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10-20-2008, 09:21 AM   #1
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Best practises for focus adjustments.

Got the chart. Just was "getting it" that the reason the pictures are soft is that they are out of focus. Yep my 50mm front focuses.

How do I do the correction best? I have a few lenses so I'll test them all soon. If they are all off by the same amount, and it is adjustable, then all is good? or does that still make it a warranty job for the camera? A k20d.

If they all turn out different but are adjustable then adjust each one and all is good? When does it become a warranty issue with the lenses?

Thanks for the advice...

10-20-2008, 09:33 AM   #2
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QuoteOriginally posted by metroeloise Quote
Got the chart. Just was "getting it" that the reason the pictures are soft is that they are out of focus. Yep my 50mm front focuses.

How do I do the correction best? I have a few lenses so I'll test them all soon. If they are all off by the same amount, and it is adjustable, then all is good? or does that still make it a warranty job for the camera? A k20d.

If they all turn out different but are adjustable then adjust each one and all is good? When does it become a warranty issue with the lenses?

Thanks for the advice...
if you K20D is under warranty you can just send it along w/ Pentax lenses to Pentax to get it (FF/BF) fixed ... no need for lenses to be under warranty - it is SAFOX' fault when you can focus the the same lens manually w/o any BF/FF and AF subsystem can't - lenses are not at fault if SAFOX (or the way AF module was placed in the body if there is
some physical misalignment) can't match your eyes and your fingers...
10-20-2008, 06:58 PM   #3
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Another Dah Moment

QuoteOriginally posted by deejjjaaaa Quote
if you K20D is under warranty you can just send it along w/ Pentax lenses to Pentax to get it (FF/BF) fixed ... no need for lenses to be under warranty - it is SAFOX' fault when you can focus the the same lens manually w/o any BF/FF and AF subsystem can't - lenses are not at fault if SAFOX (or the way AF module was placed in the body if there is
some physical misalignment) can't match your eyes and your fingers...
Maybe this can be a topic.. recount your "Dah" moments in photography!



Got the logic of how to tell which one is at fault. Thank you deejjjaaaa. Busy just now but will get this done at work tonight providing. "The River of denial runs deep in this one..." All my internalized evidence points toward the camera body...

At least my visiting sister will bring her k100d! Yep She' Cool.

It'll be back to the s5is until camera returns...

Setting up the focus test with a tripod and using a remote clicker (Which doesn't impress in terms of reach and range at all; in fact it is kind of finicky) and so get as accurate a read on the situation as I can. Also bring glasses...
10-20-2008, 09:20 PM   #4
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I never got the logic of "the lens frontback focuses", the body tells the cam what to do, at best i could see a lens being sloppy and not moving accurately enough to where the body says "nope, too far back, come to the front, duh, not that much, go back" and we then say the lens hunts. But it's the body that says "ahh, there, stop"
Having said that, being able to adjust the lenses on the body i do that, since the cam can be adjusted by me.

10-20-2008, 09:22 PM   #5
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For what it's worth Pentax says to test at 5 feet or more when adjusting and to do it at the middle focal length on zooms.

Maybe that is actually useful info.
10-21-2008, 09:50 AM   #6
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QuoteOriginally posted by morfic Quote
I never got the logic of "the lens frontback focuses", the body tells the cam what to do, at best i could see a lens being sloppy and not moving accurately enough to where the body says "nope, too far back, come to the front, duh, not that much, go back" and we then say the lens hunts. But it's the body that says "ahh, there, stop"
Having said that, being able to adjust the lenses on the body i do that, since the cam can be adjusted by me.
Yeah! That.

My dah moment was with asertaining which was the culprit: If the lens can be focused manually but the Af doesn't do it... then it is the AF. If neither can than check another lens. Either both or it is the lens.

QuoteOriginally posted by morfic Quote
For what it's worth Pentax says to test at 5 feet or more when adjusting and to do it at the middle focal length on zooms.

Maybe that is actually useful info.
That sounds useful to me. Thanks!

Work was a bit out of this world last night. Have too much to do right now to go through all my lenses. But I'll try. Have a show tonight. My little town also reveals in vaudeville! Antics, spectacles, flashy costumes...Sounds like a great photo op.

Nap now; trouble shoot later... Rule: Never trouble shoot when you're tired.
10-23-2008, 10:24 AM   #7
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Working at Adjusting the focus

This is a big weekend for the Pentax and my relationship. So tested two lenses, the FA50mm and Sigma 17-35mm. Both were front focusing quite a bit. I can manual focus better than the AF system. Have a few more lenses and a new one arriving shortly. Will be calibrating each lens and then deciding on how to go about it. Set the camera back to the minimum then make adjustments to the individual lenses.

