For example, ISO 400 film needs half as much light as ISO 200 film, so you can set the shutter one speed faster or shut the aperture down one stop when you switch from film with a speed of ISO 200 to ISO 400 film. There are plenty to choose from. Scenes with reduced lighting can often be captured with a hand-held camera without the need for a tripod. Increased depth of field can be achieved. There is a price to pay for faster film speed.
Fast film creates the image using larger grains of silver than slower films.
Clumps of these grains form the image, and their larger size can sometimes be seen in a print. Such a print is said to have poorer definition. Many of today's film manufacturers have improved their fast films so that these characteristics are less evident. For instance, some photographers may like the effect of purposefully overexposing or underexposing a certain film, and may always shoot that particular film at a different film speed setting than the manufacturer's recommended ISO number. Determining a film's effective speed for your own tastes is a matter of experimentation.