The f number is the number of times the diameter of the aperture divides into the focal length of the lens.
Most cameras have a built-in light exposure meter that is used to determine the correct exposure.
A metal cylinder placed between the camera body and the lens. This device shortens the focusing distance to a few inches. For example, an ISO 100 film requires less light than an ISO 50 film. Each time you double the film speed, half as much light is needed for a correct exposure. Faster films need less light but they produce grainier pictures.