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09-08-2013, 11:48 AM   #1
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focus correction

afterall i managed to test the AF of my K-X.. noticed a slight front focus .. only noticeable with the FA 50mm 1.4 and 2.8 macro...

after reading into the subject i'm a little bit confused about the possiblility of having a frontfocus with lens A and backfocus with lens B ... the focus problems on the 2 lenses meantioned above look like the same ammount of frontfocus ... as i understand its a problem with the precision of the AF unit below the sensor.
so it should be the same ammount of front or backfocus with every lens used ??

please help my mind ! ^^

09-08-2013, 12:02 PM   #2
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No, it can be different for every lens. Unfortunately, the k-x doesn't support per-lens adjustments, but all future cameras do.

More info here: Fixing Front and Back Focus - Introduction - PentaxForums.com

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09-08-2013, 12:19 PM   #3
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the question is ... why?

if the precision of the AF system is the fault ... it should front or backfocus more or less the same amount an every lens ... what is the physical cause for having backfocus on lens A and frontfocus on lens B ... tell me
09-08-2013, 01:14 PM   #4
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QuoteOriginally posted by paranoia23 Quote
the question is ... why?
I think there are quite a few threads that explain this. But here goes!
You see, the AF system is not wrong. But the AF system and the lens are made to certain tolerances. And sometimes those tolerances are on the other extremes and this creates a problem. And in those cases, its great that we have the option of AF adjust. Some brands have very tight tolerances and their cameras even get adjusted to the lenses you buy with them in the factory. But these brands (Leica) cost 10 times as much for every piece of gear. Oh, and even for other brands, professionals often send their camera and lenses in to a service centre for adjustment.

The K-x and K-r have global adjustments (not per lens, unfortunately), and the newer cameras will remember the adjustments of up to a certain number of lenses. Oh, and one more thing, live view uses a different type of AF, which doesn't need AF adjustment. (this is why the K-01 doesn't even have an option for lens adjustment) So you can try using live view and focus peaking. But CD AF is a little slower, especially with lenses that have a high minimum f number.

Its a little annoying that you would have to change the AF adjust every time you switch between those lenses, so you can try sending them to a service shop for them to calibrate the lenses to the camera. But this might cost you a little, and it might not even be possible with some lenses. Beyond that, your other option is to learn how the AF behaves and adjust accordingly, or use MF with those lenses. For macro purposes you should probably use MF anyway.

09-09-2013, 12:27 AM   #5
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i understand the fact that the phase AF missjudge the "real" focus plane ... because the plane of correct focus is different for the AF and the sensor ... yes thats why you should get with liveview the correct focus.

where does the lens come into play ?! ... i dont get it... the same question is why should the AF sensor should have his plane shiftet with different lenses ... also this doesn't make sense to me..

excause me but you still tell me that it is like it is and not why... where is the physical "fact" that shifts the focus plane for the AF sensor with the exchange of the lens ?
09-09-2013, 04:38 AM   #6
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QuoteOriginally posted by paranoia23 Quote
i understand the fact that the phase AF missjudge the "real" focus plane ... because the plane of correct focus is different for the AF and the sensor ... yes thats why you should get with liveview the correct focus.

where does the lens come into play ?! ... i dont get it... the same question is why should the AF sensor should have his plane shiftet with different lenses ... also this doesn't make sense to me..

excause me but you still tell me that it is like it is and not why... where is the physical "fact" that shifts the focus plane for the AF sensor with the exchange of the lens ?
It is the lens that is slightly out of whack. The PDAF is a static measuring device. AF calibration is software correcting for the lens tolerance, either globally (K-x) or per lens (following models).

It is the lens that has the moving glass element to focus. That is what is slightly mid-aligned.
09-09-2013, 11:30 AM   #7
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if it would be like u say it is ... phase and contrast AF wouldn't differ ...

Last edited by paranoia23; 09-09-2013 at 11:43 AM.
09-09-2013, 02:07 PM   #8
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QuoteOriginally posted by paranoia23 Quote
i understand the fact that the phase AF missjudge the "real" focus plane ... because the plane of correct focus is different for the AF and the sensor ... yes thats why you should get with liveview the correct focus.

QuoteOriginally posted by paranoia23 Quote
if it would be like u say it is ... phase and contrast AF wouldn't differ ...

PDAF and CDAF are analyzing the same image that passed through unadjusted lense so they are not differ (PDAF and CDAF correction), if i understand your question.

09-09-2013, 02:20 PM   #9
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QuoteOriginally posted by Aristophanes Quote
It is the lens that is slightly out of whack
Not necessarily. It could be either, or both. I had for some time 2 K20Ds, All of my lenses needed different amounts of correction on the two bodies; and the difference was consistent. i.e there was a difference of a factor of 3 between the two bodies. If a particular lens needed a -3 on body A it needed 0 on body B. A lens that needed -1 on body A needed +2 on body B.
09-10-2013, 01:44 AM   #10
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atleast somebody could handle empiric information
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