When you use your camera to automatically chose aperture and shutter speed, what your camera is actually doing is using the built in light meter and measuring how much light is being reflected to the camera.
But that doesn’t mean that’s all there is to it. You should also think about the angle of the light entering the frame, what kind of shadows you want, and whether you want to use fill-in-flash (using flash to light the subject if you have a really bright background).
