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04-03-2012, 05:58 AM   #1
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Problem with phase detection AF

What can cause that one particular copy of lens in low light have no AF, just go forth and back in narrow range, not all range from minimum to infinity just for example 0,5 m to 3m, and no focus at all, but in contrast detect live view mode I got pretty fast focus.
I tried in store other copy and works great even in near dark conditions.
Is there any electronic in lens that can be replaced, it is screw driven model.

04-03-2012, 06:08 AM   #2
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Regularly repeated patterns may confuse PDAF, although CDAF may still work on them.
04-03-2012, 07:00 AM   #3
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No this is just in low light, no mater on what is camera pointed
04-03-2012, 07:09 AM   #4
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The "other copy" you tried, was that another copy of the same lens?

04-03-2012, 07:42 AM   #5
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yes, FA35
04-03-2012, 07:52 AM   #6
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Sounds like the notorious "sample variation," maybe in the optics,
but maybe in the lens chip as you originally suggested.

I have no idea whether the chip can tell the difference
between the camera using PDAF and CDAF.

Perhaps someone who does know can chime in.
04-03-2012, 08:08 AM   #7
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in normal, daylight, lens is perfect, razor sharp but in lowlight simple no focus at all
04-03-2012, 02:24 PM   #8
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QuoteOriginally posted by lytrytyr Quote
Perhaps someone who does know can chime in.
The "chip" does not drive the camera. It just provides information regarding focal length and lens series. If the lens "hunts" it is because the image on the AF sensor has inadequate contrast to lock focus. When the light is dim the range of light (global contrast) is far less than if the same scene were brightly lit. AF failure in dim light is the norm across brands.

Here is a link that may interest you:

https://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html

As for one copy of the FA 35/2 behaving differently than the other...You have access to TWO copies?! The lens has become fairly scarce in recent years. If you have two, keep that you favor and sell the other.


Steve


(...never had an AF issue with his FA 35/2...but never really attempted shooting in available darkness either.)

04-03-2012, 02:32 PM   #9
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hmm I don't think this is how You explained because lens doesn't hunt form minimum to infinity like for example my dfa50 doing in dim light, lens just block in range something like 1-3 meters focus and go 1-3 3-1 1-3 3-1 and never confirm focus.
And second FA35 was not mine just borrowed for test.
04-03-2012, 07:16 PM   #10
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QuoteOriginally posted by stevebrot Quote
The "chip" does not drive the camera. It just provides information regarding focal length and lens series.
It seems that it's not that simple,
because people have sent their _lenses_ in for adjustment and recalibration
(e.g. to Sigma) in order to correct for FF/BF problems on certain cameras..

QuoteOriginally posted by stevebrot Quote
Thank you for the link.
However, it does not appear to address the issue about which I was uncertain,
namely the nature of the circuitry in the lens chip.

QuoteOriginally posted by stevebrot Quote
As for one copy of the FA 35/2 behaving differently than the other...You have access to TWO copies?! The lens has become fairly scarce in recent years. If you have two, keep that you favor and sell the other.
From the original post, it seems that a dealer (in Bosnia-Hercegovina?) had the second copy,
and made it available for comparison.
04-03-2012, 09:41 PM   #11
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Yes second copy was at dealer shop.
hmm it seams no one had similar problem with lens and is pretty weird.
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