Lighting102 1.2 - Position | Distance

So onto part II of the first section of Lighting102; varying the distance the flash is to the subject, and the distance the subject is to the background. The two distances make an important ratio. More of that in a second. In this triptych Fred is almost exactly 1m in front of my living room wall in each shot. I've varied the distance the flash is from him (slightly camera right as you can see by the shadows).

Feel free to click on the image to go to my flickr profile and see a larger version.

So the background is obviously more lit at 1m and at 3m because the power of the flash is higher to compensate for the distance to the subject (to get a consistent exposure of Fred). Somehow without planning it I've demonstrated the 'inverse square law' - double the distance and you get four times the light fall off. In our case doubling the distance to Fred to 1m (from 0.5m) means the power of the light has to go up two stops from 1/128 to 1/32(1 stop is doubling its output). Tripling the distance to Fred from 1m to 3m results in a 3 stop increase from 1/32 to 1/4 to get a consistently OK exposure. I was doing this by eye, by the way. No flash meters were harmed in the making of these images.

I've also noticed something else. Look how much more even the light is from a bare bulb flash at 3m than at 0.5m. Look at the shadows on Fred's face. I might use that one day if I get caught shooting without any umbrellas or material to soften the light from my flashes.

Anyway I know the maths is hurting you but one last set of numbers. The ratios worked out flash to subject/subject to background are as follows:

Left - very dark background - 0.5m/1m= 1/2
Middle - medium grey (in mono) - 1m/1m = 1
Right image - light background - 3m/1m = 3

Those ratios will work (because they are ratios) wherever and whenever I next shoot so I'm going to try to keep them in my head. Might also try to work out my average stride length to estimate distances over a few metres in case I set up with big spaces between subject, flashes, and background. Though I doubt my flashes with cheap cactus v2s radio triggers are going to work over very long distances anyway.