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01-03-2013, 12:24 PM   #1
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What is my K5 autofocus doing?

My K5 auto focus is being very inconsistent with how it judges distance of a stationary, well lit subject, photographed several times in a row. For example, if I take a dozen photos of the same street sign in bright sunlight, half of them are in focus and the other half are out of focus. I have been very careful to ensure the auto focus mode is set to "single" and the focus point is set to the center. The attached photos are but two out two dozen test shots, one demonstrating that the K5 can find the proper focus and the second showing incorrect focus. The conjoined images are 100% crops of the center portion of both photos. The camera was on a tripod and the 2-second, mirror-up timer was used. The shooting procedure was to auto focus on the subject and make an exposure, hold my hand in front of the lens and auto focus to de-focus the scene, and then auto focus on the subject again to make another exposure.

I first noticed this happening after receiving my K5 back from CRIS. The camera had gone to them along with my DA* 50-135mm and a DA 55-300mm for calibration as well as the general check CRIS would do on account of having Pentax's 2 year extended warranty. When the K5 came back my two lenses were performing better than I thought was ever possible, but my main walk-around lens, a Tamron 17-50mm was front-focusing. The Tamron went off to its respective repair facility several times because I was convinced they were not repairing it correctly. Finally, I sent in the K5 with the Tamron so they could calibrate the lens to the body. When I got the pair back I tested the combination in the same way I described in my first paragraph. Some images were spot on, some were way off. After this I got out my Pentax 18-55mm kit lens and performed the same test procedure with the same mixed results; some good, some bad. The K5 then took a trip to CRIS (with test images on disc) and came back to me about 10 days ago, performing with the same inconsistent auto focus.

I have noted, strangely enough, the problem only seems to occur when the two lenses are set to their widest focal lengths (18mm and 17mm). However, considering the focus is sometimes correct and other times not correct, it seems clear the lenses are not at fault. I have had several lenses which exhibited front/back focus problems, but they were at least consistent and could be managed with focus adjustment in-camera.

I'm presently waiting for a technician from CRIS to call me. In the mean time, does anyone have any thoughts?









Last edited by LowVoltage; 01-03-2013 at 12:29 PM. Reason: images didn't show up properly
01-03-2013, 12:34 PM   #2
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Turn your SR off That is the most likely culprit. It takes a short while to settle, so taking pics in quick succession will do it.
Secondly, be aware of the size of your focus sensors. They are far bigger than your square indicators suggest. The center point reach out all the way to the () spot metering markings.
01-03-2013, 12:39 PM   #3
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Could this be motion blur? Did the SR system "come to rest" in the image to the right before the shot (the hand symbol must be lit in the viewfinder)? I would have expected to see some fringing if the sign was out of focus.
01-03-2013, 12:50 PM   #4
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Not trying to thread jack but how long does it take for the sensor to "come to rest"? Judging from the images, the SUV and the white car are gone in the second image so my assumption is i twas at least a second or two later at a minimum. Surely it does not take this long for the SR to "reset" does it?

01-03-2013, 12:57 PM   #5
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SR should be off as he stated he used 2-second, mirror-up mode. Weirdly nothing looks in focus on the second shot, as if it was motion blur of something. Exif would help big time.
01-03-2013, 01:40 PM   #6
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QuoteOriginally posted by VisualDarkness Quote
Weirdly nothing looks in focus on the second shot, as if it was motion blur of something. Exif would help big time.
Sorry for not providing EXIF data with the originals. All of my test images from this particular series were exposed at 1/750 second at f/4, so surely it's not camera shake. I kept the aperture at f/4 because I needed better sharpness than what could be provided wide-open, yet I did not want a small aperture to skew the results with a great depth of field.
01-03-2013, 01:43 PM   #7
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QuoteOriginally posted by westmill Quote
Turn your SR off That is the most likely culprit. It takes a short while to settle, so taking pics in quick succession will do it.
Secondly, be aware of the size of your focus sensors. They are far bigger than your square indicators suggest. The center point reach out all the way to the () spot metering markings.
QuoteOriginally posted by Ole Quote
Could this be motion blur? Did the SR system "come to rest" in the image to the right before the shot (the hand symbol must be lit in the viewfinder)? I would have expected to see some fringing if the sign was out of focus.
The 2-second timer with mirror-up automatically disables SR per standard function of the DSLR, so this is not an issue with shake reduction.
01-03-2013, 01:58 PM   #8
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The wider the angle, the harder time is it going to have getting great focus on a faraway object. My question is where is the focus plane in the second picture? I can't enlarge it enough to really tell, but it looks like NOTHING in the whole parking lot is really in focus -- I'm not sure it ever moved at all after focusing on your hand. Unless there were vibrations -- even on a tripod a strong wind (or a big rumbling truck) can make enough movement to wreck things.

