Light is most easily controlled through the use of the camera's aperture (measure in f-stops), but it can also be regulated by adjusting the shutter speed. Using faster or slower film is not usually something that can be done quickly, at least using roll film. Large format cameras use individual sheets of film and each sheet could be a different speed. Also, if you're using a larger format camera with a polaroid back, you can switch between backs containing different speed polaroids. Digital cameras can easily adjust the film speed they are simulating by adjusting the exposure index, and many digital cameras can do so automatically in response to exposure measurements.