Do we agree that the diamater of the aperture in both lenses is the same? I'm sure we agree that the area of illumination created by the 8x10 lens is four times that of 4x5. So if the illumination per unit area is the same in both cases (and it has to be if exposure is "correct" in both instances), then there has to be four times as much energy getting through the 8x10 lens even though the aperture opening is no different. Yes? How does that happen?