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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras
Thread started 20 Mar 2017 (Monday) 01:46
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40D Corner focus points unreliable

 
RedGoblinz
Hatchling
2 posts
Joined Jul 2011
Mar 20, 2017 01:46 |  #1

As you can see form the pictures uploaded, the 40D often focuses on the background behind the subject (actual focus point selected is circled in black). At first I thought it might be because the some of the objects are a bit small, but it happens to big subjects as well.

These subjects are not really fast moving, and this is in bright conditions.

I use back-button focusing, lens is Sigma 17-50mm f2.8 for these pictures uploaded, however I do remember this happening to my previous lens, 50mm 1.8 II which I thought was causing these problems. AF mode was single shot.

Also another thing, sometimes if I focus and lock onto a still object, take a picture, and then ask the 40D to focus again, it shifts the focus and results in either a clearer or more blur picture, depending on luck, is this normal?

Hopefully some help can be given as this issue of bad focus is really discouraging for me, seeing so many oof photos with simple subjects.

Thank you.

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apersson850
Cream of the Crop
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11,473 posts
Joined Nov 2007
Traryd, Sweden
Mar 20, 2017 05:05 |  #2

Does this apply to all focus points or one particular one? From your two samples, I presume it's at least to two of them?
Does it apply to the center point as well? On the 40D, it's only the center point that has the longer base line, which helps setting focus correctly when using large apertures. Like f/2.8 and larger.

The reason for the problem could be one (or more) out of several, so it's difficult to tell. Since the 40D doesn't support micro adjustments, there's not much you can do if calibration is required.
Try to make sure there's not dust/fluff in the mirror box, in the light path for the AF sensor.


Anders

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RedGoblinz
THREAD ­ STARTER
Hatchling
2 posts
Joined Jul 2011
Post has been edited 3 days ago by RedGoblinz.
Mar 20, 2017 05:10 as a reply to apersson850's post |  #3

I have pictures where the top 3 have this problem, and the right side one. I hardly use the bottom points.

If I'm not wrong I think it also happened for the centre point at least once.

Should I try to clean out the mirror box/af sensor with some air?
Any other suggestions to test if this is an issue with the 40D or my technique? Though I remember often the point is on the subject when it blinks red, but yet focused the background.




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ksbal
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Joined Sep 2010
N.E. Kansas
Mar 20, 2017 15:04 |  #4

The outer points on the 40D are not cross type, they are either horizontal or vertical points... and in low contrast settings they fall down even if they confirm.

The 50D has all cross type points and a larger back display, and micro focus adjust.

If you find yourself using outer point's a lot for focusing, then it would be better to get a camera with all cross point and better af, or one that has outer cross points and you can turn off all non cross points.

Otherwise, if you need to keep the 40D, after cleaning, I would only use the center point on the 40D, and focus re-compose.. with the caveat that you aren't trying to do really shallow dof shots.


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apersson850
Cream of the Crop
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11,473 posts
Joined Nov 2007
Traryd, Sweden
Mar 21, 2017 16:17 |  #5

ksbal wrote in post #18306148 (external link)
The outer points on the 40D are not cross type, they are either horizontal or vertical points...

Oh yes they are. You describe the 30D and 400D, which share the same AF sensor.
The 40D was Canon's first with all points being cross type, and the center point also being a fully cross type with f/2.8 high precision point.


Anders

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40D Corner focus points unreliable
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