Digital Camera Focus Mode: Single Area Focus
Most consumer digital cameras have multiple auto focus modes. Single Area Focus is one of them. What it does is recognize the object that is most prominent in the screen and it focuses on that. It is usually accurate, but like all auto modes, it is not perfect and your lighting does play a factor in what your depth of field will be.
Understanding Single Area Focus
To get the focus you want, it is best to use a manual setting and measure the distance from your lens to your subject with tape measure. But, not all cameras can do this. The majority of consumer cameras only have auto focus features.
Because you're relying on the camera to do the focusing, it sometime doesn't turn out the way you want it. You might be shooting a fairly wide shot and the subject you want in focus is out of it.
To compensate for this, zoom in real close on their face and hold still for a moment. Since the camera is in single area focus mode and their face is filling the frame, it will come into focus. Now slowly zoom out into the frame you want. The subject should now be in focus. If you lost them, repeat these steps and zoom out of their face slower.