When shooting moving subjects (Sports), am I suppose to keep the back button AF pressed during a sequence of shots? Or do I press til it focuses, and then let go and shoot a sequence?
Thank you in advance!
Typically when using back button focus you will want to use a Continuous focus mode to gain the most advantage from un-linking the focus button from the shutter release button. [Believe that would be AF-C on most Nikons, AI Servo on Canon]
If you want it to change focus/for the focus to be able to change, press the button. If you want it to stop changing focus, release the button.
This allows you to shift how you're focusing on the fly as needed.
Have a player moving so they're changing distance from you?
Hold the AF button while you're shooting a burst, and the camera will try to adjust focus for each shot to make up for the change in focus distance from frame to frame.
Trying for a perfect focus shot as the player crosses a specific point?
Use the back button to focus on where you want the shot to be captured, such as on a line, then release the focus button.
As the player heads towards the 'target zone', start taking your short burst of photos without pressing the focus button again.
[The idea being that you have a frame or two that probably aren't yet in focus as they approach the zone, but you give your camera a moment to stabilize from the slight camera shake of pressing the button, then [hopefully] one or more in focus frames, followed by any extras that get captured in the time it takes you to get off the shutter button.]
Also a good tip for continuous focus modes: Check your camera's manual, and dial back the 'refocus speed' or the sensitivity of the camera to lock onto a new target. While at first it makes sense to want to have everything in your AF set to be 'as fast as possible', it is actually working against you at times if your focus point 'slips'. You want the auto focus to remain 'locked onto' a player who is probably in the mid-ground of the image, you don't want it snapping to focus on something far in the background every time the focus point slips off-target.
Having it set to the 'fastest' modes possible can result in focus trying to ping-pong between focus distances.
You'd mostly want to keep it down.... but being able to lift before shooting is sometimes the right choice, and one of the big benefits of back-button.
One of the places I use it this way is horse jumping. The shot you want to capture is mid-jump--but if you still have focus running that's exactly the moment it picks up the near-side jump standard.