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01-07-2010, 01:11 PM   #1
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K-x Mis-Focus?

In the course to taking some DNG+JPG shots for my trial processing of RAW files -
I was dismayed to find some of my shots appeared to be out of focus -
not just one which would have been upsetting enough but 6 shots with another 5 that did not look as crisp.......

I was being very careful about my shooting - tripod mounted K-x with 50-200 zoom set at 50mm - single center point focus - ISO3200 f/4 @ 1/30sec - 2sec timer which automatically turns SR Off focus is on the center of the target where there is a distinct cross -

This was the overall shot - red outlines of crop areas -


I think normally the first shot seems to be OK (or at least acceptable)
- but I do have a tendency to want to double-check my focus by half pressing the shutter button again (and again) sometimes the K-x would rack the focus motor again - but that's kind of "normal" my K100D did that too -
but I think it is on those occasions that I get MISfocus shots -
they are not grossly out of focus -
so that in practical terms I'd probably "get away" with it -
but they do not obviously give bitingly sharp results -
some actually are pretty bad when viewed at 100% -

A good Focus -


MISfocus -


EXIF data should be attached (PhotoBucket for sometimes seems to lose the metadata unpredictably)

I think when it mis-focuses it is back-focus -

Good Focus and Mis-Focus Shots showing object further back -

this time (obviously the MIS-focus shot is in better focus than the Good focus shot as this is about 4.5" behind the plane of intended focus).

At first I was willing to to put this just down to operator error - but to get 6 pretty badly focused and 5 more marginal shots out of 35 is quite a high percentage of 31%!

But in mitigation I had not noticed this before so it could just be that evening and its circumstances. However I do feel these shots were carefully shot and controlled.

Just so this is not in a vacuum -

dpReview of Pentax K-x Page 13 -

" Focus accuracy is generally not a problem but very occasionally, in low(ish) light and contrast situations, you'll get a slightly out of focus shot despite of a focus confirmation beep. Having said that, in our 1000+ sample shots this was literally only a handful. "

and Page 29 Conclusions -

" and, despite of focus confirmation, we also had a few slightly out of focus shots within our 1000+ sample shots; only a handful though and therefore well within acceptable limits. "

Anyone else notice a tendency to mis-focus?

Another thought had occurred to me - with hindsight - the OP examples in this thread -
K-X : Throughly Unimpressed (Disappointed)
seem maybe to be mis-focused?


Last edited by UnknownVT; 01-08-2010 at 08:50 AM.
01-07-2010, 04:45 PM   #2
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Do your test in daylight with a proper controlled set-up.
C'mon you can't even be sure you're directly level to the subject.
Sorry but trying to test at such extreme ISO with a small table lamp as a light source is always going to be a hit or miss affair.
01-07-2010, 06:43 PM   #3
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QuoteOriginally posted by creampuff Quote
Do your test in daylight with a proper controlled set-up.
C'mon you can't even be sure you're directly level to the subject.
Sorry but trying to test at such extreme ISO with a small table lamp as a light source is always going to be a hit or miss affair.
Sorry - but those are my normal shooting light levels
ISO3200 f/4 @ 1/30 is about average for my low light shooting -
that's why I bought a K-x for its outstanding High ISO performance.

I had done set of ISO performance tests under the same lighting condition and did not seem to have a problem then - please see:

Kx (jpg) ISO Performance

I have owned a K100D for over 3 years and I had not noticed similar problems

If these under tripod mounted conditions (they were not haphazard shots) failed to focus properly (which surprised me) -
then my normal handheld shots would be pretty hit and miss ......

Surely that's not really acceptable

I didn't mean to be "annoying",
but I am expecting better than that here,
I mean my K-x.

Last edited by UnknownVT; 01-07-2010 at 06:58 PM.
01-07-2010, 07:59 PM   #4
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vincent,

try it at 100 or 120mm. i noticed that at wider angles (ok, 50 is not wide but that depends on how far your subject is), but the wider the focal length, the more iffy the AF becomes.

i noticed that on my kx w/ 18-250 and sigma 17-70. this was the same issue with my k10 and *ist DL.

i'm still trying to put a "science" on how the AF algorithm works. if there are 2 high contrast subjects w/in the AF point, does it pick the one in front or behind?

questions like that that no forum seems to have an answer to.

sometimes shifting the AF point (center or otherwise) to force the camera to re-focus. does it make it better or worse? hard to tell. i'm still trying to understand the AF logic here.

however, my tests on a tripod 2sec timer MLU (SR off) seems to be okay so far with my 18-250.

try it at 100mm or 150mm to give the AF point a much clearer point to focus on and see if it's still a hit or miss.

regards,

jordan

01-07-2010, 08:47 PM   #5
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QuoteOriginally posted by opiedog Quote
i noticed that at wider angles (ok, 50 is not wide but that depends on how far your subject is), but the wider the focal length, the more iffy the AF becomes.

i noticed that on my kx w/ 18-250 and sigma 17-70. this was the same issue with my k10 and *ist DL.

try it at 100mm or 150mm to give the AF point a much clearer point to focus on and see if it's still a hit or miss.
Thank you for your kind input - I did not realize that AF could be so "iffy" and commonplace?

