If your camera is on AF-S, half-press the shutter button to refocus. If it is in AF-C, it should focus automatically, but always half-press the shutter button before taking the picture to be sure the picture is in focus. Be sure pre-focus is on if your camera has that setting.
Here is the manual if you need any help:
ftp://ftp.panasonic.com/camera/om/dmc-zs50_adv_om.pdf
Hope this helps!