Fill-in flash is a great way to balances dark shadows on people’s faces caused by strong sunlight, which isn’t the best lighting for people but sometimes you just gotta go with whatever you’ve got.
You can use your camera’s built in flash or a speedlight for fill-in and it’s pretty simple. Basically you’re telling your camera’s flash to fire a burst of light one or two stops below whatever its light meter considers to be the correct exposure. So the sun is lighting most of the image and your flash is only affecting the shadow areas.
By changing the power output you can control how deep (dark) you want the shadows to be. For example minus one stop will give you gentle shadows, minus 2 will give you heavier shadows. If you don’t want any shadows at all you can fire a balanced fill flash which will cancel them out altogether.
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