1. Thank for the extended explanation. There is progress, but I must admit that that I am a bit confused, partly by the Nikon terminology: the autofous mode (= AF-A, AF-S, or AF-C), on the one hand; and the "AF area mode" ( = Single, Dynamic <9, 21, or 39), or 3D tracking) on the other hand. Let's assume that the desired focus is at the center of the frame, (excpet where the subject changes, as discussed below).[br /> 2. The overall object is to maintain focus in dim light using the SB-600 for AF assit. The system requires using autofocus mode set to AF-A or AF-S, but not AF-C. But the camera, when set to AF-A autofocus mode, can act as if it were set to AF-S or AF-C autofocus modes, although always starting in the AF-S autofocus mode.
3. When shooting ballet, the object is to prevent the camera from shifting the focus to the backdrop when using AF-S autofocus mode and single point AF area mode (if the subject moves off the center point). The overall objective of this inquiry is: what is the best (1) autofocus mode setting and (2) what is the basrt autofocus area mode setting, under those condftions (when the subject is one or two persons).
4. Comment: Don't you find it strange that, to use the SB-600 for AF assist, i cannot set the camera's autofocus mode to AF-C, but the SB-600 will provide AF assist if the camera itself switches from AF-A to AF-C?
5. You say that "when using AF-A (autofocus mode), if the camera switches to AF-C (autofocus mode), then the area-AF mode will come into play and the selected mode should be used". This seems to mean that the camera WILL use the slected AF-area mode. Is this correct? You later suggest that is 9 point (& whic is what I use, but you also suggest that single point may be preferable; see below)).
6. This also implies that when using AF-A, if the camera, haivng started in AF-S, stays in AF-S (becausee the subject does not move, then the AF-area mode is automatically single point. Is that correct? I think so, from your next-to-last paragraph.
7. You state that "single point is useful with AF-C when you DO want the AF to adjust immediately to a new subject."
There are two situations:
(a) I want to maintain focus on a single person, who is moving (and if I do not move the camera to follow fast enough, the focus shifts to the backdrop, which is undesirable); and
(b) I change subject, and the new subject is at a distinctly different distance.
So what are the bast autofocus modes and AF area modes for each case? That's the whole point.
8. You state that "this is not tracking AF-area mode" Exposure is still based on the primary focus point." Is the distinction that, in the 3D area tracking mode, the focus point moves with the subject, but in AF-C the focuys point (assume it's the center remains the same, but if the subject movbes, say, to the right two feet, the focus remains at the same distance until I align the camera so that the center (focus) point is on the subject? The seems to be the intent, but if this is so, what is the point of using the surrounding area (such as nine points in the AF-area mode) to assist in maintaining the focus?