Guide Numbers—How Do They Work Again?

Started 1 month ago | Questions thread
OP rfsIII Regular Member • Posts: 348
You need the inverse inverse square law

Best answer is to check out the tutorials and blog posts at https://neilvn.com/tangents/ . If you can’t do that and you want to evenly light people at different distances you have to do the exact opposite of what you would think. Try these ideas first in TTL and if it doesn’t work switch to manual. Start at 1/32 power and work up from there.

  1. You can try zooming in your light to 85mm or whatever it’s highest number is and aim the flash at the farthest person. The idea behind this is that you’ll catch the rest of your subjects with the less-bright edges of the beam and they won’t get so blown out.
  2. If there’s a ceiling you can also point your on-camera flash upward at a 45 degree angle; the idea again is that you’ll put more light on the farthest person. Niel has some good tips for this. 
  3. If there’s a neutral-colored wall nearby you can even out the Intensity of the light by bouncing it off of the wall.
  4. The fourth method uses the inverse square law in a different way. By moving your light as far as you can from the group you are reducing the percentage of variation in amount of light. It’s insane but it works if you can get enough power from your lights.

Max5150 wrote:

If the OP doesn't mind, I'd like to ask this very basic related question. When I'm using my flash to take pictures of people at night, and dialing in the power setting manually (1/16, 1/8, 1/4) to keep the amount of added light at a minimum so I preserve the night look as much as possible, that power setting is basically f-stops and follows the inverse square law with distance, right? I was having a heck of a time getting good shots last night on my patio as my subjects were all at different distances. Maybe I should have used TTL flash. I'm using an EM1.3

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