Back button focus
According to an established tradition, autofocus is activated by pressing the shutter button halfway, and pressing it fully releases the shutter. However, it is often much more convenient to separate the shutter release and autofocus. Back-button focusing or back-button focusing is an autofocus control scheme that is not very popular among amateurs, in which autofocus is activated not by the shutter button, but by an independent button on the back of the camera.
Some cameras have a special focus button AF-ON, and some allow you to reprogram the AE-L / AF-L button so that it is responsible for autofocus, and not for exposure lock.
Unfortunately, the instructions for the cameras describe this feature so sparingly that an inexperienced amateur photographer may encounter considerable difficulties trying to understand why back button focusing is necessary and how to set it up correctly, but I’ll try to clarify the situation a bit.
Why do I need back focus
The traditional layout of autofocus control is good when the shutter should be released immediately after the camera reaches precise focus. In those cases where some time should pass between focusing and releasing the shutter, or when you need to take a picture without changing the focus distance, it is highly desirable that the autofocus and shutter release be controlled independently of each other.
Consider a couple of typical situations in which traditional autofocus control creates problems for the photographer.
Imagine that you are going to photograph a picturesque landscape. By installing the camera on a tripod, you select the appropriate focusing point and, pressing the shutter button down to the first stop, focus on some subject matter of significance, whether it is a waterfall, a tree or a pile of boulders. After that, you release the button, recompose the frame, and, having achieved a harmonious composition, fix the tripod head. Now, if you press the shutter release (on the camera or on the remote control), you have every chance of getting an unfocused shot. Why? Yes, because when you changed the frame layout, the active focus point moved from the tree or waterfall of your choice to some bushes or rocks in the background. By pressing the shutter, you again activated autofocus and allowed the camera to focus completely where it was needed.
The first way to avoid unnecessary refocusing is to simply turn off autofocus after focusing by using the switch on the lens. The second, more convenient solution is to use focus with the back button.
I will give one more example. Suppose you are shooting a portrait. You point the focusing point to the eye of the person portrayed, press the shutter button halfway and, holding it in this position, change the composition of the frame as you see fit. Then you press the button completely and take a picture. But suddenly, the person you are photographing smiles and you, not wanting to miss the photogenic moment, press the trigger again. Naturally, the picture will be out of focus! After all, the focusing point has shifted again and, when you press the shutter release, the camera zoomed in to an unknown destination. You would simply not have time to return the focus point to the eyes, lock the focus and restore the composition of the frame. If an independent button was responsible for autofocus, refocusing would not have happened.
Benefits of Back-Focusing
The whole charm of focusing with the back button is the ability to decide for yourself: when you need to activate autofocus, and when to release the shutter.
After disengaging the shutter and autofocus, half-pressing the shutter button wakes up only the exposure meter and stabilizer (if any), and autofocus is activated exclusively by the back button that you press with your thumb. By releasing the button, you lock the focus, even if the tracking AF mode is turned on, which is not possible with the traditional focus control scheme. It is focusing with the back button that allows me to use tracking autofocus for absolutely all subjects. The operation of autofocus in single mode is easy to simulate, just turning on and off autofocus via the back button.
Using the back button, you can focus on any element of the scene, and then, releasing the button, freely re-arranges the frame, without fear that the camera will decide to personally shift focus at the moment the shutter is released. At the same time, you maintain constant instant access to the tracking autofocus, in case you suddenly need to photograph some dynamic plot. This is especially true for low-cost cameras that do not have an autofocus mode switch on the body and, therefore, require frequent menu travel, which can significantly slow down shooting.
If the distance to the subject does not change, then you do not need to focus again before each frame, catching the subject with a focusing point. There is also no need to constantly keep the shutter button half-pressed to lock the focus while you wait for an opportunity to release the shutter.