Back button focus mostly became popular with sports photographers because you can imagine the scenario where... the photographer is on the sidelines, tracking the player with teh ball as they move across the field. They are trying to maintain focus on that athlete... but other people (athletes, referees, etc.) will occasionally come between the camera and the player with the ball. What you did not want is for the camera to re-focus on one of these other people.
Unfortunately the focus was originally tied to the shutter button. So to take the shot you'd have to fully press the shutter and that would cause the camera to focus (and possibly on the wrong thing). Back Button Focus (BBF) allows you to remove focus from the shutter button. Now you can control metering and shooting with your finger and focus can be controlled with your thumb. You can push the BBF only when you see that you have an unobstructed shot to your player of interest... and release the BBF when you don't have an unobstructed shot.
If you're doing portraiture, architecture, macro, etc. etc. then you probably don't have this scenario. You could still use BBF if you wanted... but there's not such a compelling reason for it in those areas of photography like there is for sports photography.
BBF has nothing to do with metering. In fact by default the metering still stays on the shutter button. When you half-press the shutter, it "meters" -- it just doesn't focus.
As for your AI Servo question... the 60D doesn't have Canon's iTR (intelligent Tracking & Recognition). That's a feature that the 1D X, 1DX II, 5D III, 5D IV, and 7D II all have. With that system, you can let the camera use all available focus points but pick your initial point... once the camera focuses on a subject, it will attempt to "recognize" that subject as they move around the frame... and transfer the focus point to keep following them if they move within the frame to a different focus point.
The 60D doesn't do that. You pick a focus point and it sticks with it.
However ... if you keep the subject on that focus point but the subject's distance from the camera is continuously changing then the AI Servo mode WILL continuously keep focus on them (and update the focus) as long as you (and this is the key) are holding down the focus button (regardless of whether you assigned focus to the shutter button vs. the back button.)
But recall from my 2nd paragraph above that you don't have to hold it "continuously" ... you would hold it continuously only for as long as you have an unobstructed view. If an obstruction gets between you and your subject, just release the back button but keep the camera pointed at the subject (the camera will stop updating focus) and when your view is clear again... press-and-hold the BBF again and the camera will resume focus as long as you keep holding the button.
Tim Campbell
5D III, 5D IV, 60Da
