An important concept to understand in lighting is the ambient light or ambient exposure.

The ambient light level is the amount of light that’s normally present in a setting. If you’re inside, the ambient light is probably provided by lamps or sunlight coming in through the window. If you’re outside in the daytime, the ambient light is probably the sun.

When you “expose for the ambient,” you’re taking a picture with the available level of light. You pick the right mix of camera settings – shutter speed, aperture, and ISO – that makes a reasonably well lit photo based on the light that is currently available to you.

This is particularly important to understand when you start working with external lights and flashes. Then, you’ve got the option of working with the ambient – i.e. exposing for the ambient and then adding some lights to fill in shadows – or overpowering the ambient – picking camera settings that underexpose the ambient and then light the whole scene with speedlights.