Could you explain your method a bit more, please? How does the back button improve focus over just using the shutter button to focus with AI Servo? Are you using the back button to activate AI Servo, or to lock it after the shutter button has already activated it? I have never gotten along with back button focus, but I'm willing to give it another try. My impression is that it always seems to require twice as much work and twice as much coordination, but I may be missing out on the advantage for certain situations. Thanks in advance.
Back-Button Focus (BBF) separates the autofocus from the shutter. You use it to activate AI Servo. AI Servo doesn't "lock" like One-Shot focus does. It continues to search for optimal focus if it detects a change in phase (subject movement).
The benefits over keeping the autofocus on the shutter button for AI Servo (or even One-Shot) are as follows:
1. Upon taking an exposure, if you want to take another right away, using the conventional shutter button to autofocus... Once you take the exposure, and your finger is coming back up off from depressing the shutter to take the exposure, that fraction of a second that you lose can be the difference between your subject still being in the *same place* within the frame (and still 'painted' by the AF point) that they were when the shutter opened to take the exposure. When this happens, AF effectively has to 're-hunt' for a subject. And if you want to take a subsequent exposure right away, you may or may not hit good focus.
If you are peppering the subject with continuous drive in AI Servo (holding the shutter button down), the AF never stops between exposures - it just keeps going. Remember a shutter for a moving subject could range between 1/250 and 1/4000th of a second. If AF is still going - either through continuous depressing of the shutter button - or through holding down the AF-ON button on the back, it's but a minute fraction of a second after the shutter closes and mirror flips that autofocus is back on the job.
So what you gain here is the agility of AI Servo *in continuous drive mode* with the flexibility of "one-shot-like" exposure control.
2. If another subject comes in front of the subject you already focused on using back-button focus, you can choose to let go of the AF-ON button and take the shot with the shutter button. If you would have otherwise been using the shutter button for both AF and Exposure, in AI Servo, the AF system likely will lose focus off your subject which is now in the background. This can lead to the wrong subject being in focus, of course. Worse, with a slower lens especially a telephoto like the 70-200 f/2.8 (non-IS) or better example the 85 f/1.2L which has notoriously slow AF, this could be very costly time wasted until your intended subject is back in focus and you may have lost "The Shot."
My method is simply to just hold down the AF-ON button 100% of the time while taking pictures UNLESS there is a good reason to let go (i.e. unwanted subject enters the foreground or I want to re-compose my frame WITHOUT altering focus). The *KEY* is to ensure that you continue to hold down the AF-ON button throughout your pressing the shutter release. If you look at your exposures in DPP afterwards and view the AF point selection for that frame (ALT+L) you will see which AF point(s) were used for an exposure. Unless you depress the AF-ON button throughout the shutter release, there was no "lock" and you will not see any illuminated (in red) AF points in DPP. I have found this is key. If you let go of AF-ON for a subject that is moving (or you moving the camera even the slightest) your shots won't be tack-sharp.
Does this help?