Thank you. I know the menu function but I do not really know what it is supposed to do and what is really does.
It can have a huge influence on how quickly AF is lost on moving subjects and how quickly it is re-aquired.
The longer the lock, the longer it tries to hang onto a subject, but it then also takes longer to re-quire focus. Shorter times help with quick re-aquiry, but might loose subjects quicker.
From your description above, me thinks the 810 and the 750 use different settings here.
All independent of 3D mode. Applies also for single AF point.
cheers
afx