The majority of black and white films are designed to give you a negative image in black silver. In other words, the latent image recorded in the camera is strengthened into visible black silver during developing, and then the remaining creamy halides are removed, leaving the film with a negative image (subject highlights darkest, shadows lightest) in clear gelatin.
General-purpose pan films range from about ISO 25/15° to ISO 3200/36°. The slower the film is, the finer the grain and the better its ability to resolve detail. Contrast – the range of grey tones formed between darkest black and complete transparency – is slightly greater with slow films than fast films. This tendency is taken into account in the recommended developing times, which tend to be shorter for slow films.
Films between ISO 100/22° and ISO 400/27° offer a good compromise between speed
and graininess. ISO 400/27° film gives prints with a just visible grain pattern (noticeable in areas of even grey tone, such as sky) when 35 mm is enlarged beyond 10 × 8 inch although this depends upon negative development and the type of enlarger light source used;
In general it is best to use the slowest film that subject conditions sensibly allow, especially when working with small negatives such as 35 mm. For occasions when you want to give long exposures to create blur in moving objects, this too will be easier on slow film.
However, slow film may prevent you from stopping down enough for depth of field, or necessitate a tripod in situations where it is impractical. Fast film is necessary for reportage photography under really dim conditions, and action subjects or hand-held telephoto lens shots which demand exceptionally short shutter speeds. Although reportage photographers might use flash for certain situations, and therefore can use a slower film, if they were working under conditions where they do not wish to be conspicuous this obviously would not be appropriate. If you are working in well-lit conditions but deliberately want to use very fast film to give a grainy texture to the image you may have the problem of too much light. In this case, use a neutral density (ND) filter to reduce exposure.
Chromogenic films. A few monochrome negative films, such as Ilford XP2 Super and Kodak T400CN, give their final image in a purple-brownish dye instead of silver. Extra components
in the emulsion layers form tiny globules of dye wherever silver is developed. The processing you must give these films is known as ‘chromogenic’, and this finally bleaches away all silver, leaving your image in dye molecules alone. Since chemicals and processing stages are the same as for colour negatives, it is easy to hand your exposed film in to a high street mini-lab anywhere for rapid black and white results.