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06-24-2008, 01:18 PM   #1
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Tamron 300mm f/2.8 + Pentax AF 1.7x teleconverter update. . .Gawd!

What's the deal Pentax?

I haven't been super pleased with the Tamron 300mm f/2.8 + Pentax AF 1.7x combination. It just wasn't sharp -- but then I wasn't sure what to expect anyway with this setup.

My K10D has that damned back focus crap -- but I calibrated it with my FA 50mm f/1.4 to +60um in the debug menu -- which made the FA 50mm uber sharp and fixed my wide angle back focus problems with my 18-250mm.

I was pleased. Now, I started playing with the focus adjustment for the Tamron 300mm f/2.8 + Pentax AF 1.7x combination. It likes -140um! Gawd.

I don't get this crap -- either the image is at maximum contrast or it's not, or the camera doesn't have enough contrast to focus. . . but apparently it's not that simple to understand.

Anyway, check out the below picture with the Tamron 300mm f/2.8 + Pentax AF 1.7x combination when it's properly adjusted -- head is full pixel.

That's sharp, too bad I need to get in the debug menu every time I want to use this lens.

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06-24-2008, 01:36 PM   #2
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well not only the camera body will have backfocus issue.. lens has too. So it can be your 300mm has backfocus also, no?
06-24-2008, 01:37 PM   #3
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QuoteOriginally posted by vizjerei Quote
well not only the camera body will have backfocus issue.. lens has too. So it can be your 300mm has backfocus also, no?
Yea. I'm gonna think about that too. And the Tamron adjustment is -140um, a front focus situation.

Last edited by konraDarnok; 06-24-2008 at 01:42 PM.
06-25-2008, 06:35 PM   #4
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It's not the tamron. It likes the +60um setting too. It's the teleconverter.

Any insights on how to fix this?

10-25-2008, 12:18 PM   #5
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Fixed!

Here is the link to the incorrect optical formula that I used when reassembling the teleconverter.

http://kmp.bdimitrov.de/teleconverters/_optics/1.7.jpg

Here's an attached picture showing the correct orientation.




If you install the elements as show by Bojidar Dimitrov, the Teleconverter will front focus badly.
10-25-2008, 12:51 PM   #6
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Wow, I'm impressed that the photos looked at all reasonable with some parts backwards! Mind you, I guess you can take good photos with whole lenses backwards, so who knows...
10-26-2008, 09:08 AM   #7
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QuoteOriginally posted by konraDarnok Quote
I don't get this crap -- either the image is at maximum contrast or it's not, or the camera doesn't have enough contrast to focus. . . but apparently it's not that simple to understand.
DSLRs don't use contrast detection to focus. That's why they focus much better than compacts, and why focusing during live view sucks - the focusing sensors aren't being used.

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