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06-17-2011, 08:01 AM   #1
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K-x with DA 40mm -- AF problem

Hi everyone,
I have K-x and the kit lens was broken by United Airline (Ya... Before I realized it, they took my carryon for check in...anyway)
So I got the DA 40mm, but there's the problem: Whenever I tried to focus on people/faces in AF mode, 80% of the time it will focus on the background, which makes the people/faces really blur...
It's perfectly fine in MF mode and Liveviewer mode... (But LV is really slow...) It focuses at the right object when it's not AF mode.
Someone told me the lens and the body are not very compatible? Is that possible?
I would really appreciate it if anyone can help me with this problem.
Thank you!!

I used AF mode in the first picture and LV mode in the second picture.

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Last edited by love_myenla; 06-17-2011 at 09:38 AM.
06-17-2011, 08:09 AM   #2
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No, the lens is perfectly compatible. However, some lens/camera combinations can "front focus" or "back focus". That can be adjusted in some models. Not sure if it can be done in the K-x.

Also, the AF point (which you can not see in the K-x) may not be lining up perfectly with your subject, so it is focusing on something else. Make sure the focus-point is set to center. Then half press the shutter with the subject dead-center to get the focus. With the shutter half pressed it will stay focused and you can move the camera to get the best composition.
06-17-2011, 11:11 AM   #3
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focus problem

I have the same setup and I have found that setting it to AF.S and lining up the shot down the middle before composing my will correct this most of the time. When it's on continuous, with the multi-point grid for focusing, it really doesn't look for the face unless it's in Live view. That's what I've noticed anyway.
06-17-2011, 01:04 PM   #4
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One thing I notice is colours between two posted shots. The camera seems to have changed its chosen White Balance / Colour Temp. I wonder if this is related to the K-r FF / Tungsten WB issue.

You might try selecting Tungsten and seeing if the AF performs better.

06-17-2011, 01:43 PM   #5
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QuoteOriginally posted by mattt Quote
One thing I notice is colours between two posted shots. The camera seems to have changed its chosen White Balance / Colour Temp. I wonder if this is related to the K-r FF / Tungsten WB issue.

You might try selecting Tungsten and seeing if the AF performs better.
I think that white balance focus shift its a exclusive k-r "feature", even more, its so subtle that is not very noticable.

In this case look like the da40 is backfocusing a lot.
06-17-2011, 02:18 PM   #6
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Point taken on the WB focus shift, but I disagree on back focus assessment.. its not back focussed at all, its focussed on the side table. Phase detect vs Contrast detect sensor input? I've experienced similar missed focus where a hard edge was the apparent AF focus point when I was actually trying to capture a face. I'm interested to see where this discussion leads.
06-17-2011, 07:00 PM   #7
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I have had a similar experience with pictures taken indoors without flash. Live view focus is very slow but when when it does focus, it seems to produce better pics than the regular AF. Lack of focus point confirmation in the K-x is a bummer.
06-17-2011, 07:12 PM   #8
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Lets rule out the most obvious cause first. Are you using centre AF point only, or are you still in Auto 11 point mode?

06-18-2011, 06:41 AM   #9
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QuoteOriginally posted by twitch Quote
Lets rule out the most obvious cause first. Are you using centre AF point only, or are you still in Auto 11 point mode?
+1 since if the camera has choosen centre AF point it will focus on the background.
06-19-2011, 08:55 AM   #10
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I tried all the different focus options (Center AF point/ Auto 5 point/Auto 11 point), but it doesnt make that much difference...
One thing I noticed when I used the MF mode is that, if i take a picture right when the focus indicator makes "the confirmation sound", it is actually blur. I need to "overfocus" it a little bit till the indicator stop making the sound, then the picture is focused in the right object. So is it the accuracy problem?
And also thank you for all the comments
06-19-2011, 11:23 AM   #11
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This might be stupid, but is your viewfinder diopter adjusted correctly?
06-19-2011, 03:58 PM   #12
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QuoteOriginally posted by love_myenla Quote
I tried all the different focus options (Center AF point/ Auto 5 point/Auto 11 point), but it doesnt make that much difference...
It makes a world of difference.

In anything other than single AF point mode, your camera will choose whatever AF point happens to be over the most contrasty part of the scene. That could just as easily be a foreground coffee mug, or a background phone sitting on the arm of a chair. With the K-x you have no idea in the viewfinder which point it has chosen and therefore what it has chosen to focus on. To conclude that "it doesn't make much difference" makes me think your testing is flawed, and you don't understand what the camera is doing.

Try centre point only again...
06-19-2011, 11:00 PM   #13
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Ya I understand that they are very different in principle, but what I meant was when I took pictures with the center point only, sometimes it also picks up things with the most contrasty at the background too, just like the Auto 5 and Auto 11... Maybe the contrasty item is too close to the target object?
I took sirjes2's advice, change it to AF.S. I'm wondering if I should also use spot AE metering instead of Center metering...
06-21-2011, 04:14 AM   #14
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Have you noticed a difference using AF.S? Also, I was wondering if you have the same issue with the kit 18-55. That is, if you have one.
06-22-2011, 09:59 PM   #15
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My kit lens was broken... thanks to United Airline..
But when I used the kit lens, the focus problem never come across...
Do you have the same problem with the kit lens?
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