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04-01-2015, 02:56 AM   #1
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HD PENTAX DA 1,4x AW and Sigma Lens - no transformation of focal length

Having unboxed my new HD PENTAX DA 1,4x AW teleconverter I had a first try with may Sigma 70-200mm F2,8 EX DG OS HSM.

Everything works fine but the focal length is not transformed when using the lens and converter on my K5. This must have the effect that the image stabilisation underperforms. Because the lens has ist own stabilisation that is no problem but:


The main idea for buying the converter was to use it for astrophotos with the O-GPS1-Astrotracer. But having the wrong focal data the Astrotracer cannot work correct, - that results in startrails.


If I could change the focal length manually (as when using manual lenses) I could correct the problem. But the menu option is grey because the lens transmits data. If I tape the data pin on the lens I can Change the focal length setting in the menu but apperture data are also blocked so I can only shoot wide open - that produces lens flare.


Has anybody an idea or trick to get me using convertrer, lens and O-GPS1?


Peter

04-01-2015, 04:33 AM   #2
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Your concern raises the question: does the in-lens stabilisation also need to know the focal length. In other words, does the addition of the 1.4X teleconverter mess around with the in lens stabilisation?

I have just confirmed that my Sigma 70mm macro is correctly identified as a 98mm focal length with the 1.4x converter on my K5.

Have you updated the firmware on the K5? Only the latest firmware takes the HD DA 1.4x AW into account.
04-01-2015, 04:51 AM   #3
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When I use the Pentax AW TC with my Sigma 105mm Macro on the k5iis with the most recent firmware the camera reports the focal length correctly.

Stabilization works correctly with that combination as well. It is my understanding that the stabilization system does need to know the correct focal length (within a few mm) to work correctly.

Last edited by Homo_erectus; 04-01-2015 at 04:52 AM. Reason: added stabilization info
04-01-2015, 07:42 AM   #4
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I'm surprised that the converter works with this lens. I was under the understanding that it hunted too much and was unusable in autofocus. I would appreciate a more detailed review of your findings on the SIGMA 70-200 EX HSM OS with the converter.

04-01-2015, 09:46 AM   #5
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I'm surprised as well... considering that the Sigma TC compatibility is ehm... very limited with Pentax bodies.... see Teleconverter Compatibility | Sigma Corporation of America
04-01-2015, 11:05 AM   #6
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Can you go into the focal length setting and edit it by hand when you use the lens? Not ideal by any means, but it would presumably resolve your issue.
04-01-2015, 11:54 AM   #7
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QuoteOriginally posted by narual Quote
Can you go into the focal length setting and edit it by hand when you use the lens? Not ideal by any means, but it would presumably resolve your issue.


No, I can not set it by hand. The appropiate menu item cannot be accessed. I can edit focal length when I use older manual lenses. Also if I put a piece of film tape over the electric data connection of the F-Mount of the lens or converter I can edit it. but then the aperture data is also blocked then.

---------- Post added 04-01-15 at 20:57 ----------

QuoteOriginally posted by bobell69 Quote
I'm surprised that the converter works with this lens. I was under the understanding that it hunted too much and was unusable in autofocus. I would appreciate a more detailed review of your findings on the SIGMA 70-200 EX HSM OS with the converter.

I did not have too much time to try out yet, but it seems to work without trouble. I go for a trip to Mallorca Island this weekend. I will report details after the journey.




---------- Post added 04-01-15 at 21:15 ----------

QuoteOriginally posted by Timd Quote
Your concern raises the question: does the in-lens stabilisation also need to know the focal length. In other words, does the addition of the 1.4X teleconverter mess around with the in lens stabilisation?

I have just confirmed that my Sigma 70mm macro is correctly identified as a 98mm focal length with the 1.4x converter on my K5.

Have you updated the firmware on the K5? Only the latest firmware takes the HD DA 1.4x AW into account.

In my opinion the internal lens stabilisation does not need to know if a converter is adapted. The internal correction corrects the angle of the movement before the "light" passes the adapter. That is why I do not expect problems with normal shooting having sensor shift switched off and in lens stabilisation on. My problems begin with the internal correction with sensor shift that is necessary for astrophotography with O-GPS1.


Yes, I have updated the Firmware. The converter recognizes the pentax lenses - not the Sigma 70-200 mm.

---------- Post added 04-01-15 at 21:17 ----------

QuoteOriginally posted by zzeitg Quote
I'm surprised as well... considering that the Sigma TC compatibility is ehm... very limited with Pentax bodies.... see Teleconverter Compatibility | Sigma Corporation of America


I will give a report after having intensively tested the thing over the weekend.


Happy Easter!



---------- Post added 04-01-15 at 21:27 ----------

QuoteOriginally posted by Homo_erectus Quote
When I use the Pentax AW TC with my Sigma 105mm Macro on the k5iis with the most recent firmware the camera reports the focal length correctly.

Stabilization works correctly with that combination as well. It is my understanding that the stabilization system does need to know the correct focal length (within a few mm) to work correctly.

I think the info about the actual focal length is essential for the camera software to perform the correct sensor shift operations. The longer the lens the smaller the field of view and the more relative movement of the sensor is necessary to counterbalance the shakes. At a given "shake-angle" the corresponding way of a "shaking pixel" on the sensor surface gets longer the longer the focal length is.


It becomes obvious in the star trailing problem (with O-GPS1 Astrotrace sensor shift). The field of view decreases with the focal length. Proportionally the "speed" of the relative movement of the stars caused by earth's rotation increases. The longer the lens the more correction (movement) must the sensor do in a given time to prevent star trails.
04-06-2015, 10:39 PM   #8
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Though I did not have a lot of opportunities (only shot with daylight) I tested the TC over the weekend and found:


- it works fine with the SIGMA 70-300 mm APO, focal length data is corrected by factor 1.4, not more focus hunting than usual.
- it works fine and precise with SIGMA 70-200 2.8 EX DG OS HSM, focal length data is not corrected, used in lens OS for stabilisation.


I am still searching for a good idea to use the 70-200 mm in a "manual mode" that allows me to set focal length by hand so that I can use the O-GPS1 with Astrotracer function.

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