It's simple.Zoom all the way in and either focus manually on some text on the bass drum or something with alot of lines and your set.The focus will be maintained throughout the entire zoom range.You don't have to focus anymore and your auto focus won't be fooled when the strobe comes on or the lights go out etc.You can easily do it the same way by zooming all the way in in autofocus mode and when the focus looks really sharp just switch over to manual focus.The setting will be maintained.
Yep. I do the EXACT same thing (focus on bassdrum head text, or backdrop if there is one). It never, ever fails. I don't really understand how/why it works but I just followed the advice and it truly does work. I've filmed in small clubs and then huge arenas and it works the same in both situations.
Just make sure when you do it, zoom ALL THE WAY in (to the max) and then set the focus.