Panasonic C-D3100ZM-S Instrucciones De Operación página 34

Special auto-focusing situations
The following objects can occasionally be hard to focus; very shiny
surfaces(e.g. glass, metal, water, mirrors), dark hair, flames, smoke,
objects smaller than the focusing frame, and items seen through a
window.
To focus on such subjects, aim at an object located the same distance
away as the desired subject and set the focus lock as described
before.