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...I was just wondering how to keep the camera at one constant brightness the entire time when videoing.
Use manual exposure.
You set the ISO, aperture, and shutter speed, not the camera's meter, and it won't change until you tell it to do so.
If you're in any of the autoexposure modes, when the sensor detects a light source -- such as a TV -- it will adjust the scene's exposure to render that light source as a middle tone, and that will change the exposure of all other elements in the scene.
As far as your trouble with the SteadyVid (it's not a Steadicam, although many people misuse that trademarked name), this may help: