The camera works out the strength needed for the flash by metering the subject. You can also dial in more or less power with flash compensation. A lower power is often useful outside: it creates a fill light that subtly lifts shadows.

DSLRcamera anatomy

The shutter button triggers the camera’s shutter, which exposes the sensor to light that enters through the lens. A half-press of the shutter triggers the autofocus and locks the metering.