inverse-square law, fall-off

Light from a point source falls-off inversely to the square of the distance. Move the light from 10-feet-away to 20-feet-away and you have only 1/4 of the intensity; 40-feet, 1/16 th. Reflectors and lenses reduce fall-off; Diffusion increases it. Tip: to minimize fall-off within a scene, use hard lights as far away as practical.