Have you ever heard someone talk about Back Button Focus? Did you wonder what they were talking about? Well, I did and wondered why would people want to focus with a button other than the shutter button. Then I did a little research, here is what I was able to find out.
Back Button Focus
This is a feature found on many modern digital cameras. It allows you to change the the focus from your shutter button to another button either defined by the camera manufacturer or one that the user defines. Higher end cameras tend to allow the user to define the button to be used for focus, or lower end cameras give you one or maybe two buttons that you can change the focus too. You should be able to find out if your camera allows this option by checking in your cameras manual.
What it does
The purpose of having another button doing the focusing instead of the shutter button is not that complicated actually. Imagine you are taking pictures of a subject that is 25 feet away, you are half pressing your shutter button and as you are about to take the photo, someone walks between you and the intended subject. The camera tries to focus on the new object in your viewfinder and focuses shorter and then longer. Maybe you hit the button before it has the chance to focus back on the subject. You now have to frame and refocus to get the shot. What if it was one of those once in a lifetime shots, sorry maybe next time.
But one of the other things I use it for quite often is when I am shooting sports. Most of the action is in about the same spot all the time. So I can use the back button to focus and then just make sure I have my shot ready and hit the shutter button. This can also be useful when shooting portraits since you don't have a lot of distance changes in that environment.
One other reason I use back button focus is I sometimes like to manually focus my lens for maximum focus control. If I forgot to change the lens to manual focus, as soon as I press the shutter button, nothing happens anyway. So I really never have to change the lens into manual mode.
While it did take me a few days to get used to back button focus. Now that I am used to it, I LOVE IT. I like the ability to separate the two processes (focus/shutter actuation) onto separate buttons. If you haven't thought about it before, give it a chance. You may find that it is exactly what you were looking for even though you didn't know it.