If your camera has spot metering capability, then spot meter on the face, if not, then there are four possible solutions.... Solution one, modified zone system exposure, you want the face in zone V, as the face is the most important part of a portrait, focus on eyes. Second is move the camera in close, so only the face is in the frame, take a meter reading on the face, see what the exposure is, then move back and frame, setting the camera manually. Third is a separate light meter, so you can meter on just the face. Fourth is a reflector or fill flash to get more light on the face. The last one is actually the ideal, it doesn't blow out the background, because it reduces the brightness differential.