01-03-2013, 02:11 PM   #9
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mmmmmm true.... but i would still make certain. This looks very much like it, but OK. it is unlikely to be a sensor size issue due to the subject, but not impossible if you were targeting a small object such as the lamppost. Despite what you say, which is indeed correct i still think its an SR issue. The front and rear focussing will no doubt be caused by the large sensors unless you are already aware and compensate for it. The only other issue that springs to mind regarding the tamron 17-50 is that it suffers greatly from severe field curvature at the 17mm end. This means the lens needs stopping down to at least F8 for an even field of sharpness, pref even F11.
Those are my thoughts anyway.
01-03-2013, 02:29 PM   #10
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QuoteOriginally posted by westmill Quote
The only other issue that springs to mind regarding the tamron 17-50 is that it suffers greatly from severe field curvature at the 17mm end.
My understanding of field curvature is that it affects the image borders and corners, i.e. not the center, which is where I was focusing.

Below are samples I sent to CRIS when I originally sent the camera in. As before, the 100% pixel cropping of the image shows off just how badly the subject is out of focus, while the wooden hand railing in the foreground is sharp. Obviously, this is NOT an issue with vibration or wind. What you are seeing is just one out of dozens of photos taken in which the same center subject was acquired via auto focus, which was sometimes successful, but other times not. The auto focus point was at the center and the subject was easily at or beyond the lens' "infinity" distance. This same effect is occurring with more than one lens, so it is not a back focus issue.




01-03-2013, 03:10 PM   #11
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Well its a little more complex than that but you are correct, in that if you used center focus point, this is not the problem either. The sharp rail also discounts SR thoughts too in a way, since SR ooomphs the whole pic as in the first set. Im wracking my brain here lol. This is a good one as Im at a loss at the mo. If you can be sure it is none of the suggested possibilities then I think the camera just has to be faulty. It is catching that bottom rail while using a center focus point that has got me. Other than a faulty camera, the only way I can see that is possible would be you accidentally locking on first pressure lol I hope you manage to get it sorted but I am fresh out of thoughts on this one.
01-03-2013, 10:51 PM   #12
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QuoteOriginally posted by westmill Quote
I am fresh out of thoughts on this one.
I am too! You can probably imagine how frustrated I am. I don't pretend to know much about auto focus technology, and for all I know this problem might go beyond the focus sensing mechanism itself. Hell, it could be a weak screw drive for all I know. I've never heard of a phase detection auto focus system becoming erratic, but googling "K5" and "auto focus consistency" only leads to discussions about Pentax's history with low- and mixed-light performance, how the K5 compares to the K5 II, et cetera, et cetera.
01-04-2013, 08:28 AM   #13
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QuoteOriginally posted by LowVoltage Quote
I am too! You can probably imagine how frustrated I am. I don't pretend to know much about auto focus technology, and for all I know this problem might go beyond the focus sensing mechanism itself. Hell, it could be a weak screw drive for all I know. I've never heard of a phase detection auto focus system becoming erratic, but googling "K5" and "auto focus consistency" only leads to discussions about Pentax's history with low- and mixed-light performance, how the K5 compares to the K5 II, et cetera, et cetera.
Yes i can understand your frustration I had problems with my K5 too. I managed to resolve them for most part. Mine was SR and finding out about the huge focus sensors. Mine was not as bad as this though. There has to be a fault here. I would take it back and demand an exchange if possable. I ended up selling mine and buying the XE1 fuji. The K5 is not that bad though. It certainly should not be doing what yours is doing
01-04-2013, 01:49 PM   #14
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QuoteOriginally posted by westmill Quote
Yes i can understand your frustration I had problems with my K5 too. I managed to resolve them for most part. Mine was SR and finding out about the huge focus sensors. Mine was not as bad as this though. There has to be a fault here. I would take it back and demand an exchange if possable. I ended up selling mine and buying the XE1 fuji. The K5 is not that bad though. It certainly should not be doing what yours is doing
Problems with SR? Huge focus sensors isn't a faulty device at least, just product design.
01-04-2013, 02:08 PM   #15
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QuoteOriginally posted by VisualDarkness Quote
Problems with SR? Huge focus sensors isn't a faulty device at least, just product design.
Tell me something I dont know lol
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