I've just noticed this about the D4's AF system, from Rob Galbraith's website (http://www.robgalbraith.com/bins/multi_page.asp?cid=7-11673-12223-12224
):"Autofocus with lenses or lens + teleconverter combos whose maximum aperture exceeds f/5.6. With the D3S, and many other Nikon digital SLR models past and present, attaching something like a 600mm f/4 + 1.7x teleconverter would result in sub-par autofocus or no autofocus, because this combination is beyond the f/5.6 AF limit of most Nikon models. The D4 can autofocus properly to f/8, the first Nikon digital SLR to be rated to do so, as long as your composition keeps the subject relatively close to the centre of the 51-point array.
If the maximum aperture of the lens or combo is between f/5.6 and f/8, the number of AF points that retain their cross-type performance drops to nine, while six others, left and right of centre, operate with single line sensitivity only. The remaining 37 AF points in the array may function, but may not detect focus distance properly if they do.
If the maximum aperture of the lens or combo is f/8, the total number of properly-operating points is 11, with the centre-most AF point retaining its cross-type sensitivity. Other points may function, but might not function well.
Note that the D4 doesn't prevent the user from selecting a non-optimal AF point. If you plan to hook up a long lens and teleconverter that puts you into the beyond-f/5.6 territory, you'll want to pay special attention to which of the 51 AF points work best."
From what is stated here, it doesn't seem that Nikon have actually made physical changes to the design of the AF sensor to incorporate f/8 sensitive lines, just that they are confident that the AF chip is capable of operating with less light and a smaller 'rangefinder' base length. Perhaps because (ibid.
): "The minimum light level for AF detection, at EV-2, is one stop better than the D3S' EV-1."
So, if the 1D X's AF system is also capable of working at EV-2... (http://cpn.canon-europe.com/content/education/technical/eos_1d_x_af_system_explained.do
):"The benefits of the new AF system include finer precision and increased AF tracking speed, increased low-light sensitivity down to EV -2"
Would it be possible for Canon to simply do the same as Nikon and not disable their AF system at f/8 and simply warn users that only the central sensors are likely to be reliable?
Of course, without actually seeing the D4's AF sensor, we can't know what changes Nikon have actually made to enable f/8 AF, so my logic may be totally wrong!
Over to Neuroanatomist...?