one thing i thought was odd... she said the 1dx supports AF expansion 4pts and 8pts and said the 7D doesn't have the 8pt expansion... the 7D has the 9pt "zone" cluster... so i wonder what the difference, if any that has to the single pt and 8pt expansion the 1dx/5d3 has..
The 7D has Zone AF and a 4-point expansion mode, the 5DIII/1D X have Zone AF, 4-point expansion, and 8-point expansion. Zone AF will lock on to the closest subject within the zone, regardless of the specific AF point. AF point expansion will try to get a lock with the selected AF point, and if a lock cannot be achieved the AF system will switch to an adjacent point to get a lock (or if the subject moves off that AF point and onto an adjacent point, it will use that adjacent point). The 8-point expansion mode means it will be able to keep a lock on a subject moving diagonally across the frame, in addition to the vertical/horizontal directions you get with the 4-point expansion.
1- AI servo AF seem to be a more refined version compared to the mkIII. They also mention it can track face but not sure if this is only in Live View;
Yes, the AI Servo mode on the 1D X is superior to that on the 5DIII (which is, in turn, superior to that of the 1D IV). The face tracking works in phase detect AF mode, which can be a huge advantage. It uses data from the metering sensor to track the face.
2- Metering is an obvious one, however I am curious to see how the Nikon D4 works because the 1DX # of zone dramatically reduce at high ISO
Not sure, mostly because I don't know that much about the Nikon metering sensor other than the basic specs. I do know that the 1D X (and 5DIII) offer one more stop of sensitivity (the D4/D800 meter at -1 EV, the 1D X/5DIII meter down to -2 EV).
The reason the 1D X reduces the number of zones in very low light is to maintain the high shutter speed. 7D users know that the frame rate slows down in very dim light - that's because the metering system needs more time to integrate the signal (temporal summation). The 1D X bins several zones together (spatial summation) to get sufficient light for metering, without slowing the frame rate.
3- sensor. This is the big question for me. Canon had a chart there saying "effective pixel" where they claim the 18.1MP of the 1DX is better then the 21MP of the 1DsIII. Does anyone know what effective pixel mean? Is the 1DX sensor better or not compared to the 5DmkII at low ISO, that is the real question to make this an exception camera!
No way to know, I suspect, until both bodies are 'in the wild' and proper RAW converters are available.
