When shooting headshots focusing needs to be incredibly precise. I never use global focus and let the the camera decide what to focus on. Single point focus on the nearest eye to the camera is where I focus every single shot. There is nothing worse than having a great shot slightly out of focus and my miss rate has gone down to close to 0% after switching to back button focus.
Back Button Auto Focus
By default on most cameras the focus is tied to the shutter button. This arrangement works out great for most types of photography(especially snap shots) but it’s not ideal for headshots. When shooting headshots what I generally do is focus and recompose slightly to get the perfect framing. Traditional shutter button focusing would require me to focus and recompose every shot- because each time I press the shutter the camera is going to want to focus on the current single point focus area that I have selected. Back button focus separates the shutter and focus action so I can focus with the back button then recompose and keep shooting(pressing the shutter button) away without fear of refocusing on the wrong area. I now only have to refocus when the subject moves.
Use the focus toggle but don’t rely on it
Why not just use the single focus point and toggle that to achieve the exact framing you need? When shooting headshots I always use single point focus and focus that one the nearest eye to the camera. I do toggle the focus point to be in the right area of the frame but there simply are not enough focus points available to get that single point in EXACTLY the right place. So toggling the focal point to the nearest eye gets me close and focus and recompose gets the framing perfect.
Give this technique a try- you won’t regret it!