With most cameras, the shutter button performs three functions. A half press starts the autofocus and analyzes the scene for the metering, and a full press releases the shutter. In many situations, having one button do all of these tasks is fine. But in situations where you want a little more creativity, especially where you have a moving subject, separating the focus from the image capture may be more useful.
Most DSLRs today allow you to redefine the functions of many of the camera’s controls. You can use this feature to move the focus activation to another convenient location, the most popular being one of the buttons on the rear of the camera, within easy reach of the thumb. This is referred to as back button focus. It’s a little tricky at first since it’s foreign to think of focusing separately but once you realize how liberating it is, it becomes natural.
Consider the following example where you have a small subject, the singer’s face, that is constantly moving. It makes sense to switch your camera’s focusing mode to constant. For Canon, it’s called AI Servo. In operation, you follow your subject around, placing the focus point on your subject while holding the back button that you redefined for focus activation. Once you have the scene you desire, you can either release the back button, thereby locking focus so you can recompose for your desired composition, or, if your subject is in constant motion, you hold the back button (that’s where the AI Servo mode comes in handy) and shoot away with the shutter button.
In this case, I was using the center focus point and was following the face, waiting for a pose that I wanted to capture. With my thumb on the back button, the focus system was constantly adjusting focus (AI Servo) while I was waiting to snap the picture. When the time was right, I simply released the back button, holding the current focus, quickly recomposed the frame, and pressed the shutter button to get several shots.
You can certainly shoot by focusing with the shutter button but separating it does give you the freedom to choose to focus when you want and lock focus quickly by releasing the button. It is especially useful when you are shooting different compositions of a scene but your distance to the subject is not changing. Keeping the focusing separate from the shutter release keeps you from inadvertently focusing on the wrong part of the scene. It’s also handy when other objects are entering the scene and may interfere with your focusing.
It takes a little while to get used to separating the focusing from the shutter release but once you get used to it you will find that you have more control over your camera. All of my camera bodies are set up in this manner now. If you are happy with your current operation then separating focus and shutter release may not be for you. If you decide to give it a try, check YouTube for all sorts of tutorials on your specific camera model.