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Old May 16, 2005, 12:40 PM   #1
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I have had my 20d for about two months now. I am having difficulty with auto focusing on one-time mode. Lately a few times it seemed to search in & out of the focused area then stopped while out of focus. The only way to get it back to focusing was to point it somewhere completely different, then it would start trying to focus again. This has happenedwhen I was outside in the sunshine, so lighting would not be a problem. The lens I use is the 75-300 usm is.

Would this be a problem with the lens or the camera body?

Has anyone else had similar problems?

If the batteries are low does that affect focusing ability?



Any input or advise would be appreciated


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Old May 16, 2005, 9:18 PM   #2
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We need more detail about the subject that you're pointing the camera at. If you want, it is possible to attach pictures to the posting (i.e. you don't need a web server to host the picture) so could you post an example picture (one that AF didn't work on is fine) so we can get an idea of what the subject looks like... a picture is worth a thousand words, and in this case its true.

AF systems work by contrast detection. If you pointed your camera at a smooth single color wall without any shadows it won't AF. But if its a brick wall (for example) that has texture and shadow and the AF system should lock on it very well.

Personally, my 20D is a 2-headed beast. Some times it staps into focus easily and quickly. Other times it hunts out to the extend of the AF range and then comes back and finds focus. This few seconds of hunting costs me the shot. Some times is starts to AF and then just gives up. Personally, I think when they made the 20D have a "Better" AF system, they also made it worse in some situations. My 10D didn't always AF very quickly but it seemed to always find it eventually (assuming a reasonable subject.) I can't say the same about the 20D.

Eric
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Old May 17, 2005, 7:42 AM   #3
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Use center point - focus on closest area and then recompose.

I have trouble in low light and solid colors - (horse shows).
It is all part of the 20D mystique.
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Old May 17, 2005, 9:23 AM   #4
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I can't post a picture of when the camera didn't focus because it never allowed me to take the picture.

It was done outside on several occassions where there is ample light & should have a lot of contrast ie trees/grass.

I do use the centre point as a focus point, in almost all situations.

I did read however that if you have a lens with a larger aperature eg 2.8 the camera will focus better. Any thoughts on this.

I still am not sure if the problem is a lens or a camera concern.


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Old May 17, 2005, 10:11 AM   #5
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F2.8 activates the high precision mode of the center focus point on the 20D.

I have been using all kinds of lenses on my 20D and have not had unusual problems. I have had the usual ones mentioned before :-) I have had troubles on "soft" foggy days with the lens searching, but I attributed that to the low contrast situation.
And more recently the camera is having a demon of a time finding focus in a raspberry bush, with a F2.8 lens using the center point. :-) You have to move it around till it finds focus, then recompose. But again that is a tough subject to decide what to focus on.

I'd do a bit more experimenting, but you may want to either compare your 20D operation to one in a store to see if they work different. Or take it directly to a Canon service center to be checked out.

Peter.
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Old May 17, 2005, 10:28 AM   #6
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i too have had my 20d hunt on occasions of vary low contrast, but thats not unusual, and it always eventually locks on... but in general i have also not had any real problems at all, in fact it focuses better than my previous canon slrs.. i have certainly not had any problems where i could not even take a picture because it was hunting.. i would tend to agree with peter, that if the problem is so bad you cant even take an example picture, its time to take it in to Canon..

g'luck, dustin
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Old May 17, 2005, 10:36 AM   #7
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Ah, so the problem isn't that it locked focus incorrectly, but that it just stopped trying to focus (when it isn't near focus.)

Those are completely different issues. That is why I asked.

I have had my 20D do what you describe. Some times, with exactly the same scene as it had worked on before. I can't explain it, but I wish I could.

Eric
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Old May 17, 2005, 10:38 AM   #8
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Has anyone tried shhoting the problem manually, ie manual focus to see sharpnes? also did you try FE lock? Then re-compose? I don't know if that will buy you anything.?
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Old May 18, 2005, 4:49 PM   #9
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i had that happen to me the other day. i was focusing fine on some birds in the bushes and then went to focus on a neighbors shed and the camera didn't even try to focus. i manually focussed and went to focus back on the bushes and it worked. tried the shed again, no focus movement. then it started working and has ever since. i was using center focus point only. as for hunting, i have had the 100-400mm hunt occaissionally as eric s. had mentioned. sometimes it hits a tree bud or something else and then hunts to the extremes and then finds focus. all in all, i think it is a great camera and i think we have to live with it's little quirks.

on another focus note, i tried using my sigma 150mm f/2.8 with a sigma 2X TC which makes it f/5.6 and it will not autofocus. without the 2X it actually focuses quickly and quietly but, sometimes hunts.

dennis
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Old May 18, 2005, 5:08 PM   #10
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Amen to that! I love it and really don't mind manual focusing. The Canon lenses in manual focus rotate fine but my Vivitar zoom is a very coarse manual focus feel. Thanks for responding.
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