BACK BUTTON FOCUS
Back button focusing allowed me to take this photo and two others of the same subject without refocusing since the distance from the camera to the shutter didn't change.
Over the past couple of weeks I've become a huge fan and advocate for making the change to focusing with the button on the back of your camera instead of pushing the shutter button down halfway, commonly referred to as back button focus. It was one of the most revolutionary changes I ever made to how I take pictures. It seems like such a small thing, yet it's a huge difference in the way you take pictures and how your camera focuses. This should really be the standard setting on all cameras, cause once you try back button focusing, you're gonna like it. In fact, you'll probably even love it! If you're one of the people who have never tried this -- STOP EVERYTHING NOW and do this. You'll thank me :). If you don't love it, let me know and I'll buy you lunch... but not an expensive lunch. Let's say a sandwich and a drink... but no chips. Okay chips, but no dessert.
One of my biggest frustrations when shooting photos is having to refocus ever time I recompose the shot. When you're taking a bunch of pictures in a short amount of time, this can be really time consuming, frustrating and just an overall pain in the ass. The biggest thing that has impacted my images in the realm of focus is switching from using the shutter button to control focus to using the back "*" button to control focus. And if you're a Nikon shooter and don't have a "*" button, don't worry -- there is still hope for you. :). Making this change takes some getting used to, mainly because for the first few times you try this after switching, you'll probably keep expecting your shutter button to do the focusing out of habit. The problem is after making the switch, you're shutter button is only going to activate the shutter... WHAT IT'S SUPPOSED TO DO!!
For this image I focused by using the back button method. Put my camera on a tripod, locked in my focus, then took this picture and a few others I recomposed three or four times but never had to refocus the camera.
The idea is to separate the focus function from the shutter function. I love it because when I'm shooting a stationary portrait, or some situation where the camera to subject distance is constant, I can lock in the focus (push the * button with the focal point on the spot I want in focus, then let go of the button) and shoot away, even re-composing, and the focus will stay dead-on. This has also freed me up from constantly changing my focus points and always refocusing every time I take a new shot, even though I didn't move the camera more that a little bit, or in some cases, not at all. I'd still have to refocus because the act of focusing was attached to the shutter button. This was one of the most frustrating things I ran into when I used the ‘shutter half way down method of focusing. Sure you can lock in focus by holding the shutter half way down, then focus will stay locked as long as you hold your shutter in shutter-limbo. But then you have to hold your finger there! If you let go or accidently lift your finger just a little bit, the camera will refocus as soon as you press it down again. Or press the shutter a little too hard and you will take a picture before you’re ready. I now leave the center focus point selected and recompose after locking in the focus using the "*" button on the back of the camera.
In the situation of a moving subject, I also like the back button because it allows me to track the subject and still get consistent focus.
RC answers a question on what is the back focus button. He talks about how to set your Nikon or Canon to back focus and explains why you'd want to.
If you want to try back-button focusing, you need to change some of your custom functions. You can very easily search online for the directions for making this change to your particular camera. I found the directions for my 5D Mark III in about three seconds and made the change in about six seconds. It may have actually been seven seconds, but I was a little distracted. You should notice that your camera will focus when pushing the * button but when you push the shutter button, your focus will not be impacted.
Please let me know if you have any questions, or need any help making this change.