The printed focus sheet is effective. I also used the brick wall next door (it has a metal disk on a bracket that makes for the perfect target) and then there is this:



As you can see it still front focuses a tad.
10-24-2008, 08:10 PM   #8
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QuoteOriginally posted by morfic Quote
I never got the logic of "the lens frontback focuses", the body tells the cam what to do, at best i could see a lens being sloppy and not moving accurately enough to where the body says "nope, too far back, come to the front, duh, not that much, go back" and we then say the lens hunts. But it's the body that says "ahh, there, stop"
Having said that, being able to adjust the lenses on the body i do that, since the cam can be adjusted by me.

Its simply because the AF sensors are a separate unit below the mirror. The mirror is semi transparent and lets the image through to another long thin mirror that bounces the image down to the AF module, this module is what says go back or go here to the lens or camera cpu then lens. However if its not aligned with the CCD/CMOS it will not be accurate and can BF/FF. Remember its two separate units seeing the image so the AF sensor may say its in focus, but the CCD/CMOS may not really be in focus. Thats why contrast detection used in compacts may be slow, but you don't get too many OOF shots, becuase the camera uses the image from the CCD/CMOS to focus and can focus anywhere on the image. But most importantly does work as you say above.

10-24-2008, 09:15 PM   #9
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QuoteOriginally posted by jamesm007 Quote
Its simply because the AF sensors are a separate unit below the mirror.
What you describe is all well and true but in line with morfic would call a body to have FF/BF issues.

How can a lens have FF/BF issues?

I was wondering about this myself. The phenomenon must exist since many report about it and the K20D offers AF correction for individual lenses.

I can only imagine that it has to do with slight colourations by the lens, similar to the FF/BF issues under tungsten light. Since the AF system, to the best of my knowledge, uses prisms to make a phase comparison, slight changes in the chromatic composition of the light hitting the AF system may cause it to determine slightly incorrect focus determination.
10-24-2008, 10:46 PM   #10
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QuoteOriginally posted by Class A Quote
What you describe is all well and true but in line with morfic would call a body to have FF/BF issues.

How can a lens have FF/BF issues?

I was wondering about this myself. The phenomenon must exist since many report about it and the K20D offers AF correction for individual lenses.

I can only imagine that it has to do with slight colourations by the lens, similar to the FF/BF issues under tungsten light. Since the AF system, to the best of my knowledge, uses prisms to make a phase comparison, slight changes in the chromatic composition of the light hitting the AF system may cause it to determine slightly incorrect focus determination.
Thanks, you just saved me from a upset "Uh, you didn't get what i said at all"

As far as AF adjustments for lenses, i'd love to go a step further, find the adjustment for each type of light, and be able to store a few per lens, now i am lazy and use AWB and then get it right in the raw processor, if the WB would be linked to the varying AF adjustments, i would make sure to switch WB even when shooting raw to have the proper lens calibration for that type light.

This is based on a feeling that the light affects how well or not, the camera focuses. And i mean light type, not intensity, so no "Duh, in dim light it won't focus well, everyone knows that replies" (I know someone was already thinking to write that ;P ) I am not interested in doing a clinical study on this, but my feeling tells me i am right.
10-24-2008, 10:53 PM   #11
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QuoteOriginally posted by metroeloise Quote
This is a big weekend for the Pentax and my relationship. So tested two lenses, the FA50mm and Sigma 17-35mm. Both were front focusing quite a bit. I can manual focus better than the AF system. Have a few more lenses and a new one arriving shortly. Will be calibrating each lens and then deciding on how to go about it. Set the camera back to the minimum then make adjustments to the individual lenses.

The printed focus sheet is effective. I also used the brick wall next door (it has a metal disk on a bracket that makes for the perfect target) and then there is this:



As you can see it still front focuses a tad.
What did you focus on that this shot shows front focus?
10-25-2008, 09:12 AM   #12
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Sorry for not understanding the first time. You guys are thinking deep, and your theory not easily disproved. I guess one way to fine out is - is there any dSLRs that use contrast detection for AF, and or can, and does it have the same problem of BF/FF with different lens?

Last edited by jamesm007; 10-25-2008 at 09:18 AM.
10-30-2008, 04:12 AM   #13
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QuoteOriginally posted by morfic Quote
What did you focus on that this shot shows front focus?
Well it was mostly meant as a joke...

The movement involved, the speed, I was going for the hip but off that much is no problem. She's swinging, twisting and this was before the "Duh!" of using AFC rather than AFS...

I am still having AF problems. Started a different thread... There's a more clear picture of the problem.

I am recalibrating the body and going at it again keeping the images once they are right on and then seeing what happens.

Thanks for the explanation of the system's structure and the expansion to consideration of different light and the effect on AF. One fall out of having such a great camera is that the tolerances are tight. to be able to clearly see the difference in focus caused by different lighting situations is tight.

As for the whole issue of more fine grain selection in the firmware program I am all for that! All the way to reassigning buttons. I only shot in raw so that button, A nice feature, would be good for switching between different user profiles, or as another green button since that one can be a chore to get to sometimes... And more user profiles...

Tired. End of my workweek. will be at this in the morning.

thanks all.
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