For the first year or so that I had the ZS100, I always shot in manual focus mode and would tap the AF icon on the LED screen once I zoomed in to focus the picture. The problem with this is that during the 2-3 seconds it takes to fully zoom in, as well as for a second or two after you've zoomed in, the picture is out of focus until you can tap the AF icon on the LED screen - but that disappears while the camera is zooming and doesn't reappear until a second or two after you stop zooming in/out. So a few months ago, I made the switch over to shooting in auto focus mode. I generally prefer this to manual focus; especially in situations where the musicians are different distances away from my camera (i.e., I am near the front left side of the stage, so the guitarist in front of me is 8 feet away, the lead singer in the middle of the stage is 15 feet away, and the bass player on the other side of the stage is 25 feet away, etc.). Shooting in manual focus mode in this scenario, I would have to refocus for all three of the musicians (four if you include the drummer).
A big drawback with auto focus mode is that I tend to have trouble staying in focus when there is a lot of dark parts in the picture. For example, I zoom in on someone playing a black piano and you can only see their head and the rest of the picture is dark. In this situation, the auto-focus just drifts in and out of focus and really drives me crazy. Also, when there are a lot of flashing lights or strobe lights - the auto focus has a hard time handling that.
As far as I can tell, you can't change the AF <--> MF setting while shooting on the fly (by pushing the MF button next to the "Menu Set" button). Of course, stopping the video in the middle of a show to change this setting is not practical.
I have read quite a bit about the AF/AE button on the back of the ZS100, but can't seem to figure out how to lock the focus when shooting in auto focus mode. If I could lock the focus, I could zoom in on that guy playing the piano and once I got a properly focused picture -- I could lock it in and not have to worry about the camera drifting in and out of focus. But I just can't figure this out. Any help with this would be greatly appreciated.