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Locking on subject in ai servo mode

Bob King , Mar 25, 2012; 07:35 a.m.

I have been playing around with ai servo in taking some photographs of the kids, but have been struggling with focus locking on the subject. From what I have read in the manual and online, I thought servo would force the camera to lock on the subject then track it. I have spot focus set on my 7d. In the simplest terms to explain the problem, if I am taking a picture of my daughter and start by pressing the shutter halfway to focus, then she moves and is no longer in the center point spot, the camera refocuses on whatever was behind her. I know the differences in af with one shot af and locking there, but thought that in servo the af point would move with the subject? Wondering if anyone has any insights there.

Thanks Bob

Responses

Eric Merrill , Mar 25, 2012; 08:53 a.m.

Bob:

I don't have a 7D. I'm guessing there are custom functions similar to the 1D series, though, that influence autofocus in terms of how quickly the camera tries to reacquire focus at the same point vs passing it off to an adjoining one.

That said, you'll need to follow the subject as the subject moves, though. It's not a simple matter of acquiring an initial lock and then the camera changes focus while the subject moves all around the frame.

I would strongly urge you to look at moving the focus off the shutter button and onto a back button. There's a custom function for that. Then, your shutter only controls the shutter and exposure, while a button under your thumb controls when the camera focuses.

Eric

Dennis Zaebst , Mar 25, 2012; 11:45 a.m.

Bob,

Consult the diagram on AF from the 7D manual (sorry, I tried to include the diagram here, but photo.net kept blocking the post for some unknown reason). I wouldn't use spot autofocus, as it is a single point as shown in the diagram. I would just use (for the time being until you gain experience) the auto select 19-point AF mode.

I would think that with your typical fast moving subjects, whether it be small children or birds in flight, that this 19-point mode combined with AI servo (where the camera tracks the subject) would yield the best results; i.e. the camera moves the focus point as necessary as the initially tracked subject begins moving.

With spot AF, the camera is locked onto a single, very small point, but in this mode, it is difficult to keep the center sensor on the subject (usually you want to focus on the eyes), since the AF point is not automatically changed by the camera as the subject moves; you have to do that yourself manually.

Other than that, I agree with Eric that you should try the back button focusing, but you have to train yourself to keep bumping the focus with the back button as you follow the subject in motion.

Hope this helps.
Dennis

Dennis Zaebst , Mar 25, 2012; 11:51 a.m.

Bob, the AF area selection modes are shown on page 87 of your 7D manual.

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