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07-09-2008, 06:23 AM   #1
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K10D focus issues (indoor)

I was playing around with my 50-135 on my K10D, when I noticed that at 50mm wide open, my shots were mostly OOF. Though at 135mm wide open, they were all spot-on. When I stop down down to f5.6 at 50mm, focus is tack sharp once again. I thought at first my K10D was experiencing some BF issues, but earlier today, I figured to shoot at 50mm wide open again, just to see how off my focusing was... when I checked, all my shots were in focus! Any idea what's going on? I'm thinking my K10D or 50-135 is having a hard time focusing indoors (something to do with the light perhaps?).

BTW, I tested the 50-135 with my K100D, and all shots wide open were tack sharp! (sharper than my K10D actually, when the K10 shoots in focus that is).

07-09-2008, 07:00 AM   #2
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QuoteOriginally posted by leadbelly Quote
I was playing around with my 50-135 on my K10D, when I noticed that at 50mm wide open, my shots were mostly OOF. Though at 135mm wide open, they were all spot-on. When I stop down down to f5.6 at 50mm, focus is tack sharp once again. I thought at first my K10D was experiencing some BF issues, but earlier today, I figured to shoot at 50mm wide open again, just to see how off my focusing was... when I checked, all my shots were in focus! Any idea what's going on? I'm thinking my K10D or 50-135 is having a hard time focusing indoors (something to do with the light perhaps?).

BTW, I tested the 50-135 with my K100D, and all shots wide open were tack sharp! (sharper than my K10D actually, when the K10 shoots in focus that is).
There is some evidence of focus inaccuracies under tungsten light but it is spotty and not well correlated. My guess is that, since the AF sensor is sensitive to IR (otherwise those light blocking ir filters wouldn't autofocus at all, ir people can correct me if this is wrong) and low light tungsten emits quite a bit of ir ther could be a focus shift (since ir does not focus in the same plane as visible light).
Now to give a counterpoint some light sources have been known to affect focus.
To be fair the amount of evidence here is about the same as for Pentax tungsten research.
See this:
Fluorecent and AF inaccurracy [Archive] - Open Photography Forums
Regarding your last comment, are you saying the k100 under tungsten is fine?
I try to avoid AF discusssions because there are just too many variables to go along with too many expectations
How a DSLR autofocus works: Canon EOS 1000D / 450D - 300D Forum: Digital Photography Review
Just for fun:
Autofocus troubleshooting by Thom Hogan

Last edited by jeffkrol; 07-09-2008 at 07:09 AM.
07-09-2008, 07:06 AM   #3
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Thanks! I kinda figured it has something to do with the type of available light... however, what I don't get is that it happens only when I am shooting at 50mm... at 70 or 135mm, shots are sharp...
07-09-2008, 07:56 AM   #4
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In my not-so-scientific tests... I seem to have the same 'indoor' focus problem. I have yet to test it under different types of light bulbs to see if that makes a noticeable difference. Last weekend I was taking photos of a friends dog (outside) in evening light (not bright light... but not artificial light) and even at f2.8 with a Tamron 28-75 the focus was on target. Had I tried the same shots indoors I doubt my luck would have been so good.

07-09-2008, 08:19 AM   #5
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I recall a thread here almost a year ago noting something like this. A lot of people found they needed to zoom to 135, focus, and then pull it back out.
07-09-2008, 08:29 AM   #6
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@jodocast: really? so was the problem inherent to the lens? or bad copies? or something to do with the light source? (used the 50-135 on my k100, it was sharp at 50/2.8)
07-09-2008, 08:39 AM   #7
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If it's an autofocus lens your trying to focus have you tried just popping the flash?
It always helps in lower light situations, then just push it back down.

That's all that I do.
07-09-2008, 08:45 AM   #8
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@littlelaker: It wasn't a lowlight situation, in fact, my desklamp was about 2 feet above the subject. Or even so, should I try to pop up the flash to help in focusing? (BTW, it's only with the 50-135 that I experience this problem...)

07-09-2008, 09:01 AM   #9
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https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/16438-da-50-13...iour-help.html Take a look for yourself.
07-09-2008, 09:31 AM   #10
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Thanks Jodokast96. Looks to be a normal occurence then with the 50-135 and K10D.

Are there K20D users here who are experiencing the same thing? Or is this native to the K10D only? My K100D with the 50-135 works great... I only experience this with the K10.
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