I have also notice a2/3rd of a stop difference on the 5D3. When I received the new camera I tested the exposure against gray targets, just to see how the new metering system was faring. I have noticed the same difference in my everyday shooting, and have set the cam at +2/3 pretty much consistently.
I don't know if this is related to the light leaked issue or simply the way the new metering system behaves, but I would be interested in knowing if other people have noticed.
Ok thanks I now know I am not crazy and this actually happens.
If I take a picture for example: Manual 4.0 / 1/200 / iso 800 & I compare the same but in P mode the manual picture is a bit darker like 2/3 also you can notice that if you check the histogram.
since I start to take pictures with my 5d mark iii I notice the picture was a bit dark like 2/3 so if I want to take the picture right I have to set my meter to +2/3
First do the test with the lenscap off.
With me and others including the test of CR no issue at all.
The metering is not responding to any light at all.
Test this first before you think there is a real problem with the 5d mark iii
I am talking when I take pictures normally