What does that say about the validity of your statements regarding the 5DIII and 1D X?
I don't go into the 1DX thread and spew hatred for the 1DX. Yet you and your butt kissers feel the need to come into a 6D thread to bash it. What does that say about you?
I wasn't passing off the image as "usable", as in professional quality. It was merely an experiment. Only a fool would imply otherwise. No need to get defensive just because the experiment worked. At the very least I would have used exposure compensation and shot in RAW, if it were meant to be printed or something (and yes that would have pushed to ISO 50k or higher). When I have said in the past, that the 6D's center AF point aids tremendously in low light autofocusing, I'm using positive exposure compensation (usually a full stop, sometimes more), in order to get the RGB curve to go farther toward the right side. And in those situations, the ISO I would consider "usable" for good professional quality, is not above 10,000. "Usable" for acceptable pro quality, could be 20,000, but not 25,600. If that means the available light I'm shooting in, is brighter than -3 EV, so be it. But that alone, does not mean this center point, is not still very usable, when compared to an autofocus point that is only rated for -2 EV. It just means that for a "normal exposure" at -3 EV (as in, a very black image), the necessary ISO combined with the lack of any significant recorded signal in the entire upper half of the RGB curve...there is not significant dynamic range from the sensor, or the file's 14 bit format itself, to produce anything other than a dark noisy image. Again though, digital photographers learn early on, that you look at the RGB curve, and expose for that. I deliberately did not do that in this case, because I wanted it to look similarly dark to what my eye saw. It still looks brighter than what my eye saw, though.
The back half of the bubble level, is within the focus plane. The index mark you keep harping on, is ABOVE the plane of the plate that the bubble is mounted in. And the camera, is angled diagonally to the plane the plate exists in, both left-right and front-back axis. Stop ignoring that. That alone explains why the index mark is near the plane of focus. What about the right side of the plate? It's in the focus plane. Draw a diagonal line perpendicular to the sensor, and it runs right through the rear half of the bubble level. You can say it's not because the focus plane is "infinitely small", and thus exists between the subatomic particles that make up the atoms of the plastic of the bubble. But that just smacks of desperation. The plane of focus is effectively more than 1 pixel in width on this sensor (due to the limited resolution of the lens itself, but also the high ISO). And since there is motion blur, it's more like 4 pixels in width. That's a lot bigger than "infinitely small". I know one thing that's infinitely small though, and you keep compensating for it, lol.
Since you say your 1DX could autofocus in -2EV light (as you claim this was)...how about trying it on a similar black object with a tiny light colored object for contrast, and posting the results? Make sure to shoot only as a jpeg (with no in-camera NR), that the exposure is not faster than 1/13 second, there's no image stabilization, and that you are hand-holding it while bending over a bit, and that your subject distance is ONLY 4 inches in front of the front lens element. Also, make sure to select ONLY THE OUTERMOST side autofocus point (don't use any groupings). Your results had better be better than mine, because you won't be needing to focus recompose. Make sure the available light in the room, is equal to 3 lit candles (with no reflectors), placed 8 feet away from the camera.