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10-30-2011, 05:37 PM   #1
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AF adjustment debugging needed

After reading numerous posts on the forums I decided it was time to attempt an autofocus adjustment on my K-5 for my Sigma 70-200mm F2.8 II APO EX DG Macro HSM. I use it mainly for indoor shots at swim meets and I've noticed FF frequently. I used the chart and approach so thoroughly described by Jeffry Friedl on his blog. Many thanks to him for the chart and instructions. Anyway, I set my camera up on a secure tripod about 2 meters from the chart. The chart was angled at about 40 degrees. I used a 2.8 aperture at about 1/50 sec. I think ISO was around 100-200. I had the zoom set at 200 as that is where I normally shoot. I decided upon 2 meters as that is the distance that allowed me to ensure the AF was not focusing on the bold text at the sides. I tested the set-up repeatedly to make sure the AF could not pick up the bold areas of the chart. I used the 2 sec timer to make sure I was not moving the camera at all. The lighting was somewhat week natural light. I reset the focus to near or far between each shot. I used center focus for all shots. The initial results confirmed FF and I tried adjustments of -5, -7 and finally -10. Even with -10 I could not get the focus point to the target line although it was significantly closer than the earlier settings. So I'm concerned that I can't eliminate the FF and a greater concern is the lack of focus on the right side of the chart. Regardless of where the sharp focus was on the left side, the right side was never very clear. Even at the final adjustment the right side lacks sharp focus. I tried to make sure I was square to the chart at all times and made a few adjustments as I proceeded. I even turned the chart upside down to ensure that the chart was not somewhat blurry on one side. Bottom line is that I feel I cannot correct the focus and that my lens appears to be flawed on much of the right side. I've attached a representative shot at -10 adjustment. I appreciate any advice on my problems.

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10-30-2011, 05:49 PM   #2
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Sure looks like you have some decentering with a really soft right side. I would send it in to Sigma to fix it.
10-30-2011, 06:20 PM   #3
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To my eye, looking at the sides of the chart, it doesn't look like you're square to the chart at all. At f2.8 and 200mm, the smallest angular misplacement of the lens will certainly produce dodgy results.
10-30-2011, 09:03 PM   #4
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I agree with SJ - you're not square with the paper so that could account for the decentering look you're getting.

That said - if you look around this forum, you will see several people reporting the same exact FF issue with both the 24-70 EX DG APO HSM and 70-200 EX DG HSM II lenses.

I own both of these lenses, purchased about 3 months ago, and both couldn't be sufficiently calibrated using my cameras' built-in AF adjustments. Both my K5 and K20, set at -10, couldn't bring either of these two lenses in.

So once my season slowed down enough, I sent both my lenses and one of my cameras into Sigma, where they're currently being worked on. Last I heard read on the support site, both were calibrated and had firmware upgraded.

I will report once they get back.

My next step is to send one of my cameras and my workhorse, the DA*16-50, to Pentax for the same treatment.

10-31-2011, 04:29 PM   #5
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I set-up my testing configuration at least 3 different times in two locations. I carefully considered if I was square or not and each time it appeared that I was square. I guess I could repeat the testing and actually measure the distance to the edges of the test sheet. Or I could put a mirror over the test sheet and confirm the lens appears in the center of the mirror. As I think about this more, I realize that with the test sheet at an approximate 45 degree angle, part of the right side should be in focus even if I was not square. While the line of focus would not be horizontal if I was not square, there should certainly be an area on the right side that is just as clear as the left side, correct?

If any one has experience with Sigma's repair process and turnaround time I'd like to hear it as I will have to send it in between scheduled shoots.

Last edited by dave2k; 10-31-2011 at 04:48 PM.

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