Home Blog Portfolio Client Login

Friday, August 26, 2011

Flash Facts - Calculating manual flash

guide number = f/stop x flash to subject distance

flash to subject distance = guide number / f/stop

f/stop = guide number / flash to subject distance

These three equations are all that you need to know in order to manually calculate flash exposure.

For instance the LumoPro LP160 has a guide number of 140 @ ISO 100. So, if I place the flash 10 feet away from my subject the equation would look like this:

f/stop = 140(guide number) / 10 (flash to subject distance)

If you solve the equation you would set the aperture to f/14 to have the correct exposure.

You can plug in any two variables to get the correct flash exposures needed. Just keep in mind that these formulas are a good starting point. The end decision of exposure is up to you.

No comments:

Post a Comment