Rev. Sidney Flack wrote:
...It works like this. If you halve the distance, the light intensity
increases by four. If you double the distance it falls off by 1/4. In either case the change is 400% or 1:4.
The decrease in intensity would be reduced TO 1/4 of the original not by 1/4.
Thus, if you have two lights of equal intensity lighting a subject with one light 4 (4x4=16) feet away and the other 8 (8x8=64) feet the ratio is 1:4 (or 4:1) (64/16=4).
This ratio is equivalent to a 2 stop change in lighting since 1 stop change is 1:2 and 2 stops is 1:4. This will make the following clearer.
Now to the original example with a ratio of 1:3 instead. Keeping the closer light at 4 feet, the farther light would then be about 6.93 feet away and represents roughly 1 2/3 stops (6.93/4=1.7325) less light than the first.
This corresponds to something less than a 2 stop difference, i.e., a 1:3 ratio.
Peace! Sidney
