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11-17-2010, 08:19 PM   #1
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Phase detect auto focus inaccurate

I recently purchased a K5 and was doing some focusing tests tonight and noticed something strange. The phase detect auto focus seems inaccurate and consistently provides less sharp images than if I use the camera in live view. Is this normal? I have two lenses the 18-55 kit lens and the sigma 30mm 1.4. Both lenses exhibit the same behavior with the live view focused images being much sharper.

I know the sigma's have focusing issues which is why I was testing it in the first place but it is behaving the same as the kit lens. I played around with the fine focus adjustment to some success but am concerned there may be a problem with the camera.

11-17-2010, 09:15 PM   #2
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K5 fine tune AF on a per lens basis. Its in the last page of the custom settings.

Give it a try and report back,

Cheers,

John
11-17-2010, 09:37 PM   #3
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It is quite normal.
You dial in the phase focus with the AF Fine Adjustment.
11-17-2010, 11:58 PM   #4
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because contrast focus in LV is read off the sensor vs phase read off a separate sensor in a different location, it can be less accurate. This is why you have the focus adjuster.

11-18-2010, 11:44 AM   #5
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I guess you learn something new every day! I noticed the same thing last night. When I used the AF looking through the view finder, the K-5 focus was not nearly as sharp as using the AF button during live view.

So the fine AF adjustments that you can do in the custom settings only affect the "phase read" focusing?
11-18-2010, 02:30 PM - 1 Like   #6
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QuoteOriginally posted by Trig Quote
I guess you learn something new every day! I noticed the same thing last night. When I used the AF looking through the view finder, the K-5 focus was not nearly as sharp as using the AF button during live view.

So the fine AF adjustments that you can do in the custom settings only affect the "phase read" focusing?
Correct, the AF adjustment feature will only offset phase detect focusing. Live View focusing (contrast detect, face detect) will not be affected, as these are sensor based algorithms.

- Jason
11-18-2010, 05:08 PM   #7
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Original Poster


Alright so tonight I more carefully controlled the auto focus adjustment using a tripod and just slowly varied the adjustment from -10 to 10 (the results are above). The good news is that this does seem to fix the problem. The bad news is that I have to dial in a -10 on the sigma and -9 on the kit lens. With these settings the sharpness is very close to the live view focusing.

These are at the very edge of the adjustment scale however and therefore if focus ever shifts or I get a lens that needs a little more (-) adjustment I am out of luck. Should I return the camera while I still can and get another copy?
11-18-2010, 05:21 PM   #8
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Yeah, that's a little farther than you want. I'd return it.

11-18-2010, 05:44 PM   #9
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I would dial in a -5 as a global number, that will still give you -5 to play with at that end and +15 at the other end. If the camera is fine and gives good results then there is little point in returning it, The next one may be better (or worse)


At least be thankful you have a Pentax with individual per lens adjustment. Many cameras do not have this.
11-18-2010, 06:13 PM   #10
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unless this changed, it did not work like that on K20D... the global and individual number were not cumulative.

For K20D, you can unlock the "debug" mode however which than gives you a true "global" adjustment which can go quite far, doubt this is already available for K-5, and even if was - for a new K-5 which needs -10 straight away, to focus properly, I would return it as well and get a replacement...
11-18-2010, 06:42 PM   #11
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yes the global and single lens adjustment doesn't sumes up. I had exactly the same problem with moste of my lenses -5 to -10 correction needed but my 50-135 needed more at artificial light - it made me to send it to the service (it's my second k5 the first one was even worse)
11-18-2010, 08:31 PM   #12
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I hope this isn't too off-topic but while we are on the subject of LV AF (contrast) versus phase AF, should I expect to get the exact same exposure from both? In other words, if I use LV AF should my resulting histogram be the same as the phase focused exposure (with all else being the same; lighting, all on a tripod, etc.)?
11-18-2010, 11:03 PM   #13
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Please read these before doing anything:

LensRentals.com - "This lens is soft" and other myths

LensRentals.com - "This lens is soft" and other FACTS

In summary, it might be your lens not your camera that is the problem, and that it was working fine before might just mean your old camera was out further in the opposite direction. And / or the extra resolution of the K-5 is showing the faults more.

Also stuffed toys are not an accepted test chart :P
11-18-2010, 11:48 PM   #14
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QuoteOriginally posted by WerTicus Quote
Please read these before doing anything:

LensRentals.com - "This lens is soft" and other myths

LensRentals.com - "This lens is soft" and other FACTS

In summary, it might be your lens not your camera that is the problem, and that it was working fine before might just mean your old camera was out further in the opposite direction. And / or the extra resolution of the K-5 is showing the faults more.

Also stuffed toys are not an accepted test chart :P
I had to make adjustments on my K-5 for my FA limiteds which do not need adjustments on my K-7.

cheers
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