their use may occur in contrasty lighting conditions (under a street light at night, for example), the film’s inherent lower contrast can be an advantage. This film is not intended for use in extremely bright conditions, but can be used for just about any other level of illumination. Because of that, it is a useful back-up film to carry since it will perform where other films can’t when flash equipment fails. If it must be used in bright light where over-exposure is a possibility, a dark neutral-density filter can be fitted to the lens.
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