Using the back button AF is a technique that allows you to focus on your subject without having to press the shutter button halfway. It involves assigning a different button on the back of your camera to activate the autofocus system, instead of using the shutter button. Normally, when you press the shutter button halfway, you activate the autofocus system and lock the focus on your subject. When you press it fully, you take the picture. However, if you move your camera after locking the focus, or if your subject moves, you may lose focus and end up with a blurry image. By using a separate button for focusing, you can avoid this problem and have more control over when and where to focus. This way, you can lock the focus on your subject and recompose the frame without losing focus. Using back button AF can be useful for situations where you want to have more control over the focus point, or when you are shooting moving subjects and want to track them continuously. Some of the benefits of using back button AF are:

• You can avoid accidental refocusing when you press the shutter button fully.
• You can switch between single and continuous autofocus modes without changing the settings on your camera.
• You can prevent focus hunting when shooting in low light or low contrast scenes.
• You can use manual focus and autofocus interchangeably without switching modes.

To use the back button AF, you need to assign a button on the back of your camera to activate the autofocus system. The exact procedure may vary depending on your camera model, but generally, you can find it in the custom functions menu. Once you have assigned the button, you need to disable the autofocus function from the shutter button. This way, the shutter button will only be used to take the picture, and the back button will be used to focus. To use the back button AF, you simply press and hold the back button to focus on your subject, and then release it when you have achieved focus. You can then recompose the frame and press the shutter button to take the picture.