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11-23-2012, 07:28 PM   #1
Elise
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TC comparability

Hi
I have a Pentax K-r that I'm using with a Sigma 28-300mm f/3.5-6.3 DG IF Macro Aspherical Lens. I'm wondering if the Sigma1.4 TC will be compatible and retain AF capability.
Thanks

11-23-2012, 09:20 PM   #2
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In my opinion, the hit on IQ on kind of consumer zoom will be so great that you will find a simple crop will provide a better picture. Especially at the long end where your max aperture with the TC would be f9 or whereabouts. As far as compatibility and AF, it would help to know the specific Sigma TC version but regardless, you will find that kind of lens is better off without one. It seems most of us have to go through this learning curve with trying to use teleconverters with our consumer zooms. Welcome to the club!
11-24-2012, 01:01 AM   #3
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I assume the thread title should read "TC compatibility"?

Is this the TC to which you are referring: https://www.pentaxforums.com/userreviews/sigma-1-4x-dg-ex-apo-teleconverter.html

Last edited by p38arover; 11-24-2012 at 01:15 AM.
11-24-2012, 10:54 AM   #4
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QuoteOriginally posted by Elise Quote
Hi
I have a Pentax K-r that I'm using with a Sigma 28-300mm f/3.5-6.3 DG IF Macro Aspherical Lens. I'm wondering if the Sigma1.4 TC will be compatible and retain AF capability.
Thanks
Depending on which model you are talking about.

The current Sigma tele converter "APO EX DG 1.4x TC" is only approved for a specific list of lenses. This list can be found here: Notice to customers using Sigma Tele Converters for Pentax mount | sigmaphoto.com I believe this is because the TC glass extends into the lens and can conflict with the lens glass.

But as noted above using a TC with that lens, even it it fits, is not going to get you very good pictures. Quite likely the auto-focus will not work either as the lens is already too slow. Adding a TC will cost you at least one full stop and probably a bit more on the 1.4x and two stops on the 2x. Note that in the list they say that only lenses with constant f/2.8 and f/4 aperture will work with AF. Some other, slower ones will work but only in manual focus.

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