@douglaswise, you are basically correct. If the back button is depressed, the camera will try to focus on whatever is under the active focus point(s). When you leave the button, it stops focussing (it doesn't lock though, it just stops where it is).
@Dave Williams, on my camera, it starts focusing whatever other buttons are pressed or not pressed, so I am not sure whether there is something wrong, or if some other setting isaffecting your results. On Nikon (I know very little about using the other brands). The front button is thus used for both metering and shutter activation.
The advantage is that, in operation, you have access to AF-C (when pressing the button) and AF-S (when you let go of the button). Thus, if the subject is stationary, you can focus and then let the button go. You can then change the composition (either on purpose or by accident) without losing focus.
Conversely, if you are tracking a moving subject, and you see it is about to move behind some other object (or you can't track it well enough and you see it is about to move out from under the active focus points), you can simply let the button go.
Speaking for myself, this last is about the biggest advantage for me. Little birds move far too fast for me to keep track of, so in situations where I can expect the bird to return to a specific place (birds building a nest or feeding chicks for example), I can focus on the nest and the let the button go, but if a different opportunity comes along I haven't shot myself in the foot by having to change to AF-S.
@Dave Williams, I don't know what camera you use, but on Nikons (at least the better models) there are a number of settings that can change the behaviour of how the camera chooses what focus point to use, but in most cases, the "OK" button (in the middle of the multi-selector button) can be programmed to make the centre point the active one.
@douglaswise, if the bridge camera give you results that you are happy with, why not stay with just that? Or maybe use that for birds and wildlife, and the Nikon for landscapes and such? If you have a system that works for you, there is no need to have a second one for the same job?
Oh, and I have a theory about the saying about tradesmen and tools: I believe that good tradesmen who don't blame their tools, it is because they have, over time, collected the right tools for the jobs they are doing, and learnt how to use them too. After all, they didn't become good on their first day on the job. And they don't borrow other people's tools either.
Edited by Peter Connan, 12 January 2017 - 03:35 PM.
Ek oefen skelm.