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.