You can but it's not reliable. mozCamera returns ids for the faces detected. Same id means same face but it is extremely unreliable. Any slight movement (even your hands shaking due to heart rate pulse) of camera makes the hardware think that the faces have changed when they're actually the same. This results on frantic focus on the same faces again and again. Not good UX. I realized that in practice it doesn't matter if you focus first on 3 faces and later other 3 different faces are thought to be the same. In pictures with a lot of people and at a distance that the camera can detect tend to be aligned so all the faces end up in focus. I'm sure there's edge cases where this logic wouldn't work but the user can fall back on touch to focus to manually control the focus.