Trap focusing guide says:
- Set custom setting #A5 to AF-ON ONLY
- Set the focus mode to SINGLE SERVO
- Set Auto focus area mode to SINGLE POINT (9 point, 21 point, 51 point or 51 point 3-D will all work)
- If your lens has an A/M switch, make sure it is set to A for autofocus mode.
- Pre-focus the lens for a particular distance. Don't forget that a more convenient way to use this trick is to pre-focus using not the shutter button, but the FOCUS button on the back of the camera can be used. I find this easier and more effective.
- Press and hold the shutter release ALL the way down, the camera shoots only when the selected focus bracket is in focus.
Now, I can see a clear gap between #5 and #6. If I pre-focus with AF-On and depress the shutter, it should fire readily unless I somehow make it hunt for focus. The instructor then clarifies:
- Compose your shot and set the focus by aiming the centre focus icon at a definite target at the precise distance you want, for example, a branch where a bird is about to land, or on second base where the baseball player's foot will land. Press the "AE-L/AF-L" button near the viewfinder.
- This will focus the lens, then let go of AE-L/AF-L button.
- Turn away, press and hold the shutter button all the way down.
- Now turn the camera back to the pre-focused point and wait, when the subject gets in the sweet spot the camera will take the shot.
However, another question pops up in my mind. Let's say, I want to photograph a bird which is expected to sit on a branch. Of course, I have to pre-focus on the branch, right? At #3, the instructor asks us to
Turn away. I presume, this is tantamount to recomposing the frame. I guess the reason is to forcefully defocus the shot so that the camera does not fire when I depress the shutter button. The crucial point is, at step #4 when I am asked to
turn the camera back to the pre-focused point and wait (i.e. the branch where I pre-focused initially), why on earth the camera would even wait? Won't it readily capture the branch itself into focus and fire?
As a side note, even this post, too, couldn't clarify my doubt unfortunately.