#2. "RE: Metering and exposure compensation on the D800..." In response to In response to 0
ISO is essentially an amplifier. It's like turning up the volume on a distant radio station. But if there are noisy pixels, they are also amplified. So if you had a radio with a little static, the static would be more easily noticed if it was louder.
Exposure Comp signals the camera to change the exposure by changing one of the variables - Aperture, Shutter, or ISO - depending on which automated mode you have selected. If you are in Aperture Priority with a fixed ISO, exposure compensation changes the shutter speed. With Manual Mode, Exposure Comp just changes the scale.
By dialing in exposure compensation, you can account for scenes or subjects that need more or less light than metering suggests. The meter suggests a neutral exposure based on the metering mode selected. A dark subject dominating the frame would need less light since it needs to be rendered darker than neutral. A subject in a brightly lit scene - such as a snow scene - would need positive compensation to make the snow white rather than neutral gray.