Using a 16-85 3.5-5.6 lens. Taking a head and shoulders video at 1/60 and f5.6 in mode "M". This is the exposure I want to lock in. When I zoom out and include some darker areas in the frame, the aperture will change to f4 and f3.5 thereby overexposing my subject. How can I lock my exposure? (Even in aperture priority mode it will do the same thing.)
This happens because you are using a variable max aperture lens. The aperture iris is not actually opening or closing as you zoom out – its actual opening size is staying constant. It's just that the entrance pupil, the apparent size of the aperture opening when viewed through the front of the lens, is changing size due to the movements of the lens element groups as you zoom.
It is for this reason, amongst others, that variable max aperture lenses are not the best choices for video. The only way you can lock your desired exposure is to shoot in aperture priority. If you shoot video in manual mode, you cannot avoid the exposure changing as you zoom.