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03-09-2009, 11:52 PM   #1
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Autofocus Inconsistency with K20D

The recent post by MaKettle, "AF Question", got me wondering about a problem with my recently-purchased K20D. I had originally thought it was a lens problem, but I now think it's the camera. This, after putting my replacement DA*50-135 through it's paces. I submit this post for your comments / suggestions.

It all began after Jsherman999 "talked" me into the FA 35 (well, it wasn't too hard! ). I was disappointed with it's seeming AF inconsistency- one shot clear, tack-sharp and in focus, even wide-open; yet the very next shot,
(with all settings the same, and also composition, focus point, etc.), would be way out of focus.

Well when it happened with the 50-135, I started checking. So far it does the same thing with the FA 35, DA*16-50, Tamron 28-75, and DA*50-135. I'd test the rest of my lenses as well, but today I had rotator cuff surgery on my primary shoulder, so it'll be a while. (Yea and hunting and pecking this whole post with my left hand was one huge pain in the ass!)

These sample shots just happen to all be with the 50-135 but are representative. All are crops to show the point of focus. All handheld, at 135 mm. For each, focus lock was indicated in the viewfinder. The stove shots used on-board flash, to reduce the likelihood of camera shake, 1/50th at f4, ISO 400, and focus point was the down arrow below the word, "Hour". On the basketball net shot, focus point is the little round basketball logo. This shot was taken to cover a few more bases- outdoor light, farther distance, and higher shutter speed (1/500th, f4, ISO 400) to help rule out any
"camera shake" argument.

I pretty much plan to send this K20D in to Pentax, and to include these sample photos (and more) when I send it. That is, unless somebody on the forum has any other suggestions as to the cause. FWIW, I haven't had any problems like this with my K10D (and darn glad I am that I kept it as a back-up!). Thanks for any assistance!
Paul

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03-10-2009, 12:13 AM   #2
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Not to doubt your steady hands, but I'd like to see the test replicated with the camera mounted on a tripod, so that

(1) you definitely are not introducing hand shake
(2) you are not accidentally AF locking in different areas (e.g., the first photo of the second pair looks like you focused on one of the branches).


You can force the AF to move after each shot by putting a hand in front of the lens. So

1. setup tripod, point to something, use 2 second timer, focus and shoot
2. put a hand in front of lens, half press the shutter to move AF
3. remove hand, use 2 second timer, focus and shoot

perhaps repeat this 1~2 times per subject, then point the camera at something else, repeat, so on, and try 3~4 different kinds of subjects.
03-10-2009, 06:40 AM   #3
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Bet I know what it is...when you shoot, do you look at the viewfinder and wait until the little hand lights up? That indicates that SR has activated completely. If you just point, focus, and press, it may not have had enough time and the SR mechanism will give you blurry shots...
Try turning off SR and do your tests again?
03-10-2009, 08:35 AM   #4
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which af mode have you made the pics with? there is nothing similar visible in the first 2 crops, however, comparing last ones, the first shows the branch appearing crisper than the basketball plate, the last opposite. this may indicate, that the focus point isn't actually targeting the same part in different pics of the same subject/situation whatsoever but another, due to the area it covers. this doesn't explain the behaviour in the first example though. if you havn't done so yet, give the center af mode a chance and wait for the af movement and the SR to complete - thanks for that hint, that can be very valuable).
I have a similar thing with all lenses when mointing them onto my old istDL. I assumed, the stepmotor and gear that drive the AF is just not precise enough to tackle certain situations. I had much better results with K100Ds, yet still had to argue with that model. K20d seems to no better. For all I found meanwhile, manual adjustment is even harder to acchieve accurately. Wonder how the Nikon guys speak about their gear. Is this only Pentax specific?


