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07-23-2009, 04:22 AM   #1
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K20d AF adjust: Help me choosing the correct value.

I'm adjusting the AF of all my lenses. I've tried some of the focus test charts available for download on the net, but as many have already pointed, they give contradictory results as the AF area is pretty big.

As a result I decided to do the traditional 90 degrees newspaper shots, and that's what I got:

(the lens used is the FA 31mm ltd @ 1.8 on a tripod, all photos are 100% crops)

First, I shot using 0, +5, -5 to see which was the porblem:

+5:



0:



-5:



The -5 Shot is without a doubt the sharpest one. I did further testing and found the -8, -9, -10 to be the sharpest ones, but can't really decide which one is the best. Can you help?

-8:



-9:



-10:



Thank you

07-23-2009, 07:16 AM   #2
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If you cant tell the difference then it doesnt matter which one you go with. Take the one in the middle and get outside and start shooting again.
07-23-2009, 11:16 AM   #3
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I would say: -5
But isn't there a difference in exposure as well? I mean between the -5 and the set -8, -9 & -10?
07-23-2009, 11:17 AM   #4
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You have a defective 31ltd. It seems to be translating things into Spanish...

07-23-2009, 11:36 AM   #5
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-9 seems to be the sharpest. However, having gone through different methods myself, I suggest you redo the tests with near infinity targets (well defined buildings would be good). Always shoot the same setting with at least 2 shots with 2s timer on tripod. For instance, if you think -9 is the one now. Redo -8, -9 & -10 (at least 2 shots each from infinity & closest distances of the lens scales) with near infinity target in daylight environment and see if -9 is still the sharpest. Do & redo the tests until you get repeated results under different conditions. The problem with closeup distance is that the AF might be inaccurate near infinity (my own finding). Usually AF inaccuracy is more apparent as the distance increases.
07-23-2009, 03:13 PM   #6
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I'll redo the test with near infinity targets and see which is the sharpest.

Thank you.
07-23-2009, 04:11 PM   #7
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What about a 45degree testchart? The ones with the scale that show BF/FF issues?

PENTAX DSLRs: PART-1. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

mike
07-23-2009, 04:18 PM   #8
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-9 so far. I think this way is much easier than using a focus chart.

07-23-2009, 04:57 PM   #9
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QuoteOriginally posted by JHfwp Quote
-9 so far. I think this way is much easier than using a focus chart.
Really? trying to judge FF/BF by shooting something that is a constant distance from the camera across the whole frame and then guessing if you are FF or BF. Seems to be more work IMO.

mike
07-24-2009, 04:01 AM   #10
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QuoteOriginally posted by schmik Quote
Really? trying to judge FF/BF by shooting something that is a constant distance from the camera across the whole frame and then guessing if you are FF or BF. Seems to be more work IMO.

mike
You're probably right. I grew frustrated with focus charts with a lens that I could've sworn was FF. Eventually I confirmed that it wasn't when I adopted my "textbook" method, i.e., shooting a textbook, adjusting, then confirming my results on the brick outside my house. I've assumed that all my lenses have needed an adjustment since then and have played around with each so much that I'm pretty quick at it now. Of course only one has actually required any adjustment. I can't say that it's the best way, or if I was doing something terribly wrong with the focus charts, but it's worked for me.
07-24-2009, 04:30 AM   #11
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-9.........but.

I have some experience in the field of annoying AF tuning.

-9 is the best from the samples you have shown.
But you will need to try the same test for Tungsten light.
I have a F50mm 1.7 lens. Its -7 in Tungsten light and +2 in daylight.
07-24-2009, 04:32 AM   #12
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QuoteOriginally posted by nostatic Quote
You have a defective 31ltd. It seems to be translating things into Spanish...
I love you....
07-25-2009, 08:04 PM   #13
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QuoteOriginally posted by skawi Quote
I'll redo the test with near infinity targets and see which is the sharpest.
I would suggest you calibrate with the distance you are going to use most often and/or which is the most critical to get the focus right. If you optimised for infinity, your close focusing AF precision may not be optimal.

Here are some hints for the AF adjustment.

QuoteOriginally posted by Steelski Quote
-9 is the best from the samples you have shown.
But you will need to try the same test for Tungsten light.
I agree with both statements. BTW, the deviation under Tungsten light is not a property of the lens but one of the K20D's AF system. The K-7 is the first Pentax DSLR to address this problem.
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