So I've had my 5d3 for a couple years now and I'm becoming more and more convinced that the metering is off. My first clue was that almost every image was needing +2/3 to +1 1/3 exposure in post. I don't believe that this post-adjustment is subjective as the histograms confirm the underexposure. This has been occurring for quite some time, eventually forcing me to keep the camera dialed in for +1 EC. Of course, this limits the ability to effectively use Manual Mode by setting aperture and shutter, while letting the camera decide the ISO. This is a major negative IMO. I've skimmed through the sticky on Light Leak, but it seems the consensus is that its a non-issue under normal shooting conditions.
The next thing I tried was placing the camera on a tripod with a large window about 5ft away to the right of the camera. I switched to Manual, evaluative metering, fixed ISO to 100, and fixed aperture to f/2.8. Half shutter-press and the camera selects a shutter of 1/400. Then, by simply using my hand to shade the viewfinder, the camera drops the shutter down to 1/250. Same exact results when using the rubber viewfinder cover instead of my hand. Simply putting my eye up to the viewfinder also changes the metered shutter speed, but it varies anywhere between 1/3 to 1-stop. Same results when using center-weighted and spot metering as well.
Is this the cause of my constant underexposing or is it normal behavior? This certainly seems consistent with the fact that I'm having to push exposure by a full stop in post. I'm thinking about contacting Canon but I really want to get my facts straight and a clear understanding of what is going on before raising the issue.