you can only make changes after you start recording
when in video mode you are essentially in a special mode of live view. Any time you make a change to the aperture the camera will compensate so you can still view the image on the screen. So the order is .....
Install manual lens and turn the camera to video mode. start recording your subject then lock exposure. Once exposure has been locked then you can adjust your aperture to fine tune exposure. If you do not lock exposure the camera will adjust ISO or shutter to what it thinks is the correct exposure.
Its a PITA but is the same method you had to use when the 5D originally came out.
Originally posted by greenless I got my first manual 35mm lens in today (f/2.8 135mm) and it's not working with the manual aperture ring.
The camera asks me for the focal length of the lens when I turn it on, but that's the only real difference I'm seeing.
Turning the aperture ring does nothing to my exposure whether I've locked it in-camera or not.