Strobist Lighting 102, Lesson 1.2

Fri, Aug 16, 2013
Continuing on with the Strobist's Lighting 102 series, here's my second entry.  Initially, I'd shot this exercise along with the first lesson and used the same badass robot warrior guy.  However, as I looked through the results I wasn't happy, so I decided to shoot it again: this time with a happy, dusty little buddha statue.  Here's the result:

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So here's how this exercise works: a single, off-camera flash stays roughly the same angle from the subject, but moves from far away (12ft) closer (6ft), and then in tight (1ft).  In this case 1ft might have been too tight;  I certainly overexposed the subject.  Nonetheless, these three images demonstrate the point of the lesson pretty well.  As the light source gets closer, and therefore more powerful, the subject receives more light. This makes sense, right.  The thing that's a bit less intuitive is this: relatively speaking, the amount of light hitting the wall falls in comparison to the amount of light hitting the subject.  So as the camera is stopped down to let in less light and keep the subject properly exposed, the background wall gets darker.

Explained another way, shot-by-shot, with some loose math included for effect:
  • For all three shots, the buddha and the wall behind the buddha remain the same distance apart, 6ft.
  • First shot: the flash is about 12 ft away from the buddha and 18ft away from the wall:
    • The wall is receiving 50% less light than the buddha.  The seems like a lot, but wait!
  • Second shot: the flash is 6ft away from the buddha and 12ft from the wall:
    • The wall is receiving 75% less light than buddha.  Wow, big drop off - notice it went grey?
  • Third shot: the flash is less than 1ft away from the buddha and 6ft away from the wall: 
    • The wall is receiving 97% less light than the buddha, so the wall goes nearly black (it probably would have been black if I wouldn't have overexposed the subject).
Keep in mind that throughout, the flash is actually getting closer to both the subject and the wall.  So there's more light overall on both the buddha AND the wall.  It's just that because the buddha gets so much closer to the light than the wall ever does, the amount of light on the buddha skyrockets compared to the wall.

Mathematically this effect of relative intensity of light over a given distance is explained by the inverse square law.  It's a big deal in a lot of fields including astronomy.  It basically describes why it is that our average little yellow daystar (the sun) lights up the whole sky, when super massive blue stars far off in space appear as tiny little dots in the dark night sky; answer: it's A LOT closer!