If you're using the green light to confirm manual focus (this works on the D80, even with manual lenses that won't meter), it seems to use the currently selected focus point.
To answer your question about the camera choosing focus points: it checks all the focus points and tries to identify a moving target within the frame. It then attempts to predict the movement of the target, and uses all of the available focus points to keep tracking that target. It can confirm (by illuminating in red, and boxing the focus points) which points are in focus.
So, in your soccer game, presumably you would point at a player. The camera would find the moving player in the scene, and try to use all available focus points to keep them in focus. That's the theory anyways.
The D80 has the same autofocus module as the D200, supposedly better at this sort of task. It may also be better at low light focus. Perhaps someone who has tried both could comment.
Remember, it seems that the D80/D50/D40series matrix meter uses a higher weighting on the selected focus point, so if you focus using the centre point as your main subject and recompose, it is possible that the meter will apply the weighting to something which is now background, and get a wrong exposure. You could help prevent this by locking exposure and recomposing, but potentially the entire scene could be changed enough to get an incorrect value this way as well. Moving the focus/meter point to the subject avoids this. That said, I haven't noticed this as a significant problem with my D80 so far.
Furthermore, in the time it takes to lock and recompose, the scene can change enough that the locked focus is no longer correct. I've found this sometimes with birds/wildlife and small children (or candids when your target won't sit still for you). Particularly here you want the focus to hit the eyes (or face depending on distance etc). It is sometimes easier to put the focus point at a predicted spot, and squeeze the shot when it intercepts the target.
I have also sometimes found it to be advantageous to move the point when moving the camera itself is likely to change the focus (eg macro without a tripod). If you're sufficiently skilled/steady, you could avoid this, but it's easier to just move the focus point where you want in my experience.
For action the more focus points the better. For stationary
subjects most people use the center focus point and recompose after
focusing. For manual focus you use your eye.
Can you expand on this Morris? If I'm taking shots at a soccer
game, how does the camera decide which of the available focus
points I actually want to use? Did you mean that we should use the
toggle to move the active focus point?!
Sorry, I'm with the other guys: happy to use the centre spot &
recompose. In fact, my only regret is that I can't lock the focus
point contol on my humble D50!
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Colin Malsingh
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Images:
http://www.pbase.com/cmalsingh
Food:
http://www.mangoidli.blogspot.com/
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Someday maybe I'll think of something useful to put in here...
