Choosing the right film for a situation can open up exciting new photographic possibilities. The speed of a roll of film is measured according to an International Standards Organization (ISO) scale, with the most common varieties being ISO 100, 200, 400 and 800. These numbers are referred to as the film's "speed" because each level is twice as sensitive to light as the last and will expose more quickly, allowing a faster shutter speed to be used. In very high light situations, the lower-speed films will allow you to have a greater variety of shutter speeds, while for sports or low light situations faster film will let you capture the action with a fast shutter speed. ISO 400 film is often considered the best all-purpose film. Many digital cameras will also let you choose a film speed, even though no film is used.
Besides the differences in film speed, normal black and white films can be divided into three classes: conventional films, tabular or other 'designer grain' films with specially adapted crystal shapes, and chromogenic films. For more about these types see the previous feature on film types (box at right.)
For sports photography or other pictures involving moving subjects use a fast film - generally 400ASA and for action pictures in artificial light or poor light without flash, or use an ultra-speed film - 1600 ASA or faster. For a gritty, photojournalistic look, use a conventional 400ASA film (Tri-X or HP5 Plus), preferably uprated to increase the grain; For portraiture, where tonality is most important, you will generally get best results from 100ASA film if using 35mm. For large format, a fast film may be easier to use. Many photographers prefer to use a 'traditional' emulsion. Chromogenic 400ASA materials are another possibility - and one I particularly like for medium format.
For landscape and architectural photography with 35mm cameras, where detail, fine grain and sharpness are vital, a slow 100ASA film is generally suitable. Conventional black and white films are easier for home processing than chromogenic films for beginners in home processing. If you intend to scan your negatives to make prints, only chromogenic films enable you to use the infrared dust removal channel in good film scanners. Conventional films and chromogenic films are generally more forgiving to errors in exposure than tabular grain films. For general use modern 400ASA emulsions - tabular grain or chromogenic -generally good results under a wide range of conditions.