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07-09-2015, 10:36 PM   #1
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Pentax k-x weird focus problem

Hi there,
this is my first post on this forum and i apologize if i posted in the wrong place.

I have an pentax k-x and i use it with 18-55 samsung lens and now i mounted an tamron 17-50 / f2.8 . The problem is that on both lenses, sometimes the camera does backfocus ( not always ). On the 18-55 i ajusted the AF system from the debug menu, but sometimes still does the backfocus, let's say 1 photo from 10 has backfocus.

On the tamron 17-50 lens, at f 2.8 the picture is not so clear, a little bit blurry, like is not focused corectly, but i don't see any back/front focus. If i incresse to f4 or more, everything is ok.

My question is:

The problem is from the lenses or from the body?How can i solve this?


thank you very much and i'm sorry for my bad english.

07-09-2015, 11:12 PM   #2
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With the Tamron 17-50, at F2.8, you are likely observing some softness and lower contrast that is a normal characteristic of the Tamron lens shot wide open. By F4 is sharpens up nicely and contrast also improves significantly. It is probably at its best at F5.6.

Do remember that you can only apply a single global AF adjustment via the debug menu that applies to all lenses, so you may need to go back into the debug menu each time you changes lenses in order to set the right AF correction for the lens now in use. Bit of a pain on the K-x if you swap lenses frequently.

There is some minor variability of where AF focusses a lens shot to shot, although this variability is masked by depth of field most of the time when using narrower apertures like F4 or smaller. Shooting wide open though at F2.8 will reveal this variability more often because of the limited lack of depth of field wide open. Having said that, I would get the occasion shot from the K-x/Tamron 17-50 combination where it seems the AF just got it all wrong even at F5.6 or F8. By occasional, I would guess less than 1 in 100 images where it would miss focus by a wide margin. Might well be operator error although nothing stood out as being unusual about how I took that particular image
07-10-2015, 08:43 AM   #3
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What type of focus point are you using? Multi-point? Single point?
07-12-2015, 10:08 PM   #4
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i was reading that the AF adjustments apply only for pentax lenses. is it true?

Also, on the tamron i don't see any back or front focus, just a little bit blurred/shaked at 100% zoom on f2.8. is that ok? Also i notticed that at f4 - f6.3 the picture is very sharp and the contrast verry good.

Also i have tested the k-x this days with a sigma 28-70 AF and a cosina M 50mm / f1.9 . The quality of image is very good. Maybe my tamroon have a problem and needs to be replaced. I will attach today an image made it with tamron / sigma / kit lenses and the cosina 50-M.

Thank you very much !

07-13-2015, 06:11 AM   #5
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Did you see my question immediately above your post?
07-13-2015, 06:18 AM   #6
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QuoteOriginally posted by cristianmiu Quote
i was reading that the AF adjustments apply only for pentax lenses. is it true?

Also, on the tamron i don't see any back or front focus, just a little bit blurred/shaked at 100% zoom on f2.8. is that ok? Also i notticed that at f4 - f6.3 the picture is very sharp and the contrast verry good.

Also i have tested the k-x this days with a sigma 28-70 AF and a cosina M 50mm / f1.9 . The quality of image is very good. Maybe my tamroon have a problem and needs to be replaced. I will attach today an image made it with tamron / sigma / kit lenses and the cosina 50-M.

Thank you very much !
I know that with my K-x shutter speed between 1/80 and 1/125 (from memory...) were problematic creating some heavy shutter blur most of the time. It doesn't really sound like your problem, but it could still explain some of your blurry pictures.
07-13-2015, 09:40 AM   #7
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Where is K-x debug menu for lens? AF adjustment...
07-13-2015, 11:40 PM - 1 Like   #8
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@pishta, i hope this will help you

RiceHigh's Pentax Blog: K-x Debug Mode Tutorial - AF Adjustment

07-14-2015, 11:30 AM   #9
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QuoteOriginally posted by cristianmiu Quote
i was reading that the AF adjustments apply only for pentax lenses. is it true?
On the K-x any AF adjustments in debug mode will be global. Per lens adjustment is not possible.

QuoteOriginally posted by pishta Quote
Where is K-x debug menu for lens? AF adjustment...
QuoteOriginally posted by cristianmiu Quote
@pishta, i hope this will help you
RiceHigh's instructions will give you access to the debug menu. Be aware that that is a dangerous place unless you are sure you know what you are doing. Playing with the AF adjustment is relatively safe, but other things accessible through Dev/Debug had potential to "brick" your camera.


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07-14-2015, 01:02 PM   #10
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QuoteOriginally posted by cristianmiu Quote
Awesome, thanks! I thought I bricked a TV once in test mode. I entered a value and the screen turned into a bright pencil thin line, in a panic, i just pulled the plug and let it set for a few minutes. Plugged it back in and it recovered, i guess it never had a chance to write the change, thank goodness.
07-14-2015, 03:26 PM   #11
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I was having similar issues. I switched the camera off of automatic focus point to centre focus point and the problem seems fixed. In auto detect mode I would get some images that focused on the background even when the subject was in the centre of the frame. Not sure if all the Kx's do this.

Thanks

Randy
07-14-2015, 05:24 PM   #12
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QuoteOriginally posted by slip Quote
I was having similar issues. I switched the camera off of automatic focus point to centre focus point and the problem seems fixed. In auto detect mode I would get some images that focused on the background even when the subject was in the centre of the frame. Not sure if all the Kx's do this.
As far I've seen, all cameras behave that way in multi focal point mode. The camera doesn't have the artificial intelligence to always know where the photographer wants the focus. That's why I was asking the OP how he has his focus points set, but he never answered.
07-14-2015, 10:30 PM   #13
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Sorry for my late answer, single point focus ( center ).
07-15-2015, 06:06 AM   #14
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QuoteOriginally posted by cristianmiu Quote
Sorry for my late answer, single point focus ( center ).
Thanks! So often this kind of question is posed by somebody who has not selected a single point, so I was wanting to eliminate the obvious.
07-15-2015, 07:48 AM   #15
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In multi point focus, AF tends to "guess" focus on closest objects with highest contrast in the frame. Now, without the visible AF points of KX, this can be a problem. One has to magnify view at least 8x just to ensure that intended focus was indeed attained. Of course, this can be solved by center AF, or better yet manually move the focus using the arrow pad.

---------- Post added 07-15-15 at 10:52 PM ----------

A wild guess, the recomposition using center point tends to alter focus, especially using big apertures, that's why it may be evident on f/4 or bigger. Try to use the arrow pad instead of recomposition. Let us know if this solves your problem
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