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12-19-2010, 05:42 PM   #1
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deema's Avatar

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Teleconverter and Sigma 50-500mm

As I read the information from Sigma, the 1.4 Sigma t-con for this lens is not AF on my Pentax camera. I have the non-OS version of the F4-6.3 APO DG lens. Is this correct?

Also, can I use a Tamron 1.4 F T-con with this lens?

I have a Pentax K20D. I also have the K200D.

Thanks for any assistance/advice you can share with me.

12-19-2010, 07:19 PM   #2
Lowell Goudge's Avatar

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The issue is that the lens plus any tc makes the lens too slow to AF
this is the case any time you go beyond f6.7 wide open
It is not that it won't AF at all just not reliably
12-20-2010, 05:44 PM   #3
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Hi Lowell Goudge
If you are a Pentax shooter then there is, I think, only one adapter worthy of consideration and that is Pentax's own "F SMC 1.7 AF Adapter". This is an Adapter Pentax made many many years ago and because of the realization by them that it works so very well with their DSLRs they have resurrected the manufacture of them. The problem is, they are quite popular and Pentax is not set up to make many of them, hence to secure one is relatively difficult. But if you look hard enough you will find one.
The Adapter works like this:
Once installed between the camera body and the lens it immediately disables the focus screew of the lens, (Even though the Camera setting is still set to AF) and the Adapter will take over all AF function. However since the Adapter's AF throw is very minimal (because the Adapter is so small) you need to manually focus the lens close to correct focus and then let the Adapter do the rest. The Adapter will focus very quickly.
I use my Adapter on the Pentax DA* 60-250 and must say the IQ is outstanding. I have also used the Adapter on my Tamron 24-135 f1:3.3-5.6 and at max zoom the Adapter focuses very well.
The other good thing about this Adapter is; If you use it on a fast manual prime it will AF this lens.
The photo of the traffic sign is approx. 200m away and I took the shot with the Adapter attached to the Pentax 60-250 at full zoom
(425mm). The photo is as it was taken (DNG) without any PP, only converted to JPG.
Hope you will find this info of use.

Last edited by Schraubstock; 07-24-2011 at 03:09 AM.
12-20-2010, 05:58 PM   #4
Lowell Goudge's Avatar

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I know the 1.7x well I have used it with the SMC 300F4 since 1991. Check my posts and you'll see lots of examples. But it still won't AF on a combined aperture slower thanF6.7 reliably as this is a function of all camera AF limitations not just pentax

12-21-2010, 07:14 AM   #5
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Thanks everyone. This information is extremely helpful. I am going to watch for this adapter. I also have a great MF lens that this would help.

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