If 50mm f/4 causes inconsistent focus problems -
then by logic the 18-55 kit zoom would never stand a chance of being in focus?
Just doesn't seem right - not meant to be argumentative or disagreeable -
just a confused comment :ugh:.

But I did do what you suggested - tried at 73mm and 100mm
and all the shots were just fine -

However I do shoot a lot at 50mm when I have the 50-200 zoom on - as always when I have the 50-200 on there always presents a shot when I wish had the wider angled zoom on instead - typical Murphy's law.

I use central spot focus and the "target" has a very definite and what seems to be a very easy cross to focus on.

So the mis-focus albeit slight at 50mm is a mystery and a little disconcerting.

I am sure I can live with it, since most of the time there's bound to be larger errors due to the operator than the K-x -
however if the mis-focus happens on a really good shot that I took particular care over (which unfortunately I'd like to think are most of my shots) I would be quite upset.
01-08-2010, 12:49 AM   #6
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It's unfortunately normal that under strongly colored light like typical tungsten bulbs, AF is not as accurate as in better balanced light, unless the camera has special circuitry/algorithms to correct for this. The K-7 is the only Pentax camera to offer that. So there will be cases where focus cn be slightly off under tungsten light. Normally not that much of an issue as DOF covers the difference, but on some occasions it might be more noticeable than others.

As for what subject the camera picks as a focus target when there are multiple possibilities, thereis simply no way to be sure. Sometimes is picks the closer, sometimes the nearer - depends, I would assume, on which it finds "easier" (either because it is already closer to being in focus, or because it has better contrast, or because it is not moving, or the light is better, or whatever). Basically, you can't expect to be able to predict that with any reliability, so you need to simply trust your eyes and if you see the camera choose soemthing other than you wanted, try again, or focus manually.
01-08-2010, 05:53 AM   #7
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QuoteOriginally posted by UnknownVT Quote
Thank you for your kind input - I did not realize that AF could be so "iffy" and commonplace?

apparently, so.
even "nikon" says so.

see this link
Answer

AF accuracy is also lens dependent. some lens just focus better than others.
that doesn't mean that your 18-55 is destined to mis-focus just because it maxes out at 55. it also depends on your subject, contrast, etc.

Last edited by opiedog; 01-08-2010 at 07:08 AM.
01-08-2010, 08:05 AM   #8
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QuoteOriginally posted by Marc Sabatella Quote
As for what subject the camera picks as a focus target when there are multiple possibilities, thereis simply no way to be sure. Sometimes is picks the closer, sometimes the nearer - depends, I would assume, on which it finds "easier" (either because it is already closer to being in focus, or because it has better contrast, or because it is not moving, or the light is better, or whatever). Basically, you can't expect to be able to predict that with any reliability, so you need to simply trust your eyes and if you see the camera choose soemthing other than you wanted, try again, or focus manually.
QuoteOriginally posted by opiedog Quote
apparently, so.
even "nikon" says so.

see this link
Answer

AF accuracy is also lens dependent. some lens just focus better than others.
that doesn't mean that your 18-55 is destined to mis-focus just because it maxes out at 55. it also depends on your subject, contrast, etc.
Thank you gentlemen for your considered replies - much appreciated -
and typical of what to expect from this wonderful forum.

Mis-focus under tungsten/artificial light is not commonly known.
(that Nikon page did not mention it - just the common mistakes when using multi-point AF and low contrast targets - neither was my case I was using single central point AF and a very high contrast target)

However I will reiterate - my K-x was tripod mounted (with 2sec time-delay), the target as can be seen has a very high contrast definite (not a repeating pattern) cross in the center of the frame - I was using Spot single point Central focus -
so unless the K-x mysteriously overrides my settings - there should be no question of the focus point selected and what it focused on (please look at the photos with PhotoME to see the AF point/framing). There was no difficulty locking focus - I was on AF-S so the shot would not have even taken if the K-x did not achieve proper focus lock - again the EXIF data should reveal all this.

In other words I had given the K-x its best possible chance to focus properly that I know how - for the given lighting conditions.

Yes, I was under dim artificial light - what could make it worse is that it was not just tungsten, but a warm white CFL (Compact Fluorescent spiral) - however I did not choose my target and conditions haphazardly or by mere whim - as those really are close to my normal shooting light levels -
even the distance to target was deliberately set up for the 50mm focal length so that the details in the test target were just resolved - and some parts not - that's why it was easy for me to see the misfocus because the definition was off.

I also deliberately used CFL because I also wanted to see how well the K-x AWB worked under what I considered the most demanding conditions -
and as can be seen the K-x AWB works very well indeed.

Thank you both for your valuable input.


Last edited by UnknownVT; 01-08-2010 at 08:53 AM.
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