A flashgun's power is quoted as a 'Guide Number' (GN), usually set at ISO 100. This offers a useful, standardised measurement of flash power, and this number can also be used for exposure calculations. To work out the lens aperture to use, you divide the Guide Number by the subject distance, in metres.
A typical built-in flash might have a GN of 12, so for a subject 2 metres away the aperture should be 12 divided by 2, or f6. If this sounds complicated, the good news is that in practice modern flashguns carry out these calculations automatically.
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