If the center (ie, the 4x5 film area) is the same exposure-wise for both lenses (and I would expect it to be if f16 means the same thing exposure-wise on both lenses), then we have to account for the all the extra light energy that falls outside the 6.5" circle when the 8x10 lens is used. The iris opening is the same for both lenses, so I'm not seeing how more light energy gets through the 8x10 lens.