I was in the studio the other week experimenting with some mottled grey backgrounds that are designed to be lit with coloured gels.
In the above test shot I had two flash heads gelled with blue cut filters at the top and two flash heads without gels lighting the bottom. You can do this sort of thing in Photoshop but in camera is easier.
This shot shows the issues with focusing, particularly with the common technique of focus and then recompose. The problem is I focused on the high contrast detail on the dress, that you can see is in sharp focus, I then recomposed and took the shot. As you can see the eyes are not quite in focus.
In this situation its really worth the time to use the selector on you camera to select the focus sensor closest two the eyes, usually the front eye to get things sharp.
Luckily I did not make the mistake on all my shots.