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05-24-2011, 09:29 AM   #1
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Focus: AF.C

Hi, I'm still new to DSLR. I've been studying about how to use my different focus options and trying them out since I often get the AF focusing on the wrong thing.

I put it on center single point auto focus and AF.S seems easy enough. It's AF.C that I am having a lot of trouble with.

When I'm in AF.C it doesn't seem to follow my subject like I think it should. It seems to just refocus on whatever is in the center of the frame. This is both when recomposing (which I know is better with AF.S) or when subject is actually moving. If I put out my hand and focus on it...then before releasing the shutter move it to the side, it refocuses on whatever is in the background in the center rather than following my hand.

Any ideas, or tips?

05-24-2011, 09:39 AM   #2
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That is the intended purpose of AF.C. Your description is correct.
05-24-2011, 09:54 AM   #3
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I"m sorry if this sounds dumb (remember I'm new at this), but why would it be good for action and moving subjects (like my dog or kid) then if it just going to keep refocusing on whatever is in the middle?
05-24-2011, 10:05 AM   #4
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QuoteOriginally posted by mhaws Quote
I"m sorry if this sounds dumb (remember I'm new at this), but why would it be good for action and moving subjects (like my dog or kid) then if it just going to keep refocusing on whatever is in the middle?
You're supposed to follow the subject with the focus point. Then, as it moves, it stays in focus, and you can shoot continuously with the subject always in focus. In AF.S, the camera would focus once, so subsequent shots would be out of focus, because the subject moved from that spot.

05-24-2011, 11:01 AM   #5
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QuoteOriginally posted by mhaws Quote
I"m sorry if this sounds dumb (remember I'm new at this), but why would it be good for action and moving subjects (like my dog or kid) then if it just going to keep refocusing on whatever is in the middle?
Think about if your dog or kid was running at you (therefore decreasing the distance of the subject to your camera), AF.C would continuously adjust the focus so that as the distance closes so does the focus. This is the most dramatic example. If your subject was moving sideways the distance change would not be as dramatic but unless the subject runs exactly parallel to your camera the distance from camera to subject will still change slightly. That is why AF.C is good for moving objects and as you have already figured out AF.S will focus on one thing and stay focused at that distance.
05-24-2011, 11:43 AM   #6
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So If i'm trying to catch pictures of my daughter, I'd stay in AF.C, but the center point autofocus will only work if I keep her centered. It would be better to NOT use single point AF right? and let the camera pick...even though it sometimes picks the wrong thing. Do most of you ever use single point auto focus instead of letting the camera choose when doing action? Which situations would I use that? and also...are there any exercises to practice using the AF.C mode with single point and/or mulitple point AF?
05-24-2011, 11:45 AM   #7
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QuoteOriginally posted by mhaws Quote
Hi, I'm still new to DSLR. I've been studying about how to use my different focus options and trying them out since I often get the AF focusing on the wrong thing.

I put it on center single point auto focus and AF.S seems easy enough. It's AF.C that I am having a lot of trouble with.

When I'm in AF.C it doesn't seem to follow my subject like I think it should. It seems to just refocus on whatever is in the center of the frame. This is both when recomposing (which I know is better with AF.S) or when subject is actually moving. If I put out my hand and focus on it...then before releasing the shutter move it to the side, it refocuses on whatever is in the background in the center rather than following my hand.

Any ideas, or tips?
As others have said, your AF-C is working as designed. Now for the problem of it refocusing on whatever is centered in the frame, I have a suggestion.

I do a lot of soccer games, and AF-C is great at tracking and being ready for the key moments. However, someone invariably (usually the ref) gets in the way and messes up my autofocus. So here's what I've done.

Change the AF button (the small one by your thumb) setting in the custom menu to "Cancel AF". I used to use the AF button to focus, but got frustrated because my battery grip doesn't have an AF button.

Now, I half hold the shutter button to focus and follow the play (using center point focus). When I see in my peripheral vision that someone is stepping into the center of my frame or my subject is moving out of the center point, I press the AF button. The focus system remains where it was, and I can still snap at any moment. Once the offending person is out of the center of the frame, I can release the AF button and go back to following the subject.
05-24-2011, 03:44 PM   #8
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QuoteOriginally posted by klh Quote
Change the AF button (the small one by your thumb) setting in the custom menu to "Cancel AF". I used to use the AF button to focus, but got frustrated because my battery grip doesn't have an AF button.

Now, I half hold the shutter button to focus and follow the play (using center point focus). When I see in my peripheral vision that someone is stepping into the center of my frame or my subject is moving out of the center point, I press the AF button. The focus system remains where it was, and I can still snap at any moment. Once the offending person is out of the center of the frame, I can release the AF button and go back to following the subject.
Good use of the AF button. I'll have to try that.

05-24-2011, 03:51 PM   #9
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thank you klh, for that tip. I did some practice outside today with my daughter at the park with AF.C and center AF point. I'm doing much better than I was now that I understand how it works better. I still keep getting the focus more on the wrong part of the face....like the hair, or the shoulder instead of the eyes. I think my problem is having too wide of an aperture for such an active subject.

If I increase my f stops so that I have more accuracy of focus my depth of field is a lot more deep. I like the shallow look, so if I'm trying to get this, will getting closer or using a longer length lens to make it more shallow also then make it more difficult to focus on her whole face instead of it just catching a little bit?
05-24-2011, 03:54 PM   #10
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You are correct , you need to have a smaller aperture if you want to keep the subject in focus
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