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Old Feb 10, 2011, 6:11 AM   #1
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Default Calculate the power from flashes used together?

Is there a method for calculating the guide number for a pair of flashes used together?
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Old Feb 10, 2011, 8:05 AM   #2
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The guide number is divided by the distance between the camera and the subject to suggest an aperture. Once the flash is no longer on the camera or if the flashes are no longer side by side it becomes very complex.

One solution would be to set up the camera and flashes in the desired configuration, use the histogram and highlight tools to judge the correct exposure by taking test shots, measure the camera to subject distance then multiply by the aperture to derive a guide number. Be aware that this number is only valid for this configuration and subject.

A studio photographer would use a flash meter to calculate exposure and probably still bracket.
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Old Feb 10, 2011, 10:09 AM   #3
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Quote:
Originally Posted by iowa_jim View Post
Is there a method for calculating the guide number for a pair of flashes used together?
Without modifiers:

If they are on opposite sides of the subject, treat them as individual

If they are together then:
http://photography-on-the.net/forum/...d.php?t=944815

-> The camera position has no effect on the lighting...
It's always the subject to flash distance, and not to the camera.

Last edited by NHL; Feb 10, 2011 at 4:59 PM.
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Old Feb 10, 2011, 6:58 PM   #4
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I should have specified my plan to use them side by side to drive my iso down.

The link led me to an equation that states the combined GN is the square root of the sum of the squares of the GNs being combined. That matches some other limited data I've come across.

Thanks for the help!
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