# What is Automatic Autofocus



## jrasche2003 (Jan 14, 2013)

and is it a feature on my Canon 300D?

I was just reading a SLR Camera and Photography book, and it was talking about the types of focusing you can do with a dSLR.  It of course talked about manual, auto focus, then Automatic Autofocus-it explains this as you hold the shutter halfway down, but if your subject moves it will automatically refocus.  Am I not understanding this feature, or does my camera not have it?  Part of the reason I am asking is of course sometimes you get that butterfly, or animal in focus then they move.... so this is why the subject in the book 'Automatic Autofocus" appealed to me.  But if I google that term.. I am getting nothing.. hmph!


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## SCraig (Jan 14, 2013)

Nikon defines it as AF-S (single servo), AF-C (continuous servo), and AF-A (auto servo).  AF-C refocus continuously when the shutter is pressed halfway, and AF-A is normally single-servo but switches to continuous servo as necessary.  It is available on some (most? all?) Canon's but I don't know the nomenclature.


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## jrasche2003 (Jan 14, 2013)

SCraig said:


> Nikon defines it as AF-S (single servo), AF-C (continuous servo), and AF-A (auto servo).  AF-C refocus continuously when the shutter is pressed halfway, and AF-A is normally single-servo but switches to continuous servo as necessary.  It is available on some (most? all?) Canon's but I don't know the nomenclature.



Thank You!  I did just find it in my manual, worded differently of course relating to drive modes.  I am going to have to read it over and over because I am not really grasping how I utilize the feature... or if it simply just if I use a certain mode.. seems I am brain dead today.


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## SCraig (Jan 14, 2013)

One-shot appears to be the same as Nikon AF-S.  Press the shutter halfway, the camera focuses and does not change.  Very handy for focusing on one part of the frame and then recomposing since you can move the camera without changing the focus.

AI-SERVO appears to be the same as Nikon AF-C.  Press the shutter halfway and the camera will continually focus until the shutter release is pressed the rest of the way at which time it sets the focus for the time that the shutter is open.

If you are shooting in single-shot mode (press the shutter fully for each shot) then nothing else happens until you release the shutter.  If you are shooting in continuous mode (shots are taken as long as you hold the shutter down until the buffer fills) and using AI-Servo then autofocusing continues as soon as the shutter closes.


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## pgriz (Jan 17, 2013)

There are two aspects to autofocus - what the camera focuses on, and how it behaves once the focus is achieved.  The former is determined by the AF points enabled. If you have all points enabled, then the camera selects the closest one and uses that to focus the lens. So if you're shooting a butterfly in the center of the frame, and you have a leaf in the foreground, the camera will often focus on the leaf and not the butterfly.  So, you have to tell the camera which AF point to use OR you disable all the points except for the center one (the most sensitive one, and of the cross-type), let the camera acquire the focus using THAT point, and shoot with that focus setting.  However, what if you don't want your subject to be at the center of each image?  Well, then you need to acquire focus at the center AF point, and then reframe the image so that the subject is NOT at the center.  This will work if your AF mode is set to one-shot AF.  (which is what Scott mentioned a few posts earlier).  The camera signals that it acquired focus by lighting up the selected AF point, and beeping (if you have that option turned on).

What about the other AF modes?  The continuous AF mode (AI sevro AF) tries to keep adjusting the lens following whichever AF points you've enabled. This is useful if you're following a bunch of kids running around on the field - the camera tries to keep adjusting the focus as long as your shutter button is half-pressed.  Unlike the one-shot AF, there is no feedback that the focus is properly acquired, because it is assumed that the subjects are always moving.

The last AF option is supposed to alternate between the two.


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## Png1lady (Feb 11, 2013)

I think I'm in the same boat as you.  Keep reading the manual, and I'm starting to understand what the different AF options will do.  But try as I might, I can't seem to accomplish the settings.  I'm going out to the garden today to keep trying.  I'm going on a whale watch trip soon and wondering which settings I should use?  There will be a lot of movement, it will be morning so there will be some glare off the water.  Any help would be appreciated.


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## DiskoJoe (Feb 12, 2013)

jrasche2003 said:


> SCraig said:
> 
> 
> > Nikon defines it as AF-S (single servo), AF-C (continuous servo), and AF-A (auto servo).  AF-C refocus continuously when the shutter is pressed halfway, and AF-A is normally single-servo but switches to continuous servo as necessary.  It is available on some (most? all?) Canon's but I don't know the nomenclature.
> ...



The mode you are referring to here is used when you take pictures of moving objects like cars or horses or something. Can also be handy if you are filming with your camera.


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