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02-05-2009, 03:22 AM   #1
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K20D Focus problem

I have been having a few problems with focus on my K20D. Here are a couple of test chart images taken with a Sigma 100-300 f4 lens (I hope they are big enough to be readable). The first is with Focus Correction off, and the second is with maximum rearward correction (via the menu). My inexpert reading of these is that they seem to indicate that the real focus point is way forward of the aim point. Result is easily replicated on a Sigma 70-200 f2.8 HSM, so not just one lens, and my K10D doesn't seem to have the problem. Field test this morning proved the focus on these lenses to be very sharp on my K10D, but mushy and soft on K20D.

Any thoughts or opinions? Has anyone else seen this type of problem? Hope you can help.

thanks and regards

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02-05-2009, 03:43 AM   #2
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The problem is, the bigger resolution a camera have, the more you see the misstakes the autofocus would make.

Focus corrector? you mean the focus corrector made on k20d (that work up to 20lenses)? Is this the same as the calibrate lens option?

Well, I don't really know...I have just bought a new k20d and I hope I don't see this problem with it.

Anyway, the lens is build for cameras and the k20d is build to wear lenses so some kind of solution will be....don't worry

Regards Emil
02-05-2009, 03:45 AM   #3
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Test chart is also not the same as real photos.
How many times your photos is test charts? Try it out on the real world as well.

Post a photo made with manual focus too....so we see that the problem not is the lens
02-05-2009, 03:57 AM   #4
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OK AdrianM, the autofocus points are very large and work best on sharp edged vertical lines.
Your test chart has a perfect autofocus line to the left of centre and indeed your resultant image has this thin line and the printing next to it quite sharp and in focus.
What did you see in the viewfinder? When using the autofocus hold the button half-pressed and move your point of aim around and watch goes in and out of focus.
What was your aperture? Depth of field is very very important when dealing with close targets and wide apertures.
I say again, and again, and again these are SLR cameras with viewfinders that show you what goes onto the sensor.

02-05-2009, 04:37 AM   #5
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Okay - to respond to a few of the questions and comments. The lens used for these test photographs is the 100-300 f4, set at f4 (which is the setting that I find myself using most in the real world when chasing birds). As I indicated above, this lens is perfectly crisp and sharp on my K10D, as is the 70-200 f2.8. It was real world photographs that led me to suspect a problem in the first place and do the test chart - mushy and soft on the K20D, and nothing like what I was used to with my K10D. A quick recheck this morning confirmed lots of detail and sharpness in the real world on my K10D, but not on the K20D.

What do I see in the viewfinder (when using the test chart) - the centre spot focus point right on the centre of the image. Again, this replicates how I usually use the camera in the real world. As I move the aim point left and right, the autofocus picks up when very close to objects like the vertical line or the text, but drops out immediately it is moved off the object's edge. It is behaving precisely as I would expect it to, except that the resultant image isn't actually in focus - always just slightly out. I tried a 90 degree shot and got slight fuzziness for what should have been perfect focus - agin replicating what i am seeing in the real world. What I am finding from the tests and from real world is that the vast majority of the depth of field appears to be sitting in front of the target point, where there is nothing but air to photograph. So, if I photograph a bird sitting in a tree, any leaves that are forward of the plane of the target (I'm not talking about objects directly in front of the focus sensor) are more in focus than those behind.

I have attached two photographs from the real world to show what I am talking about. They are very tight crops from photographs of a pigeon taken this morning. First one is taken with my K10D and actually shows details of feathers. Second from K20D shows detail of nothing really. This is typical of what I am seeing. Both photographs were taken with almost the same settings (K10D at 1/400th, and K20D at 1/500th, both f4, SR on) only a few seconds apart (just long enough to change lenses) with the same lens and from the same position. The shot with the K20D used a maximum rearward adjustment on the focus correction. This seems to be some improvement over no correction, but not enough. As far as manual focus is concerned, I have to rely on the autofocus telling me what is working or not because my eyesight isn't what it used to be. I tend not to get good results that way.

