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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 29 Mar 2009 (Sunday) 18:25
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Quick Autofocus Question, I'll lock the topic as soon as it's answered

 
Statement
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Mar 29, 2009 18:25 |  #1
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Hey, sorry I have to do this but I can't find the answer to this particular but seemingly simple question. I finally figured out, on my 40d, how to change the autofocus to use the center point (I think it's the button on the top right, where I can change the bracket to this: [-] ). But my question is, when I set the autofocus mode to "one shot," and I start by composing my subject in the center, and then recompose and take the picture, does the autofocus remain on my subject? I only ask because when I view the picture afterwards, the red square indicating the autofocus is still in the center. Does that mean it refocused on the new center of the shot after I recomposed the shot? Thanks for the help, all.


40d, Canon 17-55 f/2.8 IS USM, 550ex

  
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SkipD
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Mar 29, 2009 18:38 |  #2

If you're using the default trigger for autofocus (holding the shutter release button halfway down) and you let it go after focusing, then recompose, then push the shutter release button again, the camera will indeed refocus on whatever the active focus point is aimed at.

In order to focus/recompose, you need to somehow deactivate the autofocus system between the focus action and tripping the shutter after recomposing. The default way is to HOLD the shutter release button halfway down for the duration of the time between focusing and tripping the shutter.

Many of us have our cameras configured to use a button on the back of the camera (the * button in the case of my 20D) to activate auto focus. With that method, I can aim the camera so the center focus point is on whatever I want the camera to focus on, tap the * button, then do whatever I want until I trip the shutter. Without tapping the * button again, the focus will essentially be in manual focus mode and won't change by itself.

There is also a way to lock the focus, but I find that method (which I cannot remember) a pain in the you-know-where to mess with. Just using the * button whenever (and IF) I want to activate autofocus works great for me. I can, for example, take a whole series of shots with the same camera-to-subject distance and only set the focus once. I can also tweak the focus manually any time I want (with the appropriate lenses that don't have a gear train that you'd be stressing doing it).


Skip Douglas
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RDKirk
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Mar 29, 2009 18:48 as a reply to  @ SkipD's post |  #3

I only ask because when I view the picture afterwards, the red square indicating the autofocus is still in the center. Does that mean it refocused on the new center of the shot after I recomposed the shot?

If you're talking about the indicator you see when you view the image later in Zoombrowser or DPP, that indicator shows which focus point had been used for that exposure.

That information is filed in the EXIF of the image just like focal length and whether a Canon flash was used. The camera has no way of knowing, however, that you recomposed after focusing and moved your intended subject away from the focus point you used. It only knows which focus point you used when you focused for that frame, and that's what it indicates in Zoombrowser or DPP.


TANSTAAFL--The Only Unbreakable Rule in Photography

  
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Statement
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Mar 29, 2009 19:57 |  #4
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SkipD wrote in post #7624773 (external link)
Many of us have our cameras configured to use a button on the back of the camera (the * button in the case of my 20D) to activate auto focus. With that method, I can aim the camera so the center focus point is on whatever I want the camera to focus on, tap the * button, then do whatever I want until I trip the shutter. Without tapping the * button again, the focus will essentially be in manual focus mode and won't change by itself.

How do you set your camera this way? That would be useful. Thanks for the replies, both of yas.


40d, Canon 17-55 f/2.8 IS USM, 550ex

  
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SkipD
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Mar 29, 2009 20:04 |  #5

Statement wrote in post #7625301 (external link)
How do you set your camera this way? That would be useful. Thanks for the replies, both of yas.

I know the 40D is different from my 20D, so I'd have to refer you to your owner's manual. There are custom functions that can be set to modify how the camera works.


Skip Douglas
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mikeassk
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Mar 29, 2009 20:26 |  #6

SkipD wrote in post #7625363 (external link)
so I'd have to refer you to your owner's manual.

A big + 1,
If you cannot find it in your manual than by all means...


Stuff

  
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Brett
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Mar 29, 2009 22:29 |  #7

It will be listed under the Custom Functions (C.Fn) of the camera.

It gets a bit more confusing, however, because you can set the * button for AF while the shutter button locks AE, or not, or the * can set both AF and lock AE, etc.

Search the forum for "back button focus"; there are many threads detailing its use. :)

Oh, and I'm pretty sure once you start using it, you'll never go back to shutter button AF. I know I won't.



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Mar 29, 2009 22:38 |  #8
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Ok, thanks everyone.


40d, Canon 17-55 f/2.8 IS USM, 550ex

  
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ean36
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Mar 30, 2009 02:39 |  #9

check this out.
http://www.usa.canon.c​om …ArticleAct&arti​cleID=2286 (external link)
back button auto focus explained by Canon


be safe and well. George
7D, 5DIII, 5DII and too much to carry all at once

  
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SkipD
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Mar 30, 2009 06:10 |  #10

ean36 wrote in post #7627530 (external link)
check this out.
http://www.usa.canon.c​om …ArticleAct&arti​cleID=2286 (external link)
back button auto focus explained by Canon

THAT is excellent....


Skip Douglas
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RDKirk
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Mar 30, 2009 07:29 |  #11

SkipD wrote in post #7627971 (external link)
THAT is excellent....


Chuck Westfall's staff has produced quite a bit of good stuff.

Finding it is the problem.


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Brett
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Mar 30, 2009 11:43 |  #12

Exactly. I can never find that when I'm looking for it.

ean36 wrote in post #7627530 (external link)
check this out.
http://www.usa.canon.c​om …ArticleAct&arti​cleID=2286 (external link)
back button auto focus explained by Canon



flickr (external link)

  
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lungdoc
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Mar 30, 2009 13:29 |  #13

I nominate that link for inclusion in the EOS FAQ sticky - great explanation!


Mark
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Quick Autofocus Question, I'll lock the topic as soon as it's answered
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