When I bought my Olympus OM-D E-M5 in February, the first priority was figuring out the highly customizable menus in as short a time as possible. I had received the camera and housing just in time to head to Cozumel for a quick dive trip and I needed to be ready to hit the water! I searched the web and turned up a couple of posts from other underwater photographers with a great list of recommended settings, configurations, and button assignments most appropriate for underwater use in a housing. I didn't know if what I found would be complimentary to my style of shooting, but it was a good place to start. One thing I came across a couple of times that I couldn't really wrap my head around was the reassignment of the focus lock function to a button other than the shutter. Hmm.....interesting concept....but why?
This concept is what is commonly known as "back button focus", and as I mentioned, it assigns the function of triggering your autofocus process to a separate button from the shutter. This can be a function button literally on the back of the camera, or any other reassignable button that feels comfortable for you. Now, why would you want to have your focusing function separate from your shutter? Isn't it simpler to utilize the half-button-press on the shutter to lock in your focus? Surely requiring the use of two buttons to accomplish the same thing we can do with one is overly complicated, right?
True, what you were formerly accomplishing with your single shutter button is now a two button operation. This does take some getting used to when you first start to use it, but I am pretty sure that once you try it, you might just like it. With the OM-D E-M5, there are some menu choices to run through to transfer this function to another button, and most cameras that allow this capability are probably similar. I've got mine now set up such that when I elect to set the camera to any autofocus mode, things operate in the usual manner with the half-press of the shutter. However, when I choose manual focus, I can either focus the camera manually on the lens itself, or with a single press of my Fn2 button, I can accomplish a one-time trigger the S-AF (single autofocus) function. This can prove to be very handy for a few situations. The key to appreciating back button focus is realizing that you can focus once, and shoot many times! This is a real advantage for situations where you can't afford the possibility that your camera will be autofocusing on the wrong subject or at the wrong time (or not at all!!) when you are trying to get that perfect shot. If you are in any shooting situation where you can pre-focus on a specific target, and then wait for the intended subject to come into the frame. At that point you can fire away, and not having to focus at every press of the shutter can really increase the chances that you will capture the shot you want.
I have recently used the "back button focus" method when shooting both fireworks and lightning. In these situations I don't want to miss a shot because my camera is struggling to grab a focus lock on the split second it has to capture my shot. Manual focus can be a challenge with a micro 4/3 camera, when focusing on infinity in the dark. Well, OK, it's a challenge for me at least! My eyes are worse than they ever used to be :) But with no focus point stop at infinity on any of my lenses, it's just too easy to miss the perfect focal point in many situations. With my back button focus, I can aim at a very distant, but distinct point, set my autofocus selection points, and then use my Fn2 button to lock in the S-AF. After that, the focus is completely locked because I am set on manual focus as the primary focusing method. Now I can turn my attention to shot timing and exposure settings, with no frustrations with autofocus delays or missed locks.
Another very useful scenario for back button focusing is with macro photography. You can lock your S-AF one time with the back button focus, re-compose your shot as necessary, and then simply "rock" back and forth slightly to fine tune your focus without having to re-engage the S-AF. I have found that this works extremely well for many macro shooting situations, and prevents the camera from constantly making focus changes every time you take shot. When taking macro shots of animals, the key is always to have the eyes in focus. With some of my favorite underwater subjects, the eyes can be very tiny, and using this method allows me to find a good, solid focal starting point, and then with slight movement of the camera, I can fire the shutter just as the eyes come into the sharpest point of focus.
Give back button focus a try next time you are out shooting. I bet you find that in the right scenario, it will really help you get some shots that autofocus delays might otherwise cause you to miss.