Although using a fast-speed film might sound logical, medium- and slow-speed films are preferred because, even though fast films have improved over the years, slower films still produce sharper images. As a rule, use the slowest speed you can use with the light and the lens you have. If you don’t have a fast lens to compensate for the slow film speed, use a tripod and even longer exposure times.
In normal daylight conditions, the rule is that the less light there is available, the longer the exposure time you’ll need. By this logic, if you want a smaller aperture that will increase the depth of field but let in less light, you’d need a longer exposure time. With most films, this rule holds true down to 1/4th of a second. With any exposure longer than that, especially those longer than 1 or 2 seconds, film’s sensitivity to light changes. This change is known as reciprocity failure.
With black and white film, reciprocity failure typically causes underexposed, low-contrast images. With color films, however, reciprocity failure usually appears as a color shift. To compensate for reciprocity failure, you can use a wider lens aperture or, if a particular lens aperture is critital to the success of the photo, a longer exposure time than the reciprocity law would normally require. Each manufacture’s line of film reacts differently, but the films within one manufacture’s line usually react the same as others in that line. This means that some brands of film might render warmer tones, while others will render cooler tones. Information about how reciprocity failure will affect a certain brand of film can be found on the film’s package or the manufacture’s Web site.