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05-22-2009, 04:32 PM   #1
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AF adjustment (K20D)

Recently I did a weekend gig at the Dallas Arboretum taking family portraits for guests. While I was shooting I noticed that somehow I'd moved the focus option to SEL and moved the focal point to a point sort of on the left side of the frame rather than in the center. Of course, moved it back to center-point focus immediately. Thankfully the photos I took while autofocus was set to SEL were okay. There were a couple where the flowers five feet behind the subject were in focus and the subject's face was a bit fuzzy but most of them were okay. My guardian angel was watching out for me.

Anyway it got me looking closely at the autofocus acuity of my camera and I decided that, even with the settings right, the results aren't quite as good as I think they should be. So I finally broke down and tried calibrating one of my lenses (the Pentax FA 35 AL). I'm going to test a little more on the weekend but it appears that there was some slight but noticeable backfocusing going on, which I seem now to have corrected. Good news.

But I have some questions.

First, is this common? And if it is, what do folks do who don't have a focus-calibration option in their cameras? (That includes the K10D, doesn't it?)

Second, how do I decide whether to adjust for all lenses or for lenses one by one? At the moment I made the adjustment using the "all lenses" option but I haven't yet tested with other lenses.

Third, this affects only auto-focus, right? It doesn't matter to manual focus?

Fourth, does this work with non-Pentax lenses, too?

Thanks,

Will

05-22-2009, 05:06 PM   #2
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On second thought....

Well, after seeing what I THOUGHT I was seeing earlier, I wrote the previous post. And since then I've continued testing, with different lenses (added the Pentax FA 50 f/1.4 and the FA 16-45 f/4) and with various AF Adjustment values, including some extreme ones just to see if it really matters.

The attached photo was taken with the FA 35 f/2 - with an AF Adjustment value of 0 (zero). I focused on the numeral "10" on the ruler - not on the 10" line, but on the numeral. Looks tack sharp to me.

So now I'm a bit confused about what's going on.....

Will
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05-22-2009, 05:56 PM   #3
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well if the focus is accurate with a setting of zero then your lens is calibrated correctly and you don't have to worry. This is only a "just-in-case" feature thats nice to have if you have a lens that backfocuses or frontfocuses. Typically this shouldnt happen but some third party lenses are notorious for having frontfocus/backfocus issues. People that don't have this feature must either return the lens for a replacement, or send it in to the manufacturer for recalibration. If all of your lenses focus fine except for one or two, then you should do the one-by-one calibration, however if all seem to have a problem then you can do the global adjustment for all lenses. And yes this only affects AF and yes it works for third party lenses as well
05-22-2009, 06:42 PM   #4
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QuoteOriginally posted by kthung Quote
well if the focus is accurate with a setting of zero then your lens is calibrated correctly and you don't have to worry. This is only a "just-in-case" feature thats nice to have if you have a lens that backfocuses or frontfocuses. Typically this shouldnt happen but some third party lenses are notorious for having frontfocus/backfocus issues. People that don't have this feature must either return the lens for a replacement, or send it in to the manufacturer for recalibration. If all of your lenses focus fine except for one or two, then you should do the one-by-one calibration, however if all seem to have a problem then you can do the global adjustment for all lenses. And yes this only affects AF and yes it works for third party lenses as well
Thanks, kthung. I've only tested Pentax lenses so far. But as a practical matter I use my Sigma lenses pretty heavily. Think I'll have to check them and see how they're doing. Might need to calibrate them individually.

I had to scratch my head earlier because at first it looked like I'd improved things with an adjustment (using the Pentax 35) and then it looked like it was working fine when I reset the AF Adjustment to 0. I guess I'll chalk that up to my lack of familiarity with the feature.

Will

05-23-2009, 09:27 AM   #5
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Try to conduct your tests under daylight (near a window) and not under tungsten or incandesent light as that can be confusing for the AF sensor and may cause it to front focus, this could be why one time it looks better with an AF adjustment and another time looks fine with no adjustment.
05-23-2009, 09:31 AM   #6
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QuoteOriginally posted by kthung Quote
Try to conduct your tests under daylight (near a window) and not under tungsten or incandesent light as that can be confusing for the AF sensor and may cause it to front focus, this could be why one time it looks better with an AF adjustment and another time looks fine with no adjustment.
OK, thanks for that tip. I was in fact testing at my dining room table, right under a big tungsten lamp. Will test more today by a window with the sunlight. Thanks again, kthung.

