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Understanding Flash Guide Number (and Common Misconceptions)


Mystified by talk of “guide number” and “flash power”? Gerald Undone made this helpful 10-minute video that explains everything you need to know about the light from strobes and speedlights, from common misconceptions to practical formulas that will help you light your shots.

Here’s an index of subjects covered in the video along with the time they’re found at:

0:48: Guide Number Formula
1:02: Canon 600EX-RT Guide Number Explained
1:57: Misconception: Forgetting about f/1
2:57: Using Guide Number to Find Subject Distance
3:51: What If the Subject Was Closer? (Inverse-Square Law)
4:48: Practical Uses: How to Figure out Correct Flash Power
5:23: Why Flash Calculators Don’t Solve for Flash Power
5:50: How to Calculate Flash Power in Your Head
6:12: Find the Correct f-stop at Full Power
6:36: One Stop of Flash Power Equals One f-stop
6:58: Powers of 2 Make This Even Easier
7:25: How to Memorize All the f-stops Easily
7:44: A Simpler Way without Memorizing Numbers: Turn the Dials!
8:08: ISO Impact on Guide Number
8:33: What about Shutter Speed? Does It Affect Flash Power?
8:57: Final Thoughts: Know Your Lights!

“Hopefully this video will prevent you from taking too many ‘test shots’ while you get to know your lights,” Gerald says.