Facts about camera flash
A flashgun provides a brief burst of light – but the duration can be varied to alter how much flash ‘power’ is added to the scene. The amount of power needed will depend on the aperture used (the wider the f/stop setting, the less power required) and how far away they are.
The power of the camera flash falls away with distance. The maximum power varies – the built-in camera flash has much less power than add-on units – but once the subject is more than a few paces away, flash has little effect.
If your subject is within range, you can leave the camera to set the flash exposure automatically, or switch the flash to manual mode (if available) and work it out for yourself using the camera’s Guide Number.
You take the distance to your subject and divide this into the Guide Number to get the lens aperture you need for the correct flash exposure. Below we’ve provided a handy cheat sheet to understanding flash guide numbers.
The ISO setting is also a factor – the higher the sensor sensitivity, the less flash power required. More power means you can shoot at greater distances, too.
The shutter speed is often not a significant factor in the flash exposure calculation, though the shutter speed matters for other reasons.
The ‘focal plane’ shutter of your camera works in a way that means that you can’t use the full shutter speed range – ordinary flash won’t work with speeds faster than the maximum ‘sync speed’ for your camera.
If you’re using the built-in flash or a dedicated external flash, most of the factors that need to be considered when calculating flash exposure are handled by the camera. For example, if you pop up the flash on a Nikon D3100, it caps the shutter speed at 1/200 sec.