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07-06-2014, 01:51 AM   #1
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missaligned sensor or focusing screen

Hi,
I'm owning my K10d (firmware 1.31) for quite a while now (>6 years) and I'm happy with its general use. But there is one thing which is really frustrating on this camera: the auto focus. Consequently I tend to use manual focus only, which I was used to, anyway (I used an old Pentax P30 before).
At first I always thought this is a matter of calibration, but now I get the feeling, that this is a mechanical problem.

Problem description:
The focus on all pictures (independent of lens or focal length) seems to show a back focus compared to what I (and the auto focus) see in the viewfinder before taking the picture. The degree of this back focus strongly depends on the focal lenght, which is why I can't fix this by calibrating the auto focus electronically.
Accordingly I believe that either the focusing screen or the chip is mechanically shifted somehow.
=> The focusing screen seems to have a shorter optical distance to the lens than the chip and it seems to be tilted slightly clockwise...

Has anyone encountered the same problem? Any solution available?

cheers, Erik

07-06-2014, 04:26 AM   #2
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It sounds like the focus screen may merely be seated improperly. If this is the case it is a very simple matter to re-seat the screen properly. Just push back the retaining tab which releases the frame holding the screen. Then just nudge the screen or tap the camera body to get it into the proper position. If you buy a replacement screen from Pentax if comes with a special tool but you can use some wide tip tweezers. I recommend wrapping the ends of any instruments with some tape or thin vinyl hose (the sort used for aquarium air hoses). A wooden toothpick works great for releasing the tab and moving the screen around. While the toothpick can still damage your screen it is less likely to do so than a sharp steel screwdriver tip.

There are plenty of videos and posts on how to change the screen on Pentax cameras. Pretty much it is the same with most DSLRs. Here's one (not the greatest)

07-06-2014, 04:41 AM   #3
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Does the autofocus correctly focus where you think it should (not where it looks sharpest in the viewfinder, but the subject you point at when engaging the autofocus)? For example, if you do a brick wall test, does AF give the correct result?

I had to re-shim my K10D's focus screen to get the VF image to match what was being captured by the sensor, but AF always worked correctly. If your AF is off too, that's a different issue.
07-07-2014, 01:05 AM   #4
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@Not a Number: thanks for the hint! I'll go and try to realign the focus screen.
@baro-nite: as an electronic engineer I would say "the autofocus shows a non linear behaviour". When I calibrate it at 55 mm focal length and 50 cm distance (calibration value about +170 Ám) it shows a strong front focus at 35 mm or less. For higher distances (>100 m) and short focal lengths it shows a back focus again.
(I use an autofocus calibration sheet similar to this one: AF Fine Tune or Autofocus Calibration )

07-07-2014, 03:29 AM   #5
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So when you focus manually you get the result you expect, based on what you see in the VF? If that is so, don't mess with the focusing screen -- it is for manual focus (and metering) only.
07-08-2014, 01:01 AM   #6
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No, as I wrote in my first post:
"The focus [...] seems to show a back focus compared to what I [...] see in the viewfinder"
The focus of the taken images don't accord to the viewfinder AND the autofocus.

I would say that this got worse within the last year. Also this camera crashed onto the lens last year (inside a camera bag, but still broke the kit lens). Maybe this loosened something holding the chip / autofocus / viewfinder?
07-08-2014, 08:08 AM   #7
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I may have misunderstood but I interpreted your first post as saying the AF was the problem. If MF is also a problem (and shows the same non-linearity you describe) then I would guess sensor alignment is the culprit. If MF is off but in a different way then I would guess multiple issues, which could well be explained by the hard knock the camera took. My experience pretty much ends at adjusting the focusing screen, though. Good luck.
07-08-2014, 08:25 AM   #8
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QuoteOriginally posted by Agutanus Quote
Also this camera crashed onto the lens last year (inside a camera bag, but still broke the kit lens). Maybe this loosened something holding the chip / autofocus / viewfinder?
A severe jolt could well have caused internal damage to your camera.


Steve

07-08-2014, 08:28 AM   #9
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QuoteOriginally posted by stevebrot Quote
A severe jolt could well have caused internal damage to your camera.
The mirror could have been knocked out of alignment. For the price of repair you can be another used body, maybe even upgrade to a new model (used).
09-17-2014, 07:36 PM   #10
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I would agree with Not a Number. The distance from mounting ring, through mirror (reflect) to focus screen likely is different to the sensor now. Either the focus screen is not seated well (it is holding by a metal clip) or the mirror box is knock off alignment. Look around and mirror and where it rest (lift mirror carefully or use mirror up) to see if there are damages you can observe. You can search the web for service manual to better understand the structure and alignment procedure (not very easy). Since the mirror is a moving part, it is not very easy to use pad or shim but you can shim the focus screen if it is not off too much. Looking for "replace focus screen" video on web for better instruction. Replace mirror box is very complicate and may do more damage than due to the mounting, wiring and flash discharge.
09-19-2014, 12:17 PM   #11
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QuoteOriginally posted by twey Quote
I would agree with Not a Number. The distance from mounting ring, through mirror (reflect) to focus screen likely is different to the sensor now. Either the focus screen is not seated well (it is holding by a metal clip) or the mirror box is knock off alignment. Look around and mirror and where it rest (lift mirror carefully or use mirror up) to see if there are damages you can observe. You can search the web for service manual to better understand the structure and alignment procedure (not very easy). Since the mirror is a moving part, it is not very easy to use pad or shim but you can shim the focus screen if it is not off too much. Looking for "replace focus screen" video on web for better instruction. Replace mirror box is very complicate and may do more damage than due to the mounting, wiring and flash discharge.
Also the image is reflect from the secondary mirror (through the primary mirror) down to bottom of camera where the focus sensor located. If the auto focus still work well and in focus then likely is the focus screen, otherwise possible the mirrors.
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