I am having real trouble with my canon 5D mark ii in the sense it appears to look like it underexpose shots (but the histogram says otherwise!).
I also own a Canon 60D and this appears to be fine, but when taking the same shot, with exactly the same aperture, iso and shutter speed - the 5D mark ii shots are too dark...yet the 60D shots are perfect.
I struggle to get shots of say a wedding party of people dancing without having a very high iso....and needing a shutter speed to slow to let more light in but then get the motion blur I want to avoid. I am also using widest aperture and still the 5D cant cope with it - what am I doing wrong?
This sounds odd as the 5D MK II is a great camera that can handle a variety of lighting conditions and there are a number of possibilities it could be. It's hard to tell from what you've outlined in your message above the exact problem.
It may be that you set your camera to something without realising. First check you haven't knocked the exposure compensation setting down to underexpose the scene. If it's not this you could try reseting your camera to the manufacturer default settings. You can find this in the main menu. If this doesn't work then it could be that the camera is faulty or the lens?