Auto exposure modes and camera metering systems are programmed to give you an exposure that will is based upon18% reflective grey or middle grey. Point your camera at a white wall and it is programmed to expose so that white wall is middle grey. Point your camera at a black wall and it will expose the black wall so it is middle grey. Put the camera in manual exposure mode and the camera meter will still want to make these subjects middle grey, but at least you will have to set both the shutter speed setting and the aperture setting–hopefully during that process you will ask yourself; “Is the reflective value of my subject going to result in a proper exposure?”