This was also a question which had dogged me and now I feel much better about what to do when. It seems as I get older, it more and more needs to be a hands on learning experience in order to get me from HUH? to AAHAW! to DUH ! (DUH is only when I am reminded of what I learned back at AAHAW)
In the course of going thru that though the thread and David Chin's links, (Thank you David as well !!!) and then my copy of Thom Hogan's manual on this camera, I learned two other things:
1. how to use AF-C to its max potential;
2. trap focus; and
3. the purpose of FV lock.
This AE/AF button can be used for multiple purposes depending on your needs.
David seems to prefer using the AF-ON in conjunction with AF-C which makes the camera constantly focus. This is pretty cool but the shutter will not go off unless you are pushing on the AE/AF button. But the idea of having to push a button and at the same time hit the shutter sounded strange to me. Then...then I discovered/realized that the button is RIGHT NEXT to where my thimb sits when I have my finger on the shutter. Imagine that?
David's link mentions trap focus. The trap focus is amazing and perfectly suited for macro work as Thom Hogan says in his manual on the camera. (I have a D50). You preset your focus distance and then when ready, move in on your subject and the camera goes off at the predetermeined distance. I then for the first time actually got a close-up from the camera in focus. Amazing!
In FV lock, pushing this button sends a pre-flash in order to set flash exposure. While I have an SB800 which does that on its own, it made a HUGE difference with the on-board flash exposure.
I can't say last night was an epiphany night, but it was close. I musn't get another camera till I learn this one and there is so much to learn with these.
Thanks again,
gk
--
'I'm not as smart today as I will be tomorrow.'