You are assuming light meters are always correct. They are not, in fact reflected light meters are very easily fooled and tricked. For instance, what if you were trying to take a picture of something in a snowy scene with a ISO of 400 and aperture of f/2. It could well be that the correct shutter speed to render the snow white would be less than the camera's maximum shutter speed, however the light meter would think the correct shutter speed is 1/8000 (which few if any cameras can do) because it tries to render the snowy scene a neutral 18% gray. Blind reliance on light meters is a very bad way to take pictures- you can almost always get a better picture if you carefull spot meter the scene or use an incident meter. No one does this anymore in the age of digital because they can assume "I can just fix it later"- but they actually can't because of digital's low dynamic range..