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06-28-2009, 05:00 AM   #1
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k200d yellow light AF

Several times i've seen this issue being mentioned in related topics, but i have never seen or been able to find actual topic about it.

What are the figures one normally should expect? When it is just because of the light and when you have a real defect?

I am asking this because yesterday, i did some shooting indoors. The light was incandescent but not too low. Most of the shots had focus on something in foreground. Lens used DA70/2.4, Sigma 30/1.4.

Some samples here, including some from other times: http://foto.inbox.lv/tamrac/yel_ligh?language=en All with AF confirmed (Green circle lit in "status bar").

I've set up manual AF point selection and mostly use center, or set it according to subject if required.

So, should it be like that and i must use pentax only as a daytime-outdoor camera or i should prepare for another two month warranty mess?

With all those issues (already had some with k100d) im start thinking of dropping Pentax. At first i thought it was my bad skills or just some random unfortune, but the whole function seems seriously flawed.
And Pentax isnt releasing any firmware updates even after so many complaints. Do they care? From what i've red AF updates in k7 also seems to be only little tweaking to trick buyers that something has been actually done.
They should put in a god damn split prism if they are unable or not willing to make the AF right.


Last edited by ytterbium; 06-28-2009 at 06:29 AM.
06-28-2009, 09:11 AM   #2
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QuoteOriginally posted by ytterbium Quote
Several times i've seen this issue being mentioned in related topics, but i have never seen or been able to find actual topic about it.

What are the figures one normally should expect?
Differences of fractions of an inch.

QuoteQuote:
Some samples here
Would be more helpful if you could say what you think the camera focused on. The ones I looked at all appeared to show the camera clearly and suiccessfully focusing on something that seemed completely logical. Eg, the box of truffles in #3, or the bowl in #4. If you expected the camera to read your mind and focus on the people in the background, that was not a reasonable expectation with *any* camera that lacks mind reading technology. For others, like the cat in #8, it appears to be a simple case of the DOF being incredibly shallow at that distance at f/1.8; all it took was you or the cat moving a millimeter or two between when focus lock was achieved and when the shutter actually tripped.

QuoteQuote:
With all those issues (already had some with k100d) im start thinking of dropping Pentax.
The physics that cause AF system to respond differently according to color of light affects the whole world, not just Pentax. Other systems don't have magic prisms that bend light differently than the ones used by Pentax. Nor are small discrepancies in focus from other causes unique to Pentax.

QuoteQuote:
At first i thought it was my bad skills or just some random unfortune, but the whole function seems seriously flawed.
Again, it would help to point out a specific photo in which you think a problem exists. The ones I looked at from your sample set do not look problematic at all - the camera appeared to simply have chosen a different target than you intended.
06-28-2009, 09:51 AM   #3
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How can camera pick a target out of frame or even if the DOF can be shallow, but not a feet or more in front.

The same sample with bowl:
Inbox Foto (foto.inbox.lv)
Why it seems for you so reasonable and logical?
Center focus point. Aimed at pretty much isolated subject (wall several feet behind, table - almost half frame lower), with good contrast (dark hair on white clothes)

Would you consider it to be logical, if camera focussed on the near end of the guitar in your shot:
Flickr Photo Download: MJS_090603_7798p
If you had selected "top"(relative to the frame) focusing point, or used center on the mike?

And with the truffles - the box IS NOT in focus. But the few inches there are acceptable instead of a feet or so in here:
http://foto.inbox.lv/tamrac/yel_ligh/IMGP9380.jpg
Where there even isnt anything else in front. The necklace has good contrast, and the dof isn't that shallow for cat to be completely out of focus even if it moved 5..10 centimeters.


I must apologize for a rude response, but it seems that you are being unrealistic and present very biased opinion.
Or do you also get so inaccurate results and your replies acknowledge that my experiences are not anything unusual?
Btw, i've observed that you mostly use MF and split prism what eliminates most of those problems. But this does not change the fact that AF should be more or less reliable for those who cannot use other options, or else why include it at all and not sell cameras and lenses for 100..300$ less.

Last holidays i was using film chinon CE-4 with SMC Pentax M 50/1.7 and it was so pleasant to shoot with the confidence that every frame you expose will have correct focus, even at F1.7, right where i had the prism aligned or only as much off as i myself had mistaken or hurried. And even in night i was able to do it much faster than my DSLR could hunt in sub par lighting.
Taking my k200 after this, i get this insecure feeling and need to always double check focus, take multiple shots an re-trigger af after lock wich makes shooting a very unpleasant experience. And this is even more depressing when the subject gets nervous waiting while you fiddle with the camera.
I love so many aspects of pentax system, but such situations are so frustrating.

