Friday, February 7, 2014

Day 26, Learning 26: Inverse Square Law

Honestly, I thought I'd covered this one early, but I guess not.  It's one of the sides of the lighting "Rubik's Cube" I mentioned before, and will definitely, definitely get around to discussing.

Inverse Square Law.  Every pro photographer knows it and so had you.  It's simply this, if you double the distance from the light source you end up with 1/4 the light.  If your light is 4 feet from the subject and you move it to 8 feet, you end with 1/4 the amount of light, or 2 stops difference.  If you move that 4 foot light closer, to 2 feet, you have double the amount of light.

In terms of F stops it looks like this:

LIGHT:  2'          4'          8'          16'
F STOP: f16      f8          f4          f2

At 2 feet your proper exposure is f16; at 16 feet your proper exposure requires the aperture to be f2.

So yesterday I wrote that if you knew the inverse square law, you could balance the shadows and light from a single flash above and a reflector below....  If the flash is located four feet above the subject's face and the reflector 4 feet below (a total of 8 feet), then the light hitting the reflector is 1/4 the amount illuminating the face.  That's 2 stops.  But of course the light has to travel back up to the face so we're looking at closer to 3 stops difference between light and shadow (this doesn't take into consideration the amount of light lost in the reflective surface which may be white, gold, silver, or a mirror).  If you want less of a difference (i.e., a smaller ratio), then all you have to do is raise the flash higher so that the distance between the flash and the subject is closer to the distance between the flash the reflector.  If the flash is 8 feet above the subject and 12 feet above the reflector, than the shadow is closer to 2 stops.

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