Tried both cameras at ISO 6400 and RAW files are virtually the same, (the 5DIII just shows a bit a front focusing with my copy of the 24-105).
JPGs are a complete different story, but I cannot see the high ISO performance improvement everybody is talking about.
Did anybody experienced the same?
I'm not saying that I'm disappointed with the camera, in my opinion AF and silent shooting are a great improvement for wedding photography, it's just that the sensor technology seems exactly the same.
My 5D MK III is obviously better than my MK II at ISO 12800 and up. I did not bother to compare at 6400 They are the same at ISO 100 as well unless you get in a difficult NR situation..
However, I do low light photography, and when a person is comparing them in a well lighted studio, the effects of noise will appear to be a lot less.
At 12800 and higher ISO, the MK III pulls away. However, there is not going to be 2 stops difference in Raw, except maybe at ISO 58600 or higher.
I can use the MK III in the dark and as long as I print 8 X 10 or smaller, all the ISO settings look pretty good.
I took this image in near dark, about -1 lv, It was as low a light as my AF would focus and then, extremely slowly. There was a tiny bit of light from another room lighting it from the right side, but I could not read the test, it was too dark.
Its captured in raw, and has NR applied at ISO 51600 and it is noisy, but I can read the text when I pixel peep.