Um, er, ah, ignoring complexities due to pupillary magnification not equal to 1.0, the magic formula for exposure compensation given magnification is effective aperture = aperture set (f/ number) * magnification + 1

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.