Hello everyone, fisrt time poster here.
The phenomenon described by the OP is related to the strong metering bias that the 5DmkIII has on the selected AF point in evaluative metering.
This is the first thing I noticed coming from a 40D, and is my main complain about 5DmkIII metering algorithm.
When I shoot at night in urban environment, I always switch from evaluative to mean to avoid any strange behaviour of the camera.
In AF mode the level of brightness of the portion of the scene that is caught in the selected AF point area determines the bias in the exposure value.
When "all points" is selected, or when MF selected, this doesn't happen because the camera has no clue about what AF point will be actually used to focus.
This happens on every lens, and is not a flaw in the body, but a choice of the designers of the evaluative metering algorithm that I personally don't like at all.
I'll provide evidence later.
Bye for now.
Update: see the 4 pics attached, made with 135 f/2 L
the AF point is on the light on ceiling, iso is 320 fixed, aperture f/2.0 fixed
pic 1 has AF + eval metering, resulting in 1/4000 sec (strong bias on the AF point, it results effectively in a spot metering on the AF point)
pic 2 has MF + eval metering, resulting in 1/160 sec (the camera has no clue about the AF point in use, and provides a "true" unbiased evaluative metering)
pic 3 has AF + average metering, resulting in 1/160 sec (no bias on mean metering)
pic 4 has MF + average metering, resulting in 1/160 sec
As you see, in the case AF + evaluative metering the camera attemped not to clip the area of the picture on the selected AF point, thus rendering every other area of the image absolutely meaningless.
The amount of shift in this case is almost 5 stops!
If I were the designer of this feature, I would let the user to activare or deactivate it, or at least provide a "cap" on the maximum amount of correction allowable, say max 2 stops of bias.
Having it permanently operating as it is, without reasonable cap, means that you have a spot metering on whatever AF point is selected, and this is pretty disturbing to me in certain circumstances.
What do you think about it?