Back Focus: Myth or real?

marlon · 09-09-2008, 12:26 AM

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

Marc Sabatella · 09-09-2008, 07:22 AM

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.

flyer · 09-09-2008, 07:39 AM

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?).

deejjjaaaa · 09-09-2008, 07:51 AM

Originally posted by marlon

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.

Frank Fletcher · 09-09-2008, 10:34 AM

Originally posted by deejjjaaaa

... 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?

deejjjaaaa · 09-09-2008, 11:11 AM

Originally posted by Frank Fletcher

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)...

Quicksand · 09-09-2008, 11:18 AM

Originally posted by Frank Fletcher

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!

marlon · 09-09-2008, 11:25 AM

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

deejjjaaaa · 09-09-2008, 11:30 AM

Originally posted by marlon

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.

marlon · 09-09-2008, 11:54 PM

Originally posted by deejjjaaaa

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.

deejjjaaaa · 09-10-2008, 10:23 AM

I was reading penta-club.ru and the topic there ( Ãóëÿíèå àâòîôîêóñà â Ê20Ä - Ôîðóìû Ïåíòà-êëóáà ) 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...

09-09-2008, 12:26 AM	#1
marlon Inactive Account Join Date: Aug 2008 Location: Cape Town, South Africa Posts: 232	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

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09-09-2008, 07:22 AM	#2
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	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
flyer Veteran Member Join Date: Jan 2008 Location: Ste-Anne des Plaines, Qc., Canada Posts: 2,013	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
deejjjaaaa Veteran Member Join Date: May 2007 Location: steel city / rust belt Posts: 2,043	Originally posted by marlon 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
Frank Fletcher Senior Member Join Date: Oct 2007 Photos: Albums Posts: 257	Originally posted by deejjjaaaa ... 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
deejjjaaaa Veteran Member Join Date: May 2007 Location: steel city / rust belt Posts: 2,043	Originally posted by Frank Fletcher 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
Quicksand Pentaxian Join Date: Apr 2008 Location: Central Ohio (formerly SF Bay Area) Posts: 1,519	Originally posted by Frank Fletcher 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
marlon Inactive Account Join Date: Aug 2008 Location: Cape Town, South Africa Posts: 232 Original Poster	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
deejjjaaaa Veteran Member Join Date: May 2007 Location: steel city / rust belt Posts: 2,043	Originally posted by marlon 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
marlon Inactive Account Join Date: Aug 2008 Location: Cape Town, South Africa Posts: 232 Original Poster	Originally posted by deejjjaaaa 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
deejjjaaaa Veteran Member Join Date: May 2007 Location: steel city / rust belt Posts: 2,043	I was reading penta-club.ru and the topic there ( Ãóëÿíèå àâòîôîêóñà â Ê20Ä - Ôîðóìû Ïåíòà-êëóáà ) 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...