Caution ! Dynamic AF versus single point AF... !

A quick update... If you make an experiment with your D4 and D800 and put a blank white page

onto your computer monitor and insert a face in one of the corners you will see that with its autofocus system

you can come near with a selcted focus point and without the focus point being on something with contrast it still locks onto a part of the face within reach of the next focus points not more..

with automatic mode it can reach a portrait of a face almost on the full screen....

so in dynamic mode its only a  very low distance that it jumps..... but if it cant lock because of low contrast it does use a different field...

Peter