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Old March 17th, 2012, 06:33 PM
DonnaA DonnaA is offline
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Unhappy Why does my autofocus on portraits drift from point I focused on?

When taking portraits using my Canon 17-85mm lens on autofocus (focus point is set to the center point), I focus on the eyes, press shutter halfway then recompose. But when I look at where the AF point actually landed (using the Canon DPP Viewer), the center focus point never lands on the spot where I focused. It's either way above like in the hair or off the face or anywhere but where I focused! In all cases the subject was not moving and didn't move during the shot. The light was consistent. Exposure ISO 800, f5.6. Can this be happening because maybe after locking the focus, I take a few steps back when recomposing? Or after locking focus I then move the zoom setting? I can't figure out what's causing it. Is it my error, if so what? Or is it lens error/malfunction? Or something else altogether? Any help would be appreciated!
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Old March 17th, 2012, 06:58 PM
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TonyW TonyW is offline
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I'm a Nikonian so can't really help but are you sure you have it on single point focus or whatever Canon calls it. It sounds like you might have face detection turned on.

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Old March 17th, 2012, 07:27 PM
DonnaA DonnaA is offline
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Tony and MiKe, thanks so much for responding. Tony I definitely have the center focus point selected and I never change it. It lights up red when focus is achieved. I'm using the Canon 40D. Pretty sure it doesn't have face detection technology like the new 50D does.

Mike, my method is to follow all the steps you suggested, except I didn't know that locking the focus in, then changing the zoom setting, even a little, would throw the focus off. Also that I should not take any steps back and forth. Thanks for those tips. Now I'll just have to try them out. I'll let you know how it works. Thanks again!
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Old March 17th, 2012, 07:43 PM
MikeBC MikeBC is offline
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Donna,

On both my 30D and 50D, there is am option on the display playback that shows the focus point when you press the Info button to cycle thru the display options. I usually have it set to off, but turned it back on for a little testing I did.

I am not sure the 50D has facial recognition - no interest in using it so never looked.

I would suggest a little testing with 2 objects, one near and the other a little further back (say 2 feet), Focus on the closer, center the camera on the farther one and shoot. Look to see if the closer one is in sharp focus and the farther centered one is soft focus. Set your lens to max zoom and use a wide open aperture (5.6) on your lens.
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Old March 17th, 2012, 07:17 PM
MikeBC MikeBC is offline
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I have the 17/85 lens and use it a lot on my 30D, then my 50D.

I don't use DPP, but always do the following

  1. set zoom length
  2. select focus spot on subject, press and hold shutter halfway down
  3. recompose (while continuing to press and hold shutter half way down) by moving camera, never change the zoom or wander back and forth - all of these things will change the focus
  4. press shutter all the way.
I have an option on my 50D that displays the selected auto-focus point. It always shows the center point as being selected and the focus is where I set it.


Try these steps out and get back with details of camera model, etc and see what happens. (Or let us know it worked and all is well).
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Old March 17th, 2012, 07:23 PM
MikeBC MikeBC is offline
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I would add that I always use the center auto focus spot and do the aut-focus/compose/shoot thing. The red dot shows the center selected, but the focus point is where I set it - based on an observation of image sharpness.
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Last edited by MikeBC; March 17th, 2012 at 10:13 PM. Reason: typo correction
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Old March 18th, 2012, 10:14 AM
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baycruisers baycruisers is offline
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Quote:
Originally Posted by MikeBC View Post
I have the 17/85 lens and use it a lot on my 30D, then my 50D.

I don't use DPP, but always do the following

  1. set zoom length
  2. select focus spot on subject, press and hold shutter halfway down
  3. recompose (while continuing to press and hold shutter half way down) by moving camera, never change the zoom or wander back and forth - all of these things will change the focus
  4. press shutter all the way.
I have an option on my 50D that displays the selected auto-focus point. It always shows the center point as being selected and the focus is where I set it.


Try these steps out and get back with details of camera model, etc and see what happens. (Or let us know it worked and all is well).
This is what I do with my Nikon D7000 (and did with my D70S and D80).
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Old March 18th, 2012, 11:38 AM
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Codebreaker Codebreaker is offline
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I do the focus and move option quite often and don't have any problems with it but something to check is the focusing mode of your camera.

Canon cameras have at least three focusing modes:-

1. One shot AF which is good for still subjects.
2. AI Servo AF for moving subjects
3. AI Focus AF for Automatic Switching to AF mode - again for moving subjects.

The key thing about the last two options is these are for moving subjects, which is similar to having a still subject but moving the camera.

Colin
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Old March 18th, 2012, 12:00 PM
MT Stringer MT Stringer is offline
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Originally Posted by Codebreaker View Post
I do the focus and move option quite often and don't have any problems with it but something to check is the focusing mode of your camera.

Canon cameras have at least three focusing modes:-

1. One shot AF which is good for still subjects.
2. AI Servo AF for moving subjects
3. AI Focus AF for Automatic Switching to AF mode - again for moving subjects.

The key thing about the last two options is these are for moving subjects, which is similar to having a still subject but moving the camera.

Colin
What he said.

Also, why not set your focus point on your subjects eyes by using the toggle thingie on the back of the camera. I can't recall how many focus points you have available. Saves some composition time.
Also, at f/5.6, does it really matter how accurate your focus is? Just curious.

Since I shoot high school sports mostly, I always use AI Servo. Say I am shooting a volleyball game. Sometimes I set the focus point on a lower point (shooting in vertical orientation) so I grab the legs of the player on the other side of the net and not the net (volleyball).
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Old March 18th, 2012, 07:31 PM
DonnaA DonnaA is offline
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Quote:
Originally Posted by MT Stringer View Post
Also, at f/5.6, does it really matter how accurate your focus is? Just curious.
I find it matters a lot in portraits where the focus point is, particularly at f/5.6 and wider apetures. Especially in the eyes, which can look soft from the focus point being elsewhere.
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