Transcription: The aperture opening and shutter speed determine the amount of light striking the film, and the film speed indicates the film's sensitivity to light. F-stops and shutter speeds are arranged so that each step makes the same change to the amount of light striking the film. The set of steps that let in twice as much light as the previous step are called full stops. Film speeds are also arranged in full stops. Each change in film speed is twice as sensitive as the last.
(.mp3)