How many readers have the EOS 5D Mark II?
Of you, how many have tried the movie modes and have figured out the way to accurate exposure?
You essentially have two controls over exposure on the camera. Firstly, there is the AE Lock button. Just like in still shooting, the AE lock button will hold the exposure at whatever value is currently set when you press the button. The one thing to remember is that you don't need to keep the button pressed. Just press it once and leave it alone. The exposure then won't change unless you press the button again... or use the second exposure control: The quick command dial.
The quick command dial acts like exposure compensation in still shooting. Essentially it makes the scene brighter or darker and that is about as much information as it gives you.
Clearly there are a couple of different ways of getting the right exposure then - you can either point at a dark area, let the camera open up the exposure, use AE lock and then use the command dial to make it darker again, or you can start pointing at a light area, use AE lock and then open up the exposure. Both will work, but it is worth experimenting as you only have +/- 2stops of compensation available so you need to make sure it's within the range of adjustment.