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07-26-2015, 05:26 AM   #1
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Technical notes on the MTF program line

MTF program line is one of those interesting little gems of the Pentax K system, where the camera will prefer to use the aperture at which the lens is sharpest. However, I should note the following (correct me if I'm wrong):
  • MTF information is communicated by the lens, not stored by the camera. Pentax F lenses do not provide this information and therefore do not support the MTF program line. The lens communication protocol was expanded in the FA series to add MTF information. MTF program line is available when using FA, FA J, D FA, DA, and DA L lenses, including all modern Pentax lenses.
  • Because MTF information is communicated by the lens, third-party lenses can support MTF, as suggested by this thread.
My DA* 60-250mm is indicated to be best at f/4 (wide open!) at 108mm and below, f/5 from 118mm to 140mm, and f/5.6 at 153mm and above, as reported by the Exif data. These are mostly consistent with the Photozone review of the lens, which uses the much older K10D—note that on 16+ MP sensors, diffraction is likely to start degrading sharpness earlier than what the review suggests. I'm not really a fan of shooting wide-open, though, so I'm still inclined to stop down to at least f/5.6 in most situations as I do get slightly better contrast and sharpness this way.

Questions, thoughts, or comments?

—DragonLord


Last edited by bwDraco; 07-26-2015 at 05:33 AM.
07-26-2015, 06:36 AM   #2
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QuoteOriginally posted by DragonLord Quote
Because MTF information is communicated by the lens, third-party lenses can support MTF, as suggested by this thread
My K-3 supports P mode MTF with Tamron lenses. And unlike K-5 series, K-3's matrix mode exposure metering also works with Tamron lenses.
07-26-2015, 10:47 AM   #3
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QuoteOriginally posted by biz-engineer Quote
My K-3 supports P mode MTF with Tamron lenses.
How did you determine this? As with the OP, it was my understanding that the MTF data was only available from supporting Pentax-brand lenses.

QuoteOriginally posted by biz-engineer Quote
And unlike K-5 series, K-3's matrix mode exposure metering also works with Tamron lenses.
The K-5 series as with all Pentax dSLRs should support matrix metering with any lens having "A" contacts on the mount. Are you having problems with a particular Tamron lens?


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07-26-2015, 11:41 AM   #4
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I was referring to the users manual of the K-5, note at the bottom of page 117: "Center-weighted metering mode is automatically set even if you select multi-segement metering mode when using a lens other than a DA, DA L, D FA ...." . With K-5 + tamron lenses, I could never use the multi-segment metering, as described in the manual. The K-3 does not have this restriction (there is no note in K-3's manual), and I can see that multi-segment metering now works with non Pentax lenses. Regarding P mode MTF, does not need to know much about the lens, it's one stop down from wide-open. Works with Tamron 17-50 f2.8, and Tamron 28-75 f2.8, both being stopped down to f4.

07-27-2015, 08:09 PM   #5
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QuoteOriginally posted by biz-engineer Quote
I was referring to the users manual of the K-5, note at the bottom of page 117: "Center-weighted metering mode is automatically set even if you select multi-segement metering mode when using a lens other than a DA, DA L, D FA ...." . With K-5 + tamron lenses, I could never use the multi-segment metering, as described in the manual. The K-3 does not have this restriction (there is no note in K-3's manual), and I can see that multi-segment metering now works with non Pentax lenses.
??????

I looked on p117 of the K-5 manual and can see where there may be cause for some confusion. On p337 there is a full table for supported features by mount type that clearly shows multi-segment metering being available for all lens series having "A" contacts on the mount*. The equivalent table in the K-3 manual (p81) shows the same. The behavior has been the same for all Pentax dSLR bodies since at least 2007 when I bought my K10D. I own two non-Pentax AF lenses (one KAF and the other KAF2) that support multi-segment metering just fine on both the K10D and K-3. The same is true for the PKA Adaptall-2 that I run with my Tamron's of that vintage and my Pentax-A 50/1.7, neither of which provide a lens ID to the body.

I have no clue what might be going on with your Tamron + K-5 that is different with the K-3.


Steve

* The table explicitly lists KAF, KAF2, KAF3, and KA mounts as being compatible with multi-segment metering on the K-5 and K-3. The manuals for older bodies lack the KAF3.

Last edited by stevebrot; 07-27-2015 at 08:24 PM.
07-27-2015, 08:23 PM   #6
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QuoteOriginally posted by biz-engineer Quote
Regarding P mode MTF, does not need to know much about the lens, it's one stop down from wide-open. Works with Tamron 17-50 f2.8, and Tamron 28-75 f2.8, both being stopped down to f4.
I was wondering what the failover behavior might be. I generally don't use P mode much and never fiddle with the program line settings. (I know...boring...)


Steve
07-27-2015, 10:15 PM   #7
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Steve, thanks for pointing out the more detailed table page 337 or the K-5 manual. I sold the K-5, so I can't try. Now only having a K-3 body, everything usually works fine. I don't use the P mode so much either, just played with it to see what it does. I'm content with the K-3 for size and features, only wished it had a 24x36 sensor in it :-)
07-27-2015, 10:35 PM   #8
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QuoteOriginally posted by biz-engineer Quote
I'm content with the K-3 for size and features, only wished it had a 24x36 sensor in it :-)
Me too. Me too.


Steve

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