If your subject is at twice the distance from the lens than the focal length of that lens, it is a factor of 4, or 2-stop difference (e.g. 300mm distance using a 150mm lens). If the subject is 3 times the distance from the lens than the focal length, it is a factor of 2, or 1-stop adjustment. If the subject is at an infinite distance from the lens, the factor is 1, or 0-stop difference.