Question on Al Servo AF for Moving Subjects

[cecilc]

Hello, all you learned Canon gurus .....

Another post in this forum got me to thinking about Al Servo AF in regard to 1) focus, and 2) custom function 04-03.

I get the impression from the 10D manual that, if the half-shutter press is used for focusing in Al Servo AF, when you half-press the shutter on a moving subject to gain focus that the focus continually adjusts to match the moving subject. Am I reading and understanding that correctly?

Does transferring the focus to the assist button (in custom function 04-03) alter that in any way? I use the "back button" for focusing and I usually use the Al Servo AF since I shoot a lot of high school and college lacrosse games (full of moving subjects). In order for my 10D to continue to alter focus on a moving subject using the back button, do I need to hold down the assist button before shooting, or is a one push of the assist button all I need to do to keep the camera focusing on a moving subject?

Any and all help and explanations will be appreciated - my eyes are starting to unfocus .....

[scottbergerphoto]

In AI Servo, when you half depress the shutter button, the camera locks on initially with the center AF point and then follows the subject with the appropriate AF points unless you have limited your focus points to a single point only. It doesn't lock exposure until you fully depress the shutter button and take the shot. I believe if you move focus to the *, the same procedure applies except you have to hold down the *, and then depress the shutter to lock exposure and take the picture. If you realese *, the AF stops. Some people take advantage of this when using flash. Releasing the *, causes the flash to average all the focusing points giving less erratic ETTL, much like switching to MF.
Scott

[cecilc]

Thank you, Scott....

That answers several questions and probably prevented me from trying something that would have looked very stupid .....

[KennyG]

Can I add something? You must remember two important things about AI Servo - It takes time to calculate focus and estimate the changes, so you need to allow at least one second for that, plus if you use centre point only it is much faster.

[cecilc]

Can I add something? ..... plus if you use centre point only it is much faster.

True, true....

I do only use the center point - I've also found focus response time to be much faster using only that one point. And, I don't know that in shooting lacrosse (at least not with me!) that I'm ever trying to focus on something (or someone!) that isn't already in the middle of the frame. So using the other focus points in that situation is not even an issue ....