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01-06-2010, 03:00 PM   #1
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Pentax K-7 + Tamron 18-250 Problem

Greetings,

I recently bought a Pentax K-7, a Tamron 18-250 and grip. My idea was to replace my old K10D, used with the default Pentax 18-55 and a Sigma 28-200 (that came from my old MZ-7 days). As soon as the K-7 arrived, I sold the k10d.

To be honest, pics are ok, and I think that overall IQ is nice for the price range and for the fact that this glass covers from 18 to 250mm.

But Im having a strange and annoying problem: autofocus. If I let it autofocus, no matter if Im in low light or sunlight, stopped down or not, 18mm images are always blurry and out of focus. It gets gradually better up to 65 to 70mm, where the problem seems to disappear.

Not that the glass isnt sharp if I manually focus it, the problem is AF: it always "thinks" that the subject is closer than it actually is, while in th 18mm to 65 / 70mm range. Tried spot focusing, stopping down etc. Tried a tripod. Same results. Turning SR off improved results just a bit, tho.

I also tried fine tuning AF from the K-7 menus: tried up to +10 and -10. No deal.
Fact is, if I autofocus, it is clear, just by using the distance guide printed in the lens body, that something is wrong.

I was wondering if anyone came across the same behavior. Im about to return the tamron and get a Sigma 18-250. I dont have a Pentax body around to make more tests but I dont know for sure if the problem is with the glass or body.

Any ideas, thoughts, suggestions ?

01-06-2010, 08:00 PM   #2
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did you upgrade to the latest firmware?

give that a shot first.
01-06-2010, 08:18 PM   #3
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Unfortunately, yes (1.02).
01-06-2010, 08:23 PM   #4
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You're screwed, you have a zoom that is AFing differently at differnet focal lengths.

Worst case scenario I am afraid.

01-06-2010, 08:25 PM   #5
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Huh I thought 1.01
01-06-2010, 08:26 PM   #6
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Huh I thought 1.01. For what 1.02??
01-06-2010, 08:34 PM   #7
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Alfisti: it seems to be the case. I think Ill get a Sigma with the new glasses planned (a 10-20 and a 31 prime).

Danny Delcambre: Software Downloads : Support & Service : PENTAX


Thanks for the opinions... !
01-08-2010, 01:02 AM   #8
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Can you post samples with EXIF?

04-13-2010, 04:22 AM   #9
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There is a new firmware for k7
04-13-2010, 05:22 AM   #10
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QuoteOriginally posted by StealthNet Quote
Im about to return the tamron and get a Sigma 18-250.
It seems like you got a defect copy and should have it replaced. Why do you want to change brands at the same time?

It is the first time I read someone describing such behaviour for a lens. The Tamron 18-250 should not do that. I've got that lens and AF works fine at all focal lengths.
04-13-2010, 11:39 AM   #11
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What you can do is correct the AF of your K-7 for this lens at 18mm.
Have a look at http://focustestchart.com/focus21.pdf and your K-7 manual for finding the correct option in the menu and then see what the effects are at the long end of the lens.

- Bert
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