Flash guide number - an introduction
What is a flash guide number? Basically, it is a figure used to indicate the power of a flashgun. The number corresponds to the power of the flash. That is, the more powerful the flash is, the higher the number.
We can use this number to determine how far a subject should be away from the flash, in order for it to be exposed correctly. For example, for a flashgun of guide number 32 and the aperture is set to F/16, the subject must stand about 2 metres away. Basically the formula is like this:
Subject Distance = guidenumber/F-number
This is however just a rough and approximate guideline. Furthermore, the calculation is only accurate when the flash gun is fully charged.
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