The main problem appears to be that I can't get enough adjustment in to fix it. Has anyone else seen this?

cheers
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02-05-2009, 04:54 AM   #6
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The K20 image appears to be overexposed by a couple of stops over the K10 image.
Subtle shadow information would be lost in the bright areas.
Are these both in-camera jpegs? If so, what are your image settings in the K20?
I usually have natural image with a contrast boost and some fine detail sharpening on.
Many K20 users report using these settings to get best results.
Vastly different image histogram has me very puzzled.
02-05-2009, 05:05 AM   #7
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Thanks Garth. The over-exposure thing also seems to be happening and I seem to have to reduce most photos by about 1 stop. Both were RAW shots that have been cropped and converted in Pentax Photo Laboratory with no corrections made to brightness, contrast etc. I also use Natural image with an increase in contrast, but haven't checked if these affect RAW or only JPEG. Not too sure if the problems are related, but i guess it is possible.
02-05-2009, 05:14 AM   #8
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Are you using the copy of Pentax Photo Laboratory that came with the K20?

02-05-2009, 05:16 AM   #9
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In fact, Garth, when I think about it a bit more, you have perhaps hit the nail on the head with your comment on brightness. When I discussed it with the help desk at CRKennedy this morning, I pointed out that the problem had only become apparent since the main circuit card was replaced under warranty late last year (flash didn't work). The response this morning was that the sensor wasn't touched in that process, but if the computer wasn't reading the light correctly, it could be a cause of focus errors.
02-05-2009, 05:17 AM   #10
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Re Photo Lab version - I am using the latest version on the Pentax web site.
02-05-2009, 05:50 AM   #11
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AdrianM,
my K20D actually unerexpose (by ~0.5EV) but seems like metering of that model isn't its strength.
Still, I think that focus test chart tells the truth. I had problem with Sigma 17-70/2.8-4.5 - strong backfocus. I'm affraid that focus adjustment range is just too small
Or maybe I'm wrong If it's new lens, maybe Sigma will fix it?
02-05-2009, 06:18 AM   #12
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QuoteOriginally posted by AdrianM Quote

The main problem appears to be that I can't get enough adjustment in to fix it. Has anyone else seen this?

cheers
Yup. My K20 is far enough out that I can't bring some of my AF lenses into calibration.
02-05-2009, 06:27 AM   #13
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QuoteOriginally posted by Wheatfield Quote
Yup. My K20 is far enough out that I can't bring some of my AF lenses into calibration.
What I can't for the life of me figure out is how/why things are off. It seems reasonable that if the camera itself was off a bit then every lens would need adjustment in the same direction, but some lenses I tried on my K20D needed little tweaks in either direction, though they all performed just fine on a K10D.

Any theories as to that one?

And is it just me, or does it seem like the more cameras come with AF fine tuning options, the more they seem to be needing them? Seems like the Canon users who have gotten AF adjustment as an option find that more lenses than ever need tweaks on those bodies... (Even the 1Ds3 has it, though every single lens I've mounted on my 1Ds2 has been bang on -- are they just accepting sloppier tolerances and letting the end user do the final adjustments?)
02-05-2009, 07:06 AM   #14
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QuoteOriginally posted by pingflood Quote
What I can't for the life of me figure out is how/why things are off. It seems reasonable that if the camera itself was off a bit then every lens would need adjustment in the same direction, but some lenses I tried on my K20D needed little tweaks in either direction, though they all performed just fine on a K10D.

Any theories as to that one?

And is it just me, or does it seem like the more cameras come with AF fine tuning options, the more they seem to be needing them? Seems like the Canon users who have gotten AF adjustment as an option find that more lenses than ever need tweaks on those bodies... (Even the 1Ds3 has it, though every single lens I've mounted on my 1Ds2 has been bang on -- are they just accepting sloppier tolerances and letting the end user do the final adjustments?)
I think you pretty much have it surrounded, Mr. Morden.
I''m quite certain the the end user is now the only quality control inspector as well. It's the price we have to pay for wanting our cameras to be really inexpensive.
02-05-2009, 07:54 AM   #15
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QuoteOriginally posted by pingflood Quote
What I can't for the life of me figure out is how/why things are off. It seems reasonable that if the camera itself was off a bit then every lens would need adjustment in the same direction, but some lenses I tried on my K20D needed little tweaks in either direction, though they all performed just fine on a K10D.

Any theories as to that one?

And is it just me, or does it seem like the more cameras come with AF fine tuning options, the more they seem to be needing them? Seems like the Canon users who have gotten AF adjustment as an option find that more lenses than ever need tweaks on those bodies... (Even the 1Ds3 has it, though every single lens I've mounted on my 1Ds2 has been bang on -- are they just accepting sloppier tolerances and letting the end user do the final adjustments?)
canon's tolerances for consumer vs pro AF was "within the depth of focus" vs "within 1/3 the depth of focus" not depth of field btw.. subtle difference.
http://doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf
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