From what I understand on BBF it depends on what you set the cameras focus mode to.

When you press the button, it activates whatever focus mode you have active. I have mine set on continuous autofocus so when I press the button, the camera will continue to maintain and adjust focus, and will stop trying as soon as you release the button.

If your photographing a stationary object from a tripod, then pressing the button to focus then releasing the button before you press the shutter button will work perfectly well. If your subject is moving in relation to the focal plain, you can just keep your finger on the BBF button to tell the camera to continue to maintain focus while you press the shutter button.

Disclaimer: I'm a Nikon user so not certian that my comments are valid for Canon

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