Will

P.S. I like saying "kthung" aloud to myself as I type. :-)
05-24-2009, 02:20 PM   #7
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QuoteOriginally posted by WMBP Quote
The attached photo was taken with the FA 35 f/2 - with an AF Adjustment value of 0 (zero). I focused on the numeral "10" on the ruler - not on the 10" line, but on the numeral. Looks tack sharp to me.
This is a potentially misleading test, as you cannot be sure what the camera focused on. The AF sensor area is bigger than you may think and it may also be slightly off-centre. There are too many details the AF system can lock on so you cannot be sure at which position it attained focus.

You should get a focus chart or try to shoot a target where you are sure where the camera is focussing. I compiled a few AF adjustment tips. Some swear that tilted targets lead to wrong results, but I haven't been able to verify this myself.

Last edited by Class A; 05-25-2009 at 09:11 PM.
05-24-2009, 05:31 PM   #8
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Also, camera shake adds to AF focus inaccuracy: in the manual, they tell you to perform your focus adjustment on a tripod.

05-24-2009, 05:45 PM   #9
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You should download the focusing charts from Yvon Bourque's blogspot. They are a lot more accurate than a ruler, and they are free. As an aside, some Sigma lenses use the same generic chip to tell the manufacturer's name to the camera. If you happen to have a couple of different lenses with the same chips, the focus adjustment you do for one lens will automatically be applied to the other one. Don't ask how I found that out. Since then, I just kept one Sigma lens. All my other lenses are now Pentax, and I didn't need to do any focus adjustment with any of them. Good luck.
05-24-2009, 05:57 PM   #10
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Charts are here: http://www.k10dbook.com/newchart.pdf

These charts make life so much easier for the focus test.
As stated before, use a tripod. I would take at least 10shots each time and reset the focus between each shot (ie turn the focus ring to out of focus by hand).
On my k100d the result would like this:
3 shots perfect,
5 back focus
2 front focus

Doing tests like this goes to show you how inconsistent AF is (at least on my k100d).
If all the shots out of each 10 were identical (as i first expected they would be) then the whole exercise would be easy.

I don't have a K20d but i do have lenses that require different focus settings. Can't wait for a k7.

mike
05-24-2009, 06:06 PM   #11
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I found Tim Jackson's writeup and focust test methodology to be useful. Although he developed it for his Nikon equipment, it works for the Pentax line up to the K20. I don't know about the K-7 as it appears to have a different autofocus methodology. You can find it here:
Nikon D70 Focus Chart

Dave
05-24-2009, 07:39 PM   #12
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QuoteOriginally posted by flyer Quote
You should download the focusing charts from Yvon Bourque's blogspot.
Be aware though, that some (e.g. blende8) specifically advise against tilted targets. In (my) theory it shouldn't matter, but note that the service manual for the GX-10 (K10D) also specifies a flat target (with an angled ruler for the measurement).

Yvon's chart nicely avoids any potential unwanted focus targets and has a precise focus target (small point/line) but I'm not sure whether the adjustment is optimal since the measurements are so far away from the focus point. If the lens has some level of field curvature then the focus will not be spot on in the centre.

QuoteOriginally posted by davef Quote
I found Tim Jackson's writeup and focust test methodology to be useful.
I dislike the wide target. You are not sure whether you focus on the top or bottom part of the rather wide line (bar) in the middle.

A commercial solution is the LensAlign. It uses a flat target. Not sure how far away the ruler is from the centre area which is used to obtain focus. I wouldn't spend the asking price for it, but rather try to obtain a similar DIY solution. My shots haven't been bad/important enough to make me do this.

Last edited by Class A; 05-24-2009 at 07:48 PM.
05-25-2009, 03:58 AM   #13
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So why not do it this way: take a series of pictures focusing on a small (for the focal length) object with nothing around it. "Bracket" the AF adjustment, and use the best.

Seems easier and less error-prone than any chart setup.
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