Quote Originally Posted by Dan Fromm View Post

At 1:2, magnification = 0.5 so you'll need approximately 1 stop more light or exposure time or to open up one stop from whatever the meter says. At 1:1, magnification = 1.0, 2 stops more light or exposure time or open up two stops from whatever the meter says. If your camera has TTL autoexposure it will take care of this for you, whether you want it to or not.
To clarify, that's if the meter reading is not taken through the lens, or through the lens with the lens focused at or near infinity. Through the lens metering will always compensate for light loss due to close or macro focusing. If you are using through the lens metering on a manual camera and meter when at your desired focus point, it will read correctly, as it is reading the actual amount of light reaching the image plane.