« on: May 23, 2013, 06:07:30 PM »
Flipping the switch triggers metering. Since evaluative metering invokes code that looks at multiple metering points and tries to intelligently guess what to do, it's somewhat reasonable to assume that it might a different decision on what to do each time it's invoked if the light entering the lens in non-uniform across the image.
A way to test if this is happening is to point the camera so the image is completely uniform in brightness across the image — at the sky, or a flatly coloured wall, etc.
This makes perfect sense. Evaluative metering is linked to the selected AF point. Switching to MF deactivates all of the AF points, which means evaluative metering can no longer be weighted toward a selected point. The fact that you're seeing this only with a wide angle lens is not unreasonable. With such a wide FLV, there's more in the scene to affect the metering when it is no longer weighted toward the selected point.
Theory confirmed, on my 7D & EFs 15-85 as well. Standing outside, with single AF point on the top of the frame, focussing on some bright clouds, in the bottom half of the frame is my neighbour's relatively dark roof. On AF, I get 1/500s. Switch to MF, I get 1/320.
What Neuro said is also applicable to the 7D, metering is weighted more towards what is in focus (in my case, bright sky at infinity). If it doesn't know what's in focus (from being in MF), then it will give even weight to the whole frame and meter more for the dark roof. This is pretty much what the user manual states.
It's not a bug it's a feature.