I have the answer to my problem from an old post in 'dyxum' forums: the Sony Alpha cameras, unlike the Nikon cameras, when in multi segment (forty segment on my A700, A550, A580) do the exposure metering taking much consideration for optimum exposure under the focus point currently selected. Even in manual focus mode, the currently selected focus point is properly exposed. So I will just have to chase my actresses face to be under my current focus point with AF-C mode and not worry about exposure, I will leave the camera in multi segment metering and the Alpha will take care of it. I have experimented on this method using in the same plane opposite focus points, one in dark the other in light, and I confirm that this is the way multi segment metering works. With standard ISO and aperture it chooses different speeds to compensate. I wonder why I have not see it through reading the Alpha manuals.