Quick Question about AF points

I learned photography from an old Minolta XD-7, so I am considerably new to the AF world. Yes, I've had a 20D, now a 30D and a XT/350D but I guess I still tend to manual focus more often than not and my AF understanding is at exactly where I would like it to be.

Anyway, I've read many of you only use the center focus point. Which makes good sense because on most cameras including the 350D/XT and the 30D the center point is a cross point. However, since I'm sort of a Rule of Thirds shooter, with the very rare exceptions do I ever shoot otherwise, how would you use the center focus point and still adhere to the rule of thirds? Thank you in advance.

Oh lastly, I don't believe I have a strong grasp on the various AF modes, can someone explain AI Focus, AI Servo, etc. and the differences? Thanks.

Your manual is the best place to start. But in a nutshell you can do one of two things. Place the center point on the subject you want to be in focus, hold the shutter down half way locking the focus and recompose. You can also use custom function 4.1(I think that is the right number). Set it to on and one of the buttons on the back of the camera can be used to lock the focus.

AI Focus: the camera locks onto something and stays at the focus point while the shutter is depressed.

AI Servo: the focus will lock onto something and adjust as it moves. Used mostly for sports or anything that is moving while composing the shot.

Many will focus and recompose, but be wary of that with shallow depth of field as can cause missed focus in some situations, see here. Otherwise use the most suitable point.

Do some searching on AF modes for details; but the simple version is One Shot for stationary stuff - focuses when you press shutter (or * button if CF 4-1 activated) and refocuses if you press again. AI focus tries to guess whether subject is moving and choose the mode - most will say it doesn't work well and don't bother. AI servo is designed to track objects and continuously try to keep them in focus, good for moving subjects. Many will use CF 4-1 (see mega threads on this...) so a press and hold of * button will keep focusing on, but you can press and release to in effect give a one-shot type of focus. Read the manual sections several times on these issues as there are some subtle but useful points there, such as how it tries to behave in rapid-fire sequences etc.

eddarr wrote in post #5255885
Your manual is the best place to start. But in a nutshell you can do one of two things. Place the center point on the subject you want to be in focus, hold the shutter down half way locking the focus and recompose. You can also use custom function 4.1(I think that is the right number). Set it to on and one of the buttons on the back of the camera can be used to lock the focus.

AI Focus: the camera locks onto something and stays at the focus point while the shutter is depressed.

AI Servo: the focus will lock onto something and adjust as it moves. Used mostly for sports or anything that is moving while composing the shot.

I think you confused AI Focus and One Shot

I did, sorry. Thanks for catching it. I should be flogged.

So wait, if I change cf 4.1 it will no longer lock exposure and lock focus instead? I sort of misplaced the manual for both cameras, hence the question, sorry. I'm thinking of picking up one of them Magic Lantern books on both cameras, but haven't had the time. So to get this right,

Use the Center FP, use the asterisk to lock (making sure I have this set on the CFn. 4.1) and then recompose?

One Shot - This one was the obvious one, yes, simple, lens will focus on whichever FP is selected.

AI Servo - Will lock on to something and follow it as it moves, as in "panning shots", right?

AI focus - the camera tries to determine which system to use; either AI Servo or One Shot.

Right?

You have the modes correct. Manual for 30D pdf here http://www.visibledust​.com/manuals/Canon_30D​.pdf. You can focus and recompose either with shutter half press (CF 4-0) or with * button (CF 4-1).

The 30D user manual is available here. Just in case you need it.

I don't know why I'm in here. Marks got it handled

We keep answering at the same time so we don't see the other's answers. Feel free to take over, it's bedtime in the east!

But why, your doing so much better than I.

Thanks guys, last question...what the heck does the AF-On button do on the 1D(s)3 or the 40D?
Thanks.