AI Servo and back button

I know this has been said many times at many places. But can you please clarify something for me?

I got 7D recently. Been reading up good and very confusing articles on Al Servo and back button. After agonizing mental work I think I now know how what back button settings are good for what. One thing I am not sure about is the difference between a) "Metering + AF start / AF stop" with AI Servo and b)Metering start / Meter + AF start with AI Servo.

In this articles http://www.garyluhm.ne​t/bio/tips_0310.html and http://jefflynchdev.wo​rdpress.com …k-button-focus-explained/ they say the setting a) is good because you can press and hold AF-On to lock the focus, and re-compose and shoot like One Shot.

But can't setting b) do the same? In fact easier? With setting b), you can still lock the focus not by pressing and holding AF-On (in this case, the lens keeps focusing because of AI Servo, which is also useful) but by pressing it once, and recompose to shoot.

And one more question please. Why not use AI Servo all the time? It seems to me that leaving AI Servo on and Metering start / Meter + AF start is most versatile. Or at high levels, you need to switch between One Shot and AI Servo? Thank you in advance.

Wow! That is very very hard to read and follow.

I normally shooty

AI Servo on
AF-On = AF Start
Half Shutter = Metering start

Focus with AF-On normally just using the centre point release AF-On, recompose and shoot. I am struggling to understand the question.

A lot of people use AI-Servo all of the time - in conjunction with BBF, it essentially gives you the best of both worlds - a single shot focus when you hit the button, continuous focus when you hold the button.

There are as I understand it some reports that the AI-Servo focus isn't as accurate as a single shot focus, and you don't get focus confirmation with AI-Servo even on a one shot, so sometimes it's preferable to switch to one shot if you know you're not going to be focus tracking - and this is what I tend to do. But yes, some people do basically use AI-Servo most of the time.

I also prefer using "*" for the BBF, rather than "AF-On" as it's closer and easier for me to hit with my small hands.

On a 7D, the settings aren't exactly like what is described in the first post here. The settings quoted there are what you'll find on a 40D, for example.

But Metering + AF start / AF stop should be read so that what's before the / character applies to half-pressing the trigger button and what's after applies to the AF-ON (or * button, if you switch AF-ON and *, but that's another custom function).

Thus the setting above implies that the trigger button works normally, but you can interrupt the focus by keeping AF-ON pressed.
On the other hand, Metering start / Meter + AF start implies that the trigger button will not focus, just do exposure metering. Instead focusing will be active as long as you keep AF-ON pressed.

The later setting also means that you can use Servo AF more or less as One Shot, by letting go of the AF-ON button when you want to lock focus.

The disadvantages are the following:

1. Shooting isn't prevented if focusing was impossible.
   
2. No AF assist light is emitted by the flash, when that's applicable.
   
3. In low light, exposure time for the AF sensor is increased when using One Shot AF, in order to increase accuracy. Since Servo AF instead prioritizes speed, this doesn't happen. Thus Servo AF could be inferior in low light.