From a Canon spokesperson :
The 5D Mark II shoots all movie clips in a “Program AE” mode – regardless of where the mode dial is set on the camera. In movie mode, the camera automatically controls the shutter speed, aperture, and ISO. You have no true manual control of any of these settings, although there are ways of creatively manipulating how the camera will decide what settings to use. When the 5D Mark II is automatically making adjustments to these three settings, it will do so in the following order:
1st) ISO
2nd) Shutter speed (1/30~1/125 sec.)
3rd) Aperture
Because the built-in microphone picks up all of the internal camera operating noise when shooting, the camera was designed to adjust the settings in this order because adjusting the aperture is the loudest of these settings, and the ISO is the quietest.
Being aware that the 5D Mark II adjusts the exposure in the order that it does allows some degree of creative control. Knowing that the camera “doesn’t really want to” change the aperture if it doesn’t have to means that if the camera is aimed at a very dark scene when the movie is started, the aperture will typically be very wide, and will generally stay there throughout the video clip – unless there is a dramatic change in lighting conditions in mid-clip. Aiming the 5D Mark II at a bright light source when starting the video will cause the aperture to close down, assuring deep depth-of-field.
