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07-26-2012, 01:03 PM   #1
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f2.5 vs. f2.4 with the A-contacts. Has Pentax Changed the coding?

I made an intresting observation today that I wanted to share and sollict further feedback on. I was trying two copies of the Tamron Adaptall-2 PK-A mount with two Tamron Adaptall-2 lenes whose maximum aperature is f2.5 (the 28mm and the 135mm) on my K-5 with the 1.03 firmware. With both adaptors (and both lenses), the maximum aperature reported on the camera is f2.4, not f2.5. I have doubts that this is a problem with either adaptall mount, as if we review Dimitrov's chart of the A-contacts (Features and Operation of the Ka Mount), there is room for only one setting between f2 and f2.8, which he lists as f2.5. The contacts should be 011*11 to correspond to f2.5-f22. Indeed, I suspect that is the pin arrangement the Adaptall-2 mounts are giving the camera, but Pentax has changed what the contact code means from f2.5 to f2.4 at some point. After all, Pentax has two recent lenses--the DA70/2.4 and the DA35/2.4 which have a maximum aperature of f2.4 and so might have prompted Pentax to alter the coding from f2.5 to f2.4. Pentax never ended up making a PK-A lens with a maximum aperature of f2.5, so they could have easily changed it without the change affecting any of their lenses.

It would be easy for others to test: just use a lens (with A-setting) that has f2.5 as the maximum aperature (the Vivitar Series 1 105mm and the Voightlander APO-Macro 120mm come to mind besides some of the Adaptall-2) and report back what you get for the lens/camera/firmware combination. It would be great if we could get a few different camera and lens combinations, including some that are and aren't Adaptall-2 lenses.

I know the difference between f2.4 and f2.5 would be trivial to metering, but I thought the topic might be of interesting to those of us with some technical curiosity in the mount. Also noteworthy is that if I set the metering to 1/3 stops, I can set f2.5, although only when going down from f2.8, but not going up from f2.4

07-26-2012, 02:54 PM   #2
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AFAIK, the aperture display is separated from the actual metering.
07-26-2012, 03:24 PM   #3
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That's indeed strange. Lenses prepared for matrix measurement are chipped and provide much more detailed information about the lens capabilities.

I would have thought newer bodies would use the information offered by the original "A" contacts only if the chip information is missing.
Then no firmware modification would have been needed.
07-26-2012, 03:49 PM   #4
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QuoteOriginally posted by macTak Quote
...I know the difference between f2.4 and f2.5 would be trivial to metering, but I thought the topic might be of interesting to those of us with some technical curiosity in the mount. Also noteworthy is that if I set the metering to 1/3 stops, I can set f2.5, although only when going down from f2.8, but not going up from f2.4
You'll see similar results from other prime lenses whose wide open ratings are on the 1/3 stop scale but not on the 1/2 stop scale. Some examples are the SMC Pentax-A 135mm f1.8, and my KA-mount Sigma 28mm f1.8. The original KA mount was not very flexible here, and had no way to figure out a variable aperture zoom. AF lenses should send aperture information through a data pin and solve this. The newer the lens, the more data. I've read that people using newer Sigma f1.8 primes still have trouble with the camera seeing them as f1.7 lenses; in this case I think it is Pentax and Sigma disagreeing on signals sent via the data pin.

07-26-2012, 03:51 PM   #5
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QuoteOriginally posted by RKKS08 Quote
That's indeed strange. Lenses prepared for matrix measurement are chipped and provide much more detailed information about the lens capabilities.

I would have thought newer bodies would use the information offered by the original "A" contacts only if the chip information is missing.
Then no firmware modification would have been needed.
Yes, this is exactly what puzzled me; the data chip includes lens maximum and minimum aperture (as clearly seen with variable zooms). However, it's not clear how much the data contact information is used in regards to the aperture displayed (at least for constant aperture lenses). If the aperture information came solely from the data contact, then Pentax could have just stopped using the A-contacts (save the A-pin) altogether in its lenses, but they have not. I thereby suspect the A-contacts play some role (even if it is a backup role now), and therefore Pentax modified the value from f2.5 to f2.4
07-26-2012, 06:17 PM   #6
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I believe how this behaves depends on if you have your stops setup in 1/3 or 1/2 increments
07-26-2012, 08:20 PM   #7
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QuoteOriginally posted by L33tGreg Quote
I believe how this behaves depends on if you have your stops setup in 1/3 or 1/2 increments
I get f2.4 as the maximum aperture whether in 1/3 or 1/2 stop increments.

As to Just1MoreDave's remarks on Sigma and f1.8, I was aware of that, however that is somewhat different as it might have something to do with the data contact, and also the coding of the A-contacts for f1.7 has never changed. In my case, it seems like Pentax itself has changed the f-value for the 011*11 contact pattern. Dimitrov's site lists the combination as giving f2.5 and that data is from 'George de Fockert deciphered the pattern by cycling through all possible combinations on a ProgramA '. So I am suspecting that sometime between the ProgramA and the K-5 fw 1.03 Pentax changed the value.
07-27-2012, 12:55 AM - 1 Like   #8
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My youngest pentax film body (mz5n) does 2.5
My oldest pentax digital *istD does 2.4
US patent 5999753 from 1999, shows the table with 2.4

07-27-2012, 06:56 AM   #9
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QuoteOriginally posted by glasbak Quote
My youngest pentax film body (mz5n) does 2.5
My oldest pentax digital *istD does 2.4
US patent 5999753 from 1999, shows the table with 2.4
Great research! Pentax probably chose f2.5 in the first place in the event they wanted to do an A-version of the K200/2.5.
07-27-2012, 01:07 PM   #10
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I'm pretty sure the difference is in the numbers only, and never in what aperture they mean. Pentax aren't exactly good at using the right number for apertures. (Example: They use f/4.5 both for a third stop closed from f/4 and for a half stop closed from f/4. Why the latter is not f/4.8 I have no idea. Same thing with f/3.5. And before someone asks: I can see the difference in how much the blades close, so it really is just the number in this case at least.)
07-28-2012, 07:47 AM   #11
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QuoteOriginally posted by glasbak Quote
My youngest pentax film body (mz5n) does 2.5
My oldest pentax digital *istD does 2.4
US patent 5999753 from 1999, shows the table with 2.4
And on my MZ5n FA43 is identified as a f/2.0 lens, so this change might have happened to correctly identify f/1.9 FA limiteds.
07-29-2012, 08:42 AM   #12
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QuoteOriginally posted by Fogel70 Quote
And on my MZ5n FA43 is identified as a f/2.0 lens, so this change might have happened to correctly identify f/1.9 FA limiteds.
I wonder if they will (or already have) changed f1.7 to f1.8 because of the DA 50mm f1.8

Last edited by macTak; 01-26-2013 at 05:30 AM.
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