Last edited by kasv; 03-10-2009 at 08:47 AM.
03-10-2009, 10:03 AM   #5
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Thanks for the thoughtful replies. I always use center point AF, on AF-S. The information about the actual AF zone being much larger than the actual red box may be a factor, good to know. I like the idea about using a tripod and putting a hand in the frame to move the AF, I'll have to try that. And to be honest, I don't think I ever paid much attention to the Shake Reduction symbol in the viewfinder. ( I will from now on!) Thanks for the tips. There's no way I could hold a camera right now with my arm laid up, but I'm sure I can work it onto a tripod. I'll give your suggestions a try!
Paul

P.S. Yea it's probably operator error then, glad to have this forum (and many learned photographers!) as a resource. I do think this camera/lens combo (K20D/DA*50-135) is absolutely amazing. This was another one of my "test shots", taken around the house:

(135mm, 1/30th hand-held, f2.8, ISO 1250) The full-res image is amazingly sharp!
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03-10-2009, 11:46 AM   #6
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QuoteOriginally posted by NeverSatisfied Quote
The information about the actual AF zone being much larger than the actual red box may be a factor, good to know.
For the basketball shot, I'd say it is not just something that "may be a factor", it is indeed the whole story. The camera obviously focused on the branches, which would have been well within range of the sensor if that was any sort of significant crop. Harder to say in the first shot, but yeah, can't rule out simple camera shake.
03-10-2009, 12:19 PM   #7
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btw, if you install a freeware program called PhotoMe, it'll show you the focus point for the image and whether SR was active when the shot was taken. It's all stored in the EXIF info...
03-10-2009, 01:54 PM   #8
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QuoteOriginally posted by kenyee Quote
btw, if you install a freeware program called PhotoMe, it'll show you the focus point for the image and whether SR was active when the shot was taken. It's all stored in the EXIF info...
Interesting, definitely sounds worthwhile. Thanks Ken.

Yea Marc after reading MaKettle's post I was definitely suspicious that the AF zone was overlapping onto the trees. And yes that -was- a highly magnified crop. I'm going to redo the testing but I'll bet right now that was the answer. And from now on I'll be more careful with my focus points. Thanks again!
Paul

03-10-2009, 02:48 PM   #9
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Funny that this thread comes up after I blew a paid photoshoot due to out of focus images on my K20D and the FA 35mm f/2. Granted, I was shooting wide open, but my subject distance was about 10 feet, giving me around 1-2 feet of DOF. I don't recall focus errors with my old K10D.
03-10-2009, 06:21 PM   #10
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Well, now I can probably be happy with my FA 35 once again. I specifically remember when it was giving me problems, I was trying to focus on an object in the foreground which was no bigger than the red AF box in the viewfinder. Most of the resulting photos were grossly out of focus. Now I know why!
03-10-2009, 06:25 PM   #11
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Problems I was having was when I was focusing on the head of the subject. Maybe next time I should just aim for the body instead.
03-10-2009, 07:21 PM   #12
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QuoteOriginally posted by krypticide Quote
Problems I was having was when I was focusing on the head of the subject. Maybe next time I should just aim for the body instead.
Were you attaining focus and recomposing by any chance?
03-10-2009, 07:33 PM   #13
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i have random AF issues sometimes. i "reset" the focus by focusing to something to infinity or at minimum focusing distance, then i refocus on the subject. seems to work i think.
03-11-2009, 09:36 AM   #14
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QuoteOriginally posted by Damn Brit Quote
Were you attaining focus and recomposing by any chance?
Yep, but I don't think recomposing affected the focus at all, given the DOF of 2 feet and maybe only 10-15 degrees of change in camera angle.
03-12-2009, 10:40 AM   #15
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I recently ran into this same problem while trying to adjust my various lenses to my K20. I just had my K20 returned from Pentax for servicing because I couldn't bring my DA 40 into focus. Pentax adjusted the focus range and I was in the process of calibrating all of my lenses again when I started noticing the problem. A particular lens would indicate that it needed about a -3 or -4 adjustment. I would make the adjustment, take three shots and compare. One or two of the shots might show an improvment, but the other would indicate that I now needed a + adjustment of +1 or +2! Some lenses were better than others and had no problems, but some just had me chasing my tail. I don't believe that it was operator error as I had the camera on a tripod with shake reduction off and using the 2 second delay while shooting at a perfectly perpendicular test target. Oddly, I would finally give up and try again the next day and the lens would usually come back into adjustment? Not really sure whats going on as I am still in the calibrating process, but it sure looks like the focus is not consistant. The variation is small, and you may not notice the difference on a shot at any distance, but when shooting the test target is is definately noticable.
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