Back-button focusing can be quite a useful feature to use and is generally found on the majority of DSLR models from almost any manufacturer. The person who asked me about this feature is specifically using an Olympus E-5, so I'll be able to reference some page numbers in the manual --specifically pages 105-106-- but for the rest of you, simply do some digging and I'm sure you'll find it in your respective booklets. In addition, back-button focusing on the E-5 is controlled via the Auto Exposure Lock/Auto Focus Lock (AEL/AFL) button on the upper-mid backside of the camera, so again refer to your specific manual to determine what button is assigned to back-button focusing on your system.
In a nutshell, back-button focusing allows you to use that little button on the back of your camera to lock focus or exposure. But of course the question is why would it be advantageous to perform such a task using the AEL/AFL button versus the shutter button. I mean one can just as easily half-press the shutter button to lock focus and exposure, then you can compose the scene, and finally full-press to take the shot. Well allow me to use an example to explain why.
A commonly used scenario is a photographer shooting a model for some portraits. Everything is fine and dandy if the model is more or less dead center in the frame. Pressing the shutter button will likely result in a sharp picture of the model with a fairly accurate exposure. But often, asymmetrical compositions are more pleasing to the eye so the model tends to be pushed off center. In such a case, the camera might not focus on the model but instead on the background, thus leading to a potentially out of focus and unhappy model.
Enter the back-button focus option... well almost. First off, many DSLRs have several select-able options on how to behave when the shutter button is half-pressed, fully depressed, or when the back-button is used. On page 105 on the E-5's manual, there is a nifty table describing each mode. For example, if your camera is set to single auto-focus (S-AF) and you're setup to use mode 2, then when you half-press the shutter the focus is locked but the exposure isn't, and when you fully press the shutter the exposure is only then calculated and locked; if you use the back-button in this case, then only the exposure is locked, but not the focus. It's not that this very complicated, but there are certainly quite a few setup options to choose from. By the way, check the menu on your Oly camera to change these settings; off the top of my head (since my cam is currently setup for product shots and I'm lazy to go get it), these settings are hiding somewhere in the "gears" icon area and it's either the A or B section (AF/MF or BUTTON/DIAL, respectively).
Let's return to our model example and now that we know about modes, we'll stick to S-AF but in mode 3; to clarify, half-pressing the shutter button will only lock the exposure, full-pressing doesn't do anything, and pressing the back-button (that is the AEL/AFL button) the focus is locked. So let's run through this:
- Since the distance between the model and the photographer generally won't change much for a few poses and shots, we first point the camera on our subject and we'll press that back-button*. This locks the focus but not the exposure.
- Now, as the focus is locked to the plane the model is in, we can put him/her off center and start shooting.
- Every time we half-press the shutter the exposure will be calculated and locked, and upon fully pressing the shutter button the photo will be taken.
Cool hey? So an advantage we're seeing here is that the use of the back-button can make it easier to compose our shots. In mode 2 (as described above) and without using the back-button, one would have lock focus on the model (half-press) then frame the scene while will half-pressing the shutter button, then fully pressing when done... repeat. This takes extra time and if you had a good composition for one shot, you might not be able to exactly go back to that same framing due to all the extra motion required. In addition, since the focus has been locked once, taking shots becomes blazing fast as the lens does not to be refocused for each shot (even if the focus would be in exactly the same spot shot to shot).
In the resources section below I've linked to an article on Canon's website that discusses the many uses of back-button focusing. I strongly recommend reading it as it should provide you with a few more ideas and benefits to employing this method. And for those of your curious, the second link takes you to the PDF version of the Olympus E-5 manual, so even if you don't use the system, you can at least get a good idea of what I'm talking about here.
* Ok, another thing to complicate this. On most cameras, like the Oly E-5, the back-button has a memory option. If the option is set to ON then pressing the AEL/AFL button will keep the focus or exposure locked until the button is pressed again. If the option is set to OFF then the focus or exposure is only locked for as long as you hold down the button. The latter option can be useful in some cases, but as I've mentioned, check out that article from Canon on the topic.
Continuous or Predictive Focusing
Many DSLRs also have the capability to focus continually, depending again on how one's camera has been setup. On the Oly the option for continuous auto-focus is C-AF and in most modes starts working when you half-press the shutter button. So let's say you are shooting at a car race and you see a vehicle rushing at you (well hopefully not completely at you, as that could end poorly). Using C-AF you lock focus on the car by half-pressing the shutter button and as you keep half-pressing the camera continually adjusts the focus of the lens as the car, or whatever other subject in motion, rushes by. To take a photo of course, you simply fully press the shutter button. The back-button can also be used, but you might have to adjust the settings. The idea here is that you should... should have a perfect focus lock on the subject at all times, thus when you take the shot you should... should get that sharp image.
In all honesty, I think I've only used this feature on my camera for fun, merely to try it out. Those shooting action or sports subjects will likely find more use for it, but in most other cases S-AF or manual focus will do the trick. Nonetheless, go for it, try out the feature and see if works for you.
Lastly, I wanted to quickly touch upon manual focusing. I can't help but find myself using this method of focusing more and more often these days. Keep in mind, I generally shoot landscapes and things up close, so being a tad slower than the auto-focus system is ok in my case. But if you haven't given manual focusing a chance, I must suggest that you give it a try now and again.
For one, it can really speed up shooting, whether in good or bad lighting, as one of the slowest operations for almost all cameras is focusing. In manual focus mode the camera trusts that you've locked onto whatever you want and simply calculates the exposure before actually taking the photo. Calculating the exposure is extremely fast and virtually unnoticeable for us humans. When I've shot some macro images, I find that I can be much more proficient at selecting where I want the focus to be than trusting the machine to do it, and I've also realized a benefit when framing the scene. And I have to admit I get a little nostalgic about it; kind of feels nice to have more control than the silicon beast. Hopefully you'll find manual focusing as refreshing and frankly, useful, as I have.
That is it for now! L8r!