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05-10-2011, 09:08 AM   #1
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Double for pentax k20d with sigma

Can anyone verify that the bower sx4PK 2x teleconverter will work with the k20D with the sigma 70-200 2.8 hsm lens.

05-10-2011, 10:23 AM   #2
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I tried to get this confirmed by B&H in the ask B&H section, but could not. you really need someone to go into the store and look at them
05-10-2011, 10:35 AM   #3
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One of the online chat reps at BH told me that the bower would work, and she said the sigma would work also, which I know it doesn't, so I am a bit dubious of her knowledge level.
05-10-2011, 10:51 AM   #4
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Just confirmed with bower that this will work with the pentax and sigma combo.

05-10-2011, 08:18 PM   #5
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QuoteOriginally posted by ajaxmgr Quote
Just confirmed with bower that this will work with the pentax and sigma combo.
Note that even if the Bower 2x teleconverter has the Pz (PowerZoom) contacts, which are re-used for SDM/HSM power connections, nobody has reported 100% success with SDM/HSM lenses and Pz teleconverters. They've all reported issues with getting focus to lock, and excessive hunting, and at some zoom ranges, an inability to lock focus, and this seems to be related to how the lenses are being driven through the teleconverter, and perhaps due to the camera not having any knowledge that there's a teleconverter between the camera body and lens, which is affecting focal length.

Note also that with a 2x teleconverter you will loose two f-stops of light, and that will also impact AF performance with any lens. I have a Tamron 70-200mm f/2.8 lens, which is a screw-drive lens, and on a mostly sunny day my 2x teleconverter makes locking focus with AF difficult, my 1.7x hunted around a bit but locked reasonably fast, and my 1.5x worked almost as well as with no teleconverter. I would imagine that an HSM lens, which people have had trouble using with 1.5x Pz teleconverters, would not lock AF well at all with a 2x Pz teleconverter.

And FYI there has yet to be a single company to make a teleconverter specifically for SDM/HSM lenses - it just so happens that the old Pz contacts were re-used, so old teleconverter designs sort of work (at best) with SDM/HSM. (The old Pentax-F 1.7x AF teleconverter is a different beast... it does the AF in the converter after you roughly manually focus the lens, so SDM/HSM is irrelevant.)

Last edited by nater; 05-10-2011 at 08:27 PM.
05-11-2011, 05:22 AM   #6
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Nater, thanks for that info. A bit wierd that no one has developed a doubler for these faster lens, considering that is where the industry is going. I eventually will be purchasing a bigger lens but for now $$ wise, the doubler even at manual focus will be better for my trip than not having the extra reach.

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