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01-22-2011, 09:19 AM   #1
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Liveview AF back-focus with Tamrons...

Hello!

Having some time to kill, I decided to check the focus accuracy of my various lenses.

The FA50 needed a -6 to be perfect, the 18-55WR was OK, and thus I proceeded to my Tamrons 28-75 and 70-200 (both f/2.8).

Well, strange thing is that they are both spot on with phase AF, but are back-focusing like hell in liveview using the contrast AF!!!
The 70-200 focuses nearly 20cm behind a 4m target!!! The 28-75 is slightly better, with maybe only 5-10 cm of BF for a 4m range.

I don't really understand how it can be possible.

By desperation, I went to the AF calibration menu (not that it have any effect on contrast AF, but I had to check everything), and to my surprise, "Apply to One" was showing "unset" with my 70-200, instead of the usual "0"... WTF?
I successfully changed it to "0", but contrast AF is still back-focusing...

Both the FA50 and 18-55WR are focusing properly in contrast AF...

Any ideas?

01-22-2011, 12:47 PM   #2
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It looks like there is some incompatibility of your Tamron lenses with contrast AF.
There are similar reports for Sigma lenses as well. These lenses were probably designed and manufactured before the availability of contrast AF.

The only solution is to call up Tamron and hope that they have a fix for you.
01-24-2011, 03:11 AM   #3
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QuoteOriginally posted by dlacouture Quote
Well, strange thing is that they are both spot on with phase AF, but are back-focusing like hell in liveview using the contrast AF!!!
In theory that's impossible.

QuoteOriginally posted by dlacouture Quote
effect on contrast AF, but I had to check everything), and to my surprise, "Apply to One" was showing "unset" with my 70-200, instead of the usual "0"... WTF?
That's normal. The camera shows "unset" for lenses that you haven't calibrated yet (so "unset" kinda means "factory 0").

QuoteOriginally posted by dlacouture Quote
I successfully changed it to "0", but contrast AF is still back-focusing...
Are you sure you're focusing on exactly the same object with contrast AF as with phase AF? Make sure the rectangle on the LV screen is on the same object/location as the phase AF sensor and that you're not using the face detection (may think it found a face in the background? ). Of course use a flat object.
01-24-2011, 03:54 AM   #4
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QuoteOriginally posted by dlacouture Quote
The 70-200 focuses nearly 20cm behind a 4m target!!!
I have similar problem with my T70-200. For focal lengths > 135mm contrast AF doesn't work
I have tried several times to focus on these branches, but without luck.
http://my.pentax.org.pl/ldevil/K-5/AF/IMGP6324.jpg
Similar here, I was pointing at this road sign, but it was impossible to get in-focus picture.
http://my.pentax.org.pl/ldevil/K-5/AF/IMGP6320.jpg
In booth cases contrast AF confirmed that it set the focus correctly.

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