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09-09-2008, 12:26 AM   #1
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Back Focus: Myth or real?

Hi folks,
Im a bit confused, not sure I understand what back focus really is, and if I do, I don't think I know how to handle and deal with it.

I have a K20D so I have the ability to set back focus on the camera, correct?

Well, I find that alot of my photos (using my Sigma 18-50 DC EX 2.8) are not focussed where I thought they should be (on apertures between 2.8 and 6.7). So maybe it's just a DOF issue, but maybe not.

What is the best way to test/check for back focus, and if I find any, how does it work to correct it, I don't understand the k20D manual, its vague.

Maybe I am just an idiot and all of this isn't even happening, feel free to point fingers

09-09-2008, 07:22 AM   #2
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There are basically two things that can be going on. Either the camera is not choosing to focus on the object you thought it was choosing to focus on, or the camera is trying to focus on a given object but missing. The former is basically something you have to learn to control yourself, by taking advantage of the different focus points available. But the latter is back focus (if it focuses behind the intended target) or front focus (if it focuses in front). The best way to test for it is to use a well-designed test chart, like the one at

focustestchart.com

That chart, shot at a 45 degree angle according to the directions given, provides a target that AF system *cannot* possibly not choose to focus on, and then you can gauge how clsoe it came to actually hitting that target.
09-09-2008, 07:39 AM   #3
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You can also check Yvon Bourque blog for a focus test chart. He designed one that I think is among the easiest to use. Just don't forget that if you change focus adjustment for your lens, since it is a Sigma lens, the camera might not recognize the specific Sigma lens and could then apply the correction to other Sigma lens (want to know how I know that?).
09-09-2008, 07:51 AM   #4
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QuoteOriginally posted by marlon Quote

What is the best way to test/check for back focus, and if I find any, how does it work to correct it, I don't understand the k20D manual, its vague.
remember - BF/FF depends on specific camera/lens combo and BF/FF depends on daylight/tungsten light... so you if you are going to test
your camera/lens combo - do not forget to test it under both conditions
and unfortunately Pentax does not provide any way to detect/store different adjustments for both conditions at the same time... for example
my Sigma 24-135/2.8-4.5 does not require any adjustments for tungsten, but it does for daylight, but other Sigmas behave opposite way... so in many (some) cases you need to remember to change the adjustment yourself.

09-09-2008, 10:34 AM   #5
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QuoteOriginally posted by deejjjaaaa Quote
... for example
my Sigma 24-135/2.8-4.5 does not require any adjustments for tungsten, but it does for daylight, but other Sigmas behave opposite way....
I haven't run into this problem too often. Of course, that's probably because I haven't needed to shoot much indoors.

OTOH, you'd think the manufacturers could figure out how to link the chosen white-balance to the range of the lens (as automatically detected) to calculate offsets. If necessary, this too could be calibrated if necessary.

Of course, it's probably a lot more complicated than that - anybody want to chime in with some engineering knowledge?
09-09-2008, 11:11 AM   #6
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QuoteOriginally posted by Frank Fletcher Quote

OTOH, you'd think the manufacturers could figure out how to link the chosen white-balance to the range of the lens (as automatically detected) to calculate offsets. If necessary, this too could be calibrated if necessary.

Of course, it's probably a lot more complicated than that - anybody want to chime in with some engineering knowledge?
the best solution (but not the cheapest) will be to put the 2nd sensor (less resolution - bigger cells to catch faint light because most of it will be reflected by mirror towards pentaprism/mirror to go in viewfinder) in place of AF module and use contrast detecting AF (like in PS cameras)...
09-09-2008, 11:18 AM   #7
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QuoteOriginally posted by Frank Fletcher Quote
I haven't run into this problem too often. Of course, that's probably because I haven't needed to shoot much indoors.

OTOH, you'd think the manufacturers could figure out how to link the chosen white-balance to the range of the lens (as automatically detected) to calculate offsets. If necessary, this too could be calibrated if necessary.

Of course, it's probably a lot more complicated than that - anybody want to chime in with some engineering knowledge?
Oh man, I would LOVE to see that. Automatic focus compensation based on lighting conditions -- Pentax, PAY ATTENTION!



My FA 50 focuses poorly (i.e. inaccurately, not necessarily slowly) in artificial light, but is spot-on outdoors. Of course, the camera frequently guesses the wrong white-balance setting in artificial light, but if all I had to do to ensure correct focusing AND nice-looking jpegs was to manually select white-balance, I'd do it all the time!
09-09-2008, 11:25 AM   #8
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Awesome. Thanks marc and co for the chart links.
After reading your comments, I am starting to wonder if this is indeed back focus or user error, mmm.
I did notice though that I don't have any problems outdoors, tested today after I made this post.
I will test indoors tonight.
Thanks!

My engineering concern is rather, why doesn't sigma lenses report their id to the camera, what so hard about that :P

09-09-2008, 11:30 AM   #9
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QuoteOriginally posted by marlon Quote

My engineering concern is rather, why doesn't sigma lenses report their id to the camera, what so hard about that :P
because Sigma did not get a license from Pentax... so Pentax will not supply Sigma with ID#s for their lenses... Sigma has to reuse existing ID#s.
09-09-2008, 11:54 PM   #10
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QuoteOriginally posted by deejjjaaaa Quote
because Sigma did not get a license from Pentax... so Pentax will not supply Sigma with ID#s for their lenses... Sigma has to reuse existing ID#s.
Rat bastards.


Guys thanks for the input, I managed to resolve this issue last night with your advice. the Problem was 80% my fault and 20% front focussing, it seems. It seems to be better now, but will tweak on from here.

I appreciate the assistance.
09-10-2008, 10:23 AM   #11
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I was reading penta-club.ru and the topic there ( Гуляние автофокуса в К20Д - Форумы Пента-клуба ) reminded me that
solution probably exists... not cheap one - but...

B+W #486 filters ... they cut UV __AND__ IR !!! but they cost a lot... like ~$170 (or so) for 72mm... or ~$140 (or so) for 49mm... but then your
tungsten IR spectrum will be almost eliminated...
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