I've just gotten a 7D and have been shooting with it for about 4 weeks now. I've been shooting entirely in manual and recently I've noticed that the brightness level shifts if I reframe. So if I pan from one area to another area that is brighter the brightness level will slowly shift higher or if I move to a darker area the brightness level will slowly shift downward. Despite this clearly visible change the camera says that none of my settings have changed. What could e causing this?