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03-08-2011, 12:06 PM   #1
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AF inaccurate on KX?

Got my KX a few days ago, and occasionally the AF will be off by several feet. I'm using the center point, and for the example crops, I was at f5.6, 1/160, iso200, 55mm with the kit lens. Most of the time the focus misses, it happens at the wide end, but as you can see, zoomed isn't perfect either. I aimed at the lamp, and it missed the focus by 3 or 4 feet. How often is this supposed to happen? i had another incident in which I was shooting something 25ish feet away and the focus was close to 15 feet away. Is the AF just really loose in what it considers sharp? Can I tweak that?

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03-08-2011, 12:39 PM   #2
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Can you try your camera with another lens to compare,

also if you don't mind experimenting...
I've had great luck with the now 'not-so-secret' DEBUG MENU,

check out this link:

https://www.pentaxforums.com/forums/general-technical-troubleshooting/130514-...cs-please.html

Michel

Last edited by mlatour; 03-08-2011 at 01:04 PM.
03-08-2011, 12:47 PM   #3
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QuoteOriginally posted by BennyBambino Quote
Got my KX a few days ago, and occasionally the AF will be off by several feet. I'm using the center point, and for the example crops, I was at f5.6, 1/160, iso200, 55mm with the kit lens. Most of the time the focus misses, it happens at the wide end, but as you can see, zoomed isn't perfect either. I aimed at the lamp, and it missed the focus by 3 or 4 feet. How often is this supposed to happen? i had another incident in which I was shooting something 25ish feet away and the focus was close to 15 feet away. Is the AF just really loose in what it considers sharp? Can I tweak that?
You should definitely be sharper that that. Mine, though, sometimes misses focus lock and defaults to infinity when it gets confused. This usually happens in low light, though it occasionally happens if I'm not careful where I put my fingers and grab the focus ring by mistake. This is default behavior if it can't find a focus lock. If you can find a place with nice light and a way to set the camera where you don't touch the lens at all (tripod, desk, etc) run a few test shots. If you're still seeing issues... then you may want to talk to Pentax.

-Mike
03-08-2011, 12:52 PM   #4
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It's difficult with the K-x because you don't have a red spot to guide you like the other models. #2 shows that it does get it right, but for #1 there are a number of possibilities. One is that you've fixed focus and then zoomed to 55mm and took the shot, which inherently misses the focus point; or two, it's focused in on something in front of that black & white screen (which you've cropped off) and given the appearance of aberrant focusing. Of course it could be mis-focusing intermittently due to low-light or backlighting confusion, which is not a new thing for any of the camera models, so observe when the misfocusing occurs with proper technique and see how the camera behaves with multiple subjects.

Also bear in mind that stretching the kit lens to shooting wide open at 55mm is going to produce quite soft results, even at the point of focus, and especially when you're viewing at 100%.

03-08-2011, 12:59 PM   #5
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Sorry if I didn't make it clear, but those are 2 crops from the same photo. The bottom crop is just some junk about 4 feet closer to the camera than the lamp.

All I have is the kit lens. I've had a look at the debug menu, but from what I understand, that's for a consistent issue. Most of the time it gets it right, but it misses more than I'd like.

I know wide open at 55 makes soft results, but as you can see, the junk is in focus. here's the full image so you can see where everything is in relation.
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03-08-2011, 01:05 PM   #6
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QuoteOriginally posted by BennyBambino Quote
Sorry if I didn't make it clear, but those are 2 crops from the same photo. The bottom crop is just some junk about 4 feet closer to the camera than the lamp.

All I have is the kit lens. I've had a look at the debug menu, but from what I understand, that's for a consistent issue. Most of the time it gets it right, but it misses more than I'd like.

I know wide open at 55 makes soft results, but as you can see, the junk is in focus. here's the full image so you can see where everything is in relation.
The kit lens' strength isn't in low light. What you have there counts as pretty low light... indoors, low ISO, wide-open, max zoom. You're not giving the sensor much to work with. That image (seen full size) seems about par for the course given the conditions. It appears dark enough that the camera may not have been able to discern the contrast on the ribs of the lamp, same as if you tried to focus on a white piece of paper... Autofocus sensors look for contrasting lines.

Do you get focus misses in bright light (using flash, outdoors on sunny day)?

It looks like it didn't use center point as the autofocus target. Can you confirm that it's in center point mode, and not any of the auto modes?

Last edited by sgtkashim; 03-08-2011 at 01:10 PM.
03-08-2011, 01:09 PM   #7
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You may have also inadvertently got focus on the paraphernalia below the focusing chart.
Try again with multiple exposures and see if it consistently misfocuses under these conditions (ensuring centre focus and no recomposing of the shot).
03-08-2011, 01:17 PM   #8
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You said you aimed at the lamp but it still missed the focus. To me it looks like it focused on the helmet instead of the lamp. This can happen if you are NOT using the center point focus. If your focus point is set to Auto, the camera itself tries to find the focus point. Set your camera to center point focus and see if that works.

03-08-2011, 01:20 PM   #9
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Outdoors it's fine most of the time. The only thing I can get it to consistently screw up on so far is focusing on something close, then aiming at the blinds on my window. It beeps, suggesting focus lock, but the picture suggests it didn't even try. It's 3 PM here, sun is coming through my window. It manages to focus fine in this case if I hadn't focused close prior.
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03-08-2011, 01:29 PM   #10
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The K-x is a nice camera but the lack of a visible focus point does make it hard to figure out in advance if the camera is focusing where you intended it to or not. Like another poster suggested, try changing the setting to center point focus, instead of the 5 or 11 pt auto mode.
03-08-2011, 01:32 PM   #11
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I've been using center point only.
03-08-2011, 03:14 PM   #12
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Did you set your focus point to center spot focus or the center point focus on the 11 point focus mode? Is your center point still set to center point (if you set it to center point on the 11 point mode)

Could it be that a thumb or finger activated the focus point unknowingly and it was then moved down to the point UNDER center point ?
03-08-2011, 05:01 PM   #13
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I have it set to center only. It's the last option on the select af screen.

I just took it into the field, and for practical purposes, the only thing it wasn't great at was focosing on tree leaves at a distance at the wide end.
03-08-2011, 09:26 PM   #14
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Try another lens

My K-x does not do this, and I have several lenses for it. It will misfocus a little on low contrast objects from time to time, but it is quite rare.

Could you perhaps go to your local camera store and try out a lens there, using the same technique you've described here? Try some shots with your lens, then try another similar lens.
03-08-2011, 11:02 PM   #15
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I think I'll try that gebco. The only store I can think of around here is Wolf Camera, but I'll look for some small businesses.
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