Is that too much to ask from a $600 camera, $400 and $600 lenses?

Last edited by ytterbium; 06-28-2009 at 12:43 PM.
06-28-2009, 10:40 AM   #4
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The focus problem is to do with light that has a high, or higher component of infrared light. Incandescent lights are know for this. IR light, and light closer to the red end of the spectrum does not have the same focus point as does light at the cooler end of the spectrum. Its just physics. Maybe someone with a better understanding can add to this point.

The result is that the camera will miss focus due to the redder light in the scene. I have tried with my FA 40/1.4 and found that the focus will be out by about 3/4" to 1" over a 6' distance to subject.

I understand this affects almost all cameras with autofocus. Canikon's included.

From early reports, I saw that the K-7 is able to detect the colour of light, and adjust the autofocus to correct the "red shift" in focus. I'm not sure but this might be the first pro-sumer camera to do this.

06-28-2009, 10:59 AM   #5
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I went back and had a better look at the phots you posted.

Photo one, the two people at the table, do I understand that you were focusing on the wall, near the womans dark hair? Using a wall as a focus point will often cause a camera to hunt for a focus point. Generally walls have too little contrast. You should have used the contrast between the womans face and hair. Also, do you lock your focus and recompose? To me this photo looks like you took the photo while the camera was hunting for focus on the wall. I don't think this is a focus issue due to type of light.

Photo two, the guitar player, I don't see what the focus issue is in this photo. To me the focus is where you want it to be, on the woman's face. The portion of the guitar in the forground is out of focus because of the shallow depth of field you have used. I do not see this as fault of the camera. Nice photo by the way.

Photo three, cat, is just over all compleatly out of focus. Did you wait for the camera to lock focus? Or was it hunting or what happened here? Was the cat too close for the minimum focus distance for the lens used?

Overall, I see that you are frustrated with the results you are getting. But I do not think it is a camera issue. I think you need to learn how to use the autofocus with your camera. Unless your camera is not working correctly, i think this is a case where the carpenter is blaming his tools.
06-28-2009, 11:40 AM   #6
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It seems that noone actully has red my posts but just looked at the pictures and written whatever they had in their minds.

The woman with guitar was taken by Marc Sabatella.

I did not focus on wall.

The focus was locked in ALL pictures. (confirmed by STILL green hexagon)

The AF point was in centre unless written otherwise in photo.

How do you get correct focus, when 5 to 10 shots in a row are just plain wrong?
06-28-2009, 12:28 PM   #7
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QuoteOriginally posted by ytterbium Quote
It seems that noone actully has red my posts but just looked at the pictures and written whatever they had in their minds.

The woman with guitar was taken by Marc Sabatella.

Your post does not say the photo was by Marc. I did not notice the photo owner when I went to the flicker site. Marc! Nice photo!

I did not focus on wall.

This sentance to me, means you focused on the wall: "Center focus point. Aimed at pretty much isolated subject (wall several feet behind, table - almost half frame lower), with good contrast (dark hair on white clothes)". How about telling me wht the subject was?

The focus was locked in ALL pictures. (confirmed by STILL green hexagon)

Great. All I was asking with regard to "fuzzy cat" was "are you sure?"

The AF point was in centre unless written otherwise in photo.

How do you get correct focus, when 5 to 10 shots in a row are just plain wrong?
You get correct focus through two changes to what you are doing:

1. Learn to use your gear. If this is not a problem, and you appear to know everything...then go to point two.

2. Get your camera checked out. It is obviously a fault of your camera.

If the camera checks out, then go back and think about point one.
06-28-2009, 12:38 PM   #8
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The problem is that check out for me may take up to 2 months, so i was hoping others could assist me on a deeper pre-check, assuming they have working camera they know well.
And i am most likely to sell the camera now than send it to Pentax another time, because Canon has service locally unless i solve this issue in other ways (like improving my skills).
But there are too few users giving any feedback so it is hard to tell.

QuoteQuote:
Your post does not say the photo was by Marc. I did not notice the photo owner when I went to the flicker site. Marc! Nice photo!
Indeed. Changed "this" to "your".

Lol... misunderstandings.. i have to learn to make clear, short sentences. My idea was:
QuoteQuote:
This sentance to me, means you focused on the wall: "Center focus point. Aimed at pretty much isolated subject (wall several feet behind, table - almost half frame lower), with good contrast (dark hair on white clothes)".
Then i must truly ask your forgiveness. Just such technical issues and inability to nail the shot makes me a bit irritated and thus emotional and rude while describing them.

