Hey Dave,
I just realized I made a mistake when talking about reducing the power of your flash. You don't multiply the power setting by the guide number, because the guide number is not a linear value. Every time you halve the power of your flash, you divide the guide number by the square root of 2. Setting your flash to 1/2 power would require opening up your aperture by one stop, not two. The progression of opening up the aperture is as follows:
1/2 power : 1 stop
1/4 power : 2 stops
1/8 power : 3 stops
1/16 power: 4 stops
etc.
-Chip
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People who do stupid things with dangerous substances often die! -me
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