Confused with Al Focus AF and Recomposing a scene

[snowshine]

Mine Canon 40D.
AF Mode set as: Al FOCUS AF [Not One Shot AF nor Al Servo AF]
Custom Function Setting set at:
C.Fn IV -1 Shutter button/AF-ON button set for 2
2 represents:- Metering start/Metering + AF start

The above custom function setting has seperated the focus function from my shutter button to my AF-ON button on the back of the camera.

In the following scenario of mine,
When I frame a scene with an individual in the scene.
Place my camera auto focus point on the face of the individual in my scene to identify my exposure by half pressing the shutter button and then the * button on the back of the camera to lock in my exposure selection. Now while still the auto focus point is on the face achieve my focus by pressing AF-ON button on the back of my camera.

Now when I slighlty change my camera to compose the scene, will the face of my idividual that I focused will remain in focus or
Should I keep the AF-ON button pressed on while I try to move very slightly away from the face to recompose my scene?

What I am after is to know if the focus achieved will remain locked once the AF-ON button on the back of the camera is pressed and released?

What do you feel?

[Donald]

Arun

I hope I am understanding your question and, therefore, give an appropriate response.

Given all the circumstances you describe, the answer to your question should be 'yes' the focus will remain on the subject's face.

However, there are things about which I am not clear and which I think could present problems with that answer.

I appreciate that I might not be understanding how the camera is behaving because I have never used AI Focus. And I am wondering why you are? For the sort of shot you are describing, I would have thought that you should be using one-shot focus.

For me, the advantage of back-button focusing is that you can use the central point in the viewfinder for focusing and know that it is then fixed whilst you re-compose and then press the shutter. Also, for me. it's now just a matter of habit. But if you are using AI Focus, I do not know how the camera behaves in terms of thinking that objects are moving during the act of re-composing. Presumably it might think that there are moving objects and automatically go into the servo mode, thereby over-riding the focusing you have done with the back button.

As I say, the bit I don't understand is why you are using AI Focus.

[snowshine]

Hello Donald,
I will turn my camara to One Shot One-Shot AF.

In the above mode once I focus the subject using the back button [AF-ON Button] and the central point in my view finder, that focus will remain locked while recomposing my scene is what my assumption. Am I correct? I do not keep the back button pressed while recomposing then?

I thank you for help me understand it.

Regards

[Donald]

Arun

That is correct. The shutter should play no part at all in the focusing action. Once you have focused using the back button, that is where the focus will stay until you change it by, once again, pressing the back button.

If you ever do go into AI Servo mode, then just keep your thumb (well, I use my thumb) on the back button. That keeps the AI Servo action running.

[snowshine]

You are wonderful in answering as ever.
Three cheers to you & Thank you
Regards