Glad to hear you got BBF set up. It takes a little practice, but soon becomes second nature to focus that way.
I am not sure what you mean by "AI Servo choices".
Are you new to Canon cameras and their focusing systems? Just in case you are...
Like most Canon cameras, the 6D has three auto focus modes: One Shot, AI Focus and AI Servo.
One Shot is for use with stationary subjects. The focus is performed, then stops and locks, and Focus Confirmation is given (the green LED in the viewfinder, red flash of the AF point in the viewfinder, and if it's enabled, the "beep"). If anything moves, you or the subject, or if you change the focal length of most zooms, to re-focus you have to lift pressure completely off the focus buttion (whether it's BBF or half press of the shutter release), then reapply it.
One time this is quite important to remember is when using a zoom, since most modern ones are "varifocal" designs that don't maintain focus when the focal length is changed by zooming. If using One Shot and a zoom, you must either: Set the focal length first, before focusing. Or, remember to re-focus after zooming. Or, use AI Servo instead.
Because focus stops and locks after it's achieved in One Shot, it's a good mode if using a "focus and recompose" technique.
AI Servo is designed for use with moving subjects. Actually it's the mode to use anytime the photographer-to-subject distance changes for any reasons, or if using a zoom and changing the focal length. AI Servo never locks. It constantly measures the distance and updates focus, as long as you keep pressure on the back button or keep the shutter button half pressed, depending upon how you have the camera set up. AI Servo never locks, so there is no Focus Confirmation given. You need to learn to trust the camera and yourself. that it's working properly.
AI Servo cannot be used with "focus and recompose" technique, unless BBF is being used. If the shutter button still activates AF, it will cause the camera to refocus somewhere else, when you try to take the shot after recomposing. However if using BBF, all you have to do is lift the pressure of your thumb off the button to stop focus, allowing use of AI Servo with "focus and recompose".
AI Servo is also ideal for use with varifocal zooms... since the focus is continuously updated as the focal length of the lens is changed.
Though it can be very good, AI Servo is not quite as accurate as One Shot can be. So in critical focus situations, when possible use One Shot (Note: Live View and the separate focusing system it uses is even more accurate, although it's slower. A lot slower on most models.... 70D has new system that improves Live View focus quite a bit.)
AI Focus actually isn't a focus mode at all. It's supposed to decide for you whether or not the subject is moving, and then switch to using whichever is appropriate: One Shot or AI Servo. Frankly, I almost never use this mode. It may be better with modern cameras, but older ones where I tried it had a delay while the camera was "deciding" which mode to use, and that caused me to miss quite a few shots. Besides, I prefer to choose my own focus mode, and since I use BBF, I can simply leave my cameras in AI Servo most of the time. The mot "pro" oriented Canon models (1D series cameras) don't even have an AI Focus mode. They just have two: One Shot and AI Servo.
There are some "tweaks" to the AF systems, depending upon camera model, such as setting tracking priorities or how easily "distracted" the AF system is if something intrudes momentarily between you and the subject. Check your instruction manual for what specific fine-tuning can be done with 6D... I don't have one so can't say from experience.
There is a very helpful series of three videos about Canon auto focus systems on YouTube, posted by B&H Photo and featuring Canon USA's Rudy Winston... Each video is about half an hour long and the first is at: http://www.youtube.com/watch?v=iAx86nblZ2g Highly recommended, watch all three for additional info, even though these are a few years old now and don't specifically talk about the 6D focus system (5D Mark II's and the original 5D's AF systems are quite similar to 6D, except they have two fewer points and have 6 optional, hidden "assist" points available in AI Servo mode).
Hope this helps.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
