The light intensity is determined by how much reflected light is reaching the film plane. You used to have to carry a light meter before setting 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 tradeoff here is the cost of the camera, and the resulting size and weight. This is just like the iris in your eye reacting to bright sun light.
Photographic film has a limited exposure latitude.
If it is underexposed, it will not detect all the reflected light from a scene. The resultant print appears to be muddy black and lacks detail. If it is over-exposed, all the silver halide grains are exposed so there is no dicrimination 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.