Taking a Picture: Film Speed
The first step after loading the film is to focus the image on the surface of film. This is done by adjusting glass or plastic lenses that bend the reflected light from the objects onto the film. Older cameras required manual adjustment, but today's modern cameras use solid-state detectors to automatically focus the image, or else they are fixed-focus (no adjustment possible).
Next, the proper exposure must be set. The film speed is the first factor, and most of today's cameras automatically sense which speed film is being used from the markings that are on the outside of a 35-mm cartridge. The next two factors are interdependent, since the exposure to the film is the product of light intensity and exposure time. The light intensity is determined by how much reflected light is reaching the film plane. You used to have to carry a light meter to set the camera exposure, but most of today's cameras have built-in exposure meters. In addition to the brightness of the scene, the larger the diameter of the camera lens, the more light will be gathered. Obviously, the trade-off here is the cost of the camera and the resulting size and weight. If there is too much light reaching the film plane for the exposure-time setting, the lens can be "stepped down" (reduced in diameter) using the f-stop adjustment. This is just like the iris in your eye reacting to bright sunlight.
Photographic film has a limited exposure latitude. If it is underexposed, it will not detect all the reflected light from a scene. The resulting print appears to be muddy black and lacks detail. If it is over-exposed, all of the silver-halide grains are exposed so there is no discrimination between lighter and darker portions of the scene. The print appears to be washed out, with little color intensity.
There is an advantage to having a faster film in your camera. It allows you to have a smaller aperture setting for the same exposure time. This smaller aperture diameter produces a larger depth of field. Depth of field determines how much of the subject matter in your print is in focus. Sometimes, you may want to have a limited depth of field, so only the primary object is in focus and the background is out of focus.