Calculating camera flash exposure
Exposure metering for flash is handled in a different way than for non-flash ‘ambient light’ exposures. The flash fires briefly just before the shot is taken, and the light reflected back by the subject is used to work out the exposure and flash settings. This ‘preflash’ is all-but imperceptible to the human eye.
Inevitably, automatic flash will not always give the results that you want. It’s possible to switch the flash to manual mode, then choose the power you need to suit the subject distance or the aperture setting you want to use.
You can choose ½ power, ¼ power and so on – the power is adjusted in the same ‘halving’ and ‘doubling’ steps as regular exposure settings.
Most of the time, though, it’s simplest to leave the flash set to auto and use the flash exposure compensation control to reduce or increase the power as needed.
You can use flash in any of your camera’s exposure modes. Manual (M) will give you the most control over how it balances with the ambient light, but Program (P) can still give great results if you don’t want the hassle of manual adjustments.
Flash timings explained