Often misunderstood as the diameter of the lens, this is the focal length of your lens (usually measured in mm). In smaller focal length lenses the field of view is wider, allowing in more light. As the focal length increases less light can get through the lens which results in a narrower telephoto field of view and the camera detects a more detail from its target area. This works in a similar way to using a pair of binoculars.
Essentially this factor determines how much you can optically zoom in and out on a varifocal camera.