So with two replies, the conclusion is that everything is most likely fine with camera. Results visible in my samples are to be expected with k200d in conditions shown in EXIF while using said lenses in majority of shots.
I May just add, that the focus is spot on when you move closer, even with subject motion, low contrast and confusing foreground objects:
http://foto.inbox.lv/tamrac/sig_samples/IMGP9370a.jpg


Last edited by ytterbium; 06-28-2009 at 12:59 PM.
06-28-2009, 03:34 PM   #9
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Do a series of tests to isolate/understand the the problem better.

Set up a standard subject at a certain distance away with a variety of items at various distances. Choose one item as your focus target.

Put the camera on a tripod and take photos of the subject under various lighting scenarios, including different colors of light if you can do so (white, yellow, red, green, blue, incandescent etc). See if there is indeed a particular issue with autofocus under yellow light or any other light.

Also under the same test try different settings for your autofocus - Auto, multisegment or spot - to see what behaviour they exhibit under different light conditions.

You may be able to narrow down more precisely what is going on, and under what conditions it happens, and then learn to work around the issue better, or be more precise in the way you describe the defect to Pentax.

You could also use your flash to help the autofocus sensors, by sending out several strobing pulses. That might solve the problem of yellow light.

I also find the 'focus is OK' indicator in the viewfinder not very useful. Many many times it has indicated focus is OK but it has not been accurate. I do not rely on it anymore.

And to end: what's wrong with just manually focusing?
06-28-2009, 04:14 PM   #10
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what rawr said (do a controlled test, set the camera on a tripod, take notes, etc), but one more thing: this is the funkyest exifs i have seen, some "nice ones":

Scale Factor To 35mm Equivalent 0.1
Hyperfocal Distance 249.62 m
White Balance Mode Auto (Daylight?) #note: it would be best to set it to tungsten, should not have anything to do with af though
Focal Length 300.0mm (35mm equivalent: 45.0mm)
Subject Distance Range Macro #seems to point out the af system was indeed confused

i cannot begin to think what might cause this. is this indeed the original exif? can you check and confirm? (maybe the image hosting site has screwed it up somehow). were you using a manual lens, using af confirmation to focus (and SR on with the focal length set to 300mm, though the lens is probably a fast 50 or something?). more details would be wellcome.

edit: i disagree about using various focus strategies, use the center focus point for simplicity, do _not_ recompose (btw, multisegment and spot are terms used for metering usually, but that's just terminology). do not bother with various colors of light, just use tungsten (or whatever light caused you these issues), do not change the light source during the tests. maybe begin by using contrasty (easy to focus on) subjects, to see if there is some consistent error caused by wavelength.

Last edited by nanok; 06-28-2009 at 04:20 PM.
06-29-2009, 07:17 AM   #11
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Yes, the site uses to mess up exif, it shows too much. Actual one and some parsed by it wich is wrong. If you're interested you better use some Firefox add on directly on the pictures.
I'll do the testing tonight.. if i find a space, tungsten ligh and assamble my tripod.

Manual focus is too hard for my eyes and that damn screen. I cannot even see in viewfinder if the focus was off.

But the flash assist thing has rised another question. Does any cheap(-est) external flash with P-TTL has dedicated AF assist light. Does it work when flash itself is selected to OFF?
06-29-2009, 08:03 AM   #12
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i seem to recall there was talk somewhere about a special mode pttl flashes have when they do not fire, but only fire the ir assist beam. some more knowledgeable people might be able to help.

about cheap alternatives.. i'd like to know too
06-29-2009, 08:23 AM   #13
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QuoteOriginally posted by ytterbium Quote
So, should it be like that and i must use pentax only as a daytime-outdoor camera or i should prepare for another two month warranty mess?
w/ my K10D and K20D I did test all my lenses under a normal daylight and under plain tungsten light to find out necessary AF adjustments for both cases... in my case the worst lens in that respect was FA50/1.4...


QuoteOriginally posted by ytterbium Quote
From what i've red AF updates in k7.
I hope somebody with K7 (as we have people w/ production units in hands already) will make and publish a test w/ FA50/1.4 and let us know if Pentax was really able to fix the issue... FA50/1.4 because it seems that it is THE lens making SAFOX most affected by different spectrums when it is mounted... litmus test for SAFOX VIII+ so to say.
06-29-2009, 08:32 AM   #14
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Hmm. How big was the error with 50/1.4?
06-29-2009, 09:29 AM   #15
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QuoteOriginally posted by ytterbium Quote
Hmm. How big was the error with 50/1.4?
big enough to have very clear OOF under a tungsten light if AF was adjusted for a daylight and very clear OOF under a daylight if AF was adjusted for a a tungsten light.
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