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.
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.
