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07-01-2011, 09:26 AM   #1
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K-x and Tamron 28-75 issue?

I recently tested a Tamron 28-75 f2.8. When I mount it on the K-x (firmware: 1.01) and gave it whirl, I immediately noticed a few issues.

1) I couldn’t get an accurate AF wide open at f2.8. It is coming completely OOF. Manual focusing seems to be ok and actually produced good results.

2) The exif data only shows two focal lengths. Shots from 28mm up to ~45mm the exif shows 35mm throughout. From ~50mm to 75mm exif shows 55mm all the way.

3) Exposure at f2.8 seem to be darker than f3.5-f4 ….same exact setting and shooting conditions.

4) May not be an issue but worth mentioning, RAW file isn’t showing the lens model…..I honestly do not know if it should or not but I was under the impression it did for this lens.

I did about 30 or so test shots; indoors/outdoors, low light, daylight, etc. and got the same results.

Has anyone with a K-x (fw 1.01) had an issue with Tamron 28-75 f2.8, perhaps compatibility issue? Or is lens repair in order?

Cheers!

07-01-2011, 11:10 AM   #2
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I just realized I should've put this in the troubleshooting forum...
07-01-2011, 12:21 PM   #3
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07-03-2011, 05:12 PM   #4
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Sounds like a lens contact issue (solution: clean the contacts on camera and lens).

I have the Tamron 17-50 and it reports itself OK in the EXIF under the Maker Notes and the general EXIF sections, so I expect the 28-75 would too.

Metering is also always a bit under-exposed on my 17-55 but note that (theoretically) Pentax bodies like the K-x do not support full Matrix AE metering on non-Pentax brand lenses, and default to Center weighed average metering mode when third-party lenses are used. That may explain some of your metering issues, and it is certainly something to be aware of as it can make a real difference for a lot of scenes that matrix AE can handle well but center weighed averaging may not.

Note that the reporting of EXIF data values will sometimes depend on what EXIF reader you use, how up to date it is, how intelligently it can decode the EXIF codes etc.

07-04-2011, 06:52 AM   #5
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QuoteOriginally posted by rawr Quote
Sounds like a lens contact issue (solution: clean the contacts on camera and lens).
I returned the lens back to the owner but I believe he did try to clean the contacts and still had the same incorrect focal length issue on his K100D. I suspect there maybe something else going on.

QuoteOriginally posted by rawr Quote
Metering is also always a bit under-exposed on my 17-55 but note that (theoretically) Pentax bodies like the K-x do not support full Matrix AE metering on non-Pentax brand lenses, and default to Center weighed average metering mode when third-party lenses are used.

I actually wasn’t aware of that as all except one of my lenses are Pentax. Although I do shoot Center weighted most of the time

QuoteOriginally posted by rawr Quote
Note that the reporting of EXIF data values will sometimes depend on what EXIF reader you use, how up to date it is, how intelligently it can decode the EXIF codes etc.

Agreed, the lens info do show up on Photo Me but not on PDCU4.


Thanks for your help, rawr! Cheers!
07-04-2011, 10:48 PM   #6
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My Tamron 17-50 focuses perfectly, but the company has a bad reputation for sending too high a percentage of front or back focusing lenses. Though the only off lens I have that's off now now is a Pentax FA 100mm macro, which does back focus a bit. First thing I do with a new lens is test it against a focusing distance chart.
07-05-2011, 11:17 AM   #7
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QuoteOriginally posted by mysticcowboy Quote
My Tamron 17-50 focuses perfectly, but the company has a bad reputation for sending too high a percentage of front or back focusing lenses. Though the only off lens I have that's off now now is a Pentax FA 100mm macro, which does back focus a bit. First thing I do with a new lens is test it against a focusing distance chart.
Thanks for the input. I am still looking at the Tammy 28-75 preferably good used as the new ones are a little outside of budget. Though Tamron is currently having a rebate. Either way, I will surely test rigorously whatever I end up with while still within return policy.
07-05-2011, 09:27 PM   #8
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QuoteOriginally posted by K57XR Quote
Thanks for the input. I am still looking at the Tammy 28-75 preferably good used as the new ones are a little outside of budget. Though Tamron is currently having a rebate. Either way, I will surely test rigorously whatever I end up with while still within return policy.
Good plan.

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