AI Servo

[kdoc]

In this post, http://photography-on-the.net/forum/showthread.php?t=81536&highlight=Servo I think Scott Berger suggested that the multiple point tracking will only work if you start with all points active on the 20D. That is, if you start with only center point focus, only the center point will track (and the center point could lose the subject). So presumably you must start with all point focus in AI Servo, in order for it to start tracking with the center point, then follow the subject with any of the points. Is this correct? Is the best way to use AI Servo to begin with all point focus active? And if so, how do you deal with the problem that with all points active, you often end up locking onto the wrong subject (the reason I use in one shot focus, only center point focus).

kdoc

[PacAce]

In this post, http://photography-on-the.net/forum/showthread.php?t=81536&highlight=Servo I think Scott Berger suggested that the multiple point tracking will only work if you start with all points active on the 20D. That is, if you start with only center point focus, only the center point will track (and the center point could lose the subject). So presumably you must start with all point focus in AI Servo, in order for it to start tracking with the center point, then follow the subject with any of the points. Is this correct? Is the best way to use AI Servo to begin with all point focus active? And if so, how do you deal with the problem that with all points active, you often end up locking onto the wrong subject (the reason I use in one shot focus, only center point focus).

kdoc
The way it works in AI Servo mode is, when you select the Auto AF Point selection mode (or what you refer to as all points active focusing), only the center AF point is active initially. This is what the camera uses to acquire focus on a subject. Once focus has been acquired with the center AF point, the camera will then use any of the other AF points to keep tracking to subject if the subject moves out of the center AF point. As long as the subject can be covered by any of the other AF points, the camera will be able to keep tracking and focusing on the subject.

[kdoc]

Thanks: I know that is what the manual says. What isn't clear is whether you have to be in "Auto AF Point selection mode" for this to work properly--for example, if I'm in "One Shot" with Center Point focus, and then I change to AI Servo, without moving out of center point focus, does AI Servo worp properly, as you and the manual has described above? Please read Scott Berger's post I cited above to see where the question comes from. Scott?:lol:

kdoc:)

[scottbergerphoto]

You cannot have it both ways in AI Servo. If you manually select an AF point such as the center one, then that is the only point that will track the subject. If you do a search above on AI Servo, you will find my link to Canon's Chuck Westfall.
http://www.robgalbraith.com/ubbthreads/showflat.php?Cat=&Number=174243&page=&view=&sb=5&o=&fpart=all&vc=1

[PacAce]

Thanks: I know that is what the manual says. What isn't clear is whether you have to be in "Auto AF Point selection mode" for this to work properly--for example, if I'm in "One Shot" with Center Point focus, and then I change to AI Servo, without moving out of center point focus, does AI Servo worp properly, as you and the manual has described above? Please read Scott Berger's post I cited above to see where the question comes from. Scott?:lol:

kdoc:)
AI Servo will work properly with any focusing point you choose. If you want to manually select the center AF point, then in AI Servo mode, that is the AF point (and the ONLY AF point) that is used. If you want to use one of the other AF points, then again, that selected AF point is the only one used for focusing, whether you are in One Shot mode or AI Servo mode.

Auto AF point selection mode is mutually exclusive from manually selecting the AF points yourself. It's one or the other. You can't manually select the center AF point and also have be in auto AF point selection mode.

I think your confusion is coming from the fact that auto AF point selection behaves differently when the camera is in OneShot mode and in AIServo mode. In OneShot mode, you have almost no control over which AF point is selected for focusing and any one or more of the AF points can get used. However, in AIServo mode, focusing is always acquired using the center AF point. And this is true all the time. Once focus is acquired, the camera will use any of the other AF point, if necessary, to track the subject so there is no "unpredictableness" with AIServo as there is with OneShot focusing mode.

[kdoc]

Thanks Scott: That thread was excellent. And does answer the question. We're clearly left with two options according to Chuck: 1. Select automatic focusing point selection with AI Servo, and make sure that the center point is pointing at the subject to begin--and then the other points will help with the tracking. Or 2. Select center point focusing, and keep that point on the subject. If it drifts off, you'll apparently lose the AI Servo value.

kdoc

[kdoc]

And thank you too; I didn't see this post when I answered Scott's, as I was reading the thread.

kdoc

[gasrocks]

Whether it answered the quesiton or not - I would not use the combination of servo and all poionts focus. Tried it and won't do it again. It will lock onto the wrong object (not the subject) in the middle of shooting. If you need servo - I assume because the subject is moving, use one of the auto focus points that is best and track the subject with it. Subject moving too fast for that - use manual focus. Subject not moving - use 1 shot autofocus and recompose.

[CyberDyneSystems]

Gasrocks.. it depends on the situation and the Camera...
With 45 AF points the 1D handles focus tracking with multiple points much better than the 20D.

Even though the 20D's AF accuracy and speed are greatly improved over previous models in that range.. multi point predictive focus tracking remains the territory of the 1D 45 point AF almost exclusively.

As for conditions.. it only works well in certain situations... but with the 1D cameras.. when it does it works very well.

For the 20D.. a situation where it may come in handy is birds in flight,.. assuming you are shooting in an uncluttered sky.

[gasrocks]

Yes, birds in flight against clear sky (subject beignt he only thing out there,) works. I was thinking about more real life situations - people running, cars on a race track, soccer game, and such. 45 points can help.