AI - Servo - Question on how it functions

Let's say that the "Z" axis is the one that is coming towards the lens...

does the AI Servo perform well in this manner? Or does it perform better in
the "X" and "Y" axises?

Specifically, if a person is walking towards you, and you're using AI servo to try and capture a series of images of them advancing towards the lens...

Also,

I'm using back button focus, as I understand it, as long as I have that button held down,
the AI servo is active - correct?

So if I'm shooting the above mentioned scenario, I hold the back button focus down,
AI Servo starts working and then fire the shutter with the front shutter button.

So, when the mirror drops, AI servo should take over again, and attempt to focus on the subject, until the shutter is triggered again, correct?

thanks!

All correct. This is the method I use most of the time. If you just want to focus on a still object, hold back button to obtain focus and release. Focus will be locked. Hold it down and it continues to track. Works fine for subjects approaching or moving away as well as sideways from you.

stock28 wrote in post #14993719
All correct. This is the method I use most of the time. If you just want to focus on a still object, hold back button to obtain focus and release. Focus will be locked. Hold it down and it continues to track. Works fine for subjects approaching or moving away as well as sideways from you.

It doesn't work like that if the subject is moving along the x and y axes.yo need to enable all af points to track x and y

Depends upon what you mean by that.
If the object moves in just X or Y axis, then no focus adjustment is necessary, since the distance is the same.
But if the object moves so the distance changes, then you either need to keep the active AF point on the target all the time, even if the object also moves to the side, or you need a setup and surrounding conditions which allow the camera to hand over from one AF point to another whilst focusing is active and tracking. Different cameras do this more or less well.

apersson850 wrote in post #14994302
Depends upon what you mean by that.
If the object moves in just X or Y axis, then no focus adjustment is necessary, since the distance is the same.
But if the object moves so the distance changes, then you either need to keep the active AF point on the target all the time, even if the object also moves to the side, or you need a setup and surrounding conditions which allow the camera to hand over from one AF point to another whilst focusing is active and tracking. Different cameras do this more or less well.

The focus won't be the same as the af could focus on the next thing closest to the chosen af point

If the motion of the subject is purely in the X-Y plane then the lens isn't doing anything because the subject is in focus. However, if you have the camera fixed and are using manual select area AF or auto AF, then the camera will track the subject X-Y position as it moves within the field of view. During this time, if there is any movement along the Z axis, a focus correction signal will be sent to the lens. The 7D AI Servo AF system uses a predictor-corrector algorithm once it establishes a rate along the Z axis to take into account the time lag between measurement and lens focus correction. At the time of shutter actuation, another look-ahead correction is made for Z axis motion to account for the time lag from shutter button actuation and first curtain opening.

If you are using single point AF, and tracking a moving subject by panning then the task of subject tracking becomes more complicated and subject to failure if there is a lot of background or foreground clutter because there is no completely reliable way to distinguish between subject motion in the Z-axis and apparent motion caused by panning across other objects.

jonneymendoza wrote in post #14994278
It doesn't work like that if the subject is moving along the x and y axes.yo need to enable all af points to track x and y

what? Why you need to enable all AF points? What AF points you are using have nothing to do with subject movement.

bobbyz wrote in post #14994554
what? Why you need to enable all AF points? What AF points you are using have nothing to do with subject movement.

Yes it does. What planet are u living in? Go read more about af systems in the camera mate. If u select one af point and the subject moves away from that you will lose focus as the camera will only focus on the subject that's bang on the AF points u selected. Common sense

jonneymendoza wrote in post #14994581
Yes it does. What planet are u living in? Go read more about af systems in the camera mate. If u select one af point and the subject moves away from that you will lose focus as the camera will only focus on the subject that's bang on the AF points u selected. Common sense

where did I say the AF point move away from the subject, you suppose to track subject by keeping AF point on the subject. One point or more depends how big small the subject is ANd how they moving.

I shoot sports and BIFs man.I think I should know how AI servo works by now.

bobbyz wrote in post #14994593
where did I say the AF point move away from the subject, you suppose to track subject by keeping AF point on the subject. One point or more depends how big small the subject is ANd how they moving.

I shoot sports and BIFs man.I think I should know how AI servo works by now.

Yup, that's how I've always done it also.

jonneymendoza wrote in post #14994581
Yes it does. What planet are u living in? Go read more about af systems in the camera mate. If u select one af point and the subject moves away from that you will lose focus as the camera will only focus on the subject that's bang on the AF points u selected. Common sense

bobbyz wrote in post #14994593
where did I say the AF point move away from the subject, you suppose to track subject by keeping AF point on the subject. One point or more depends how big small the subject is ANd how they moving.

I shoot sports and BIFs man.I think I should know how AI servo works by now.

20DNewbie wrote in post #14994710
Yup, that's how I've always done it also.

^^ Yes, you do NOT need to enable all AF points for subject tracking. You simply move your AF point with the subject.

It's when movement is erratic and you may have problems keeping that single AF point on the target that multi-point tracking may have a value.

SoCalTiger wrote in post #14994995
^^ Yes, you do NOT need to enable all AF points for subject tracking. You simply move your AF point with the subject.

Yes but I am on about scenarios where you don't have the time and reflex to manually move your af points.

You simply need to enable all or more then one af point to track erratic behaviour where u won't have the time to manually select af or even move your camera to try and track a subject from within the af point selected

apersson850 wrote in post #14995325
It's when movement is erratic and you may have problems keeping that single AF point on the target that multi-point tracking may have a value.

Exactly

I have shot enough sports and BIFs with older cameras where we only use center AF point and know how to track. That is the difference between a rookie and someone who knows what they doing. Not that I am expert like some here on POTN. Now when I got 5dmk3, its name AF made things much easier as you can group bunch of surrounding points. Still nobody I know uses all AF points unless shooting BIFs in clear sky.