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04-04-2008, 01:25 PM   #1
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Tele converters - how does AF work?

How does focussing with a TC work? Do they transfer the AF screw drive, or is it just electrically wired and will work with SDM lenses only? Or both, screw drive AF and SDM signals?

Sigma's lens catalogue shows that only very few of their lenses can be used with a tele converter for Pentax. Does anybody know why?

For what I understand, a TC is actually K Mount on both sides, so shouldn't any K Mount lens fit on?

04-04-2008, 01:38 PM   #2
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it all depends on the tele converter

I have a good collection of them

The Sigma 1.4x and 2x DG teleconverters have a screw drive that couples the lens to the camera body. they do not have contacts for HSM. Note the sigma TCs are not universal, the design places the front element into the lens barrel slightly and as a result they interfere with the rear element of many lenses.

The Pentax 1.7x AF TC has a screw drive, but It blocks the focus information from the lens and substitutes it with the data from the TC (this TC has moveable elements that actually make an AF adjusting lens out of a MF lens you focus close in MF and then the AF locks the image in.

Older Automatic (MF) teleconverters come in 2 varients, ones with contacts for the automatic apature, and ones with mechanical apature couplings only. The ones with contacts will pass through lens apature data for matrix metering and auto exposure modes, and also permit, with A series lenses P-TTL flash

I have heard that some suppliers have HSM compatible TCs but I have also heard complaints that the contacts for the HSM don't make reliable contact and drive the lens.
04-04-2008, 11:42 PM   #3
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So that means buying an SDM/HSM lens now leaves no possibility to use it on a TC with AF?

I suppose the SDM converter from Pentax that is shown on the lens roadmap will do...?

I'd really like to see my 40 years old M42 lenses auto focussing with the 1.7 converter you mentioned That'd really cool Do you know if it's still available new?

This is really interesting.

But ... does that mean that Pentax AF works iteratively? Otherwise it shouldn't be possible to autofocus an M-lens since the lens' focus setting is unknow ...? But if it was iterative, we should not see any kind of front or back focus issues, right?

Thanks!
04-05-2008, 03:54 AM   #4
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In the Tamron TC thread ... if you can get your hands on a 1.4x Pz-AF MC4 variant (discontinued from Tamron) ... it allows SDM functionality.

I just received mine in the mail today ... so i haven't a chance to really play with it yet.

I mounted a Sigma lens to it just to play with it ... and it seems to work fine for me.
Looking forward to a nice bright day and my 75-300mm to use.

04-05-2008, 10:48 AM   #5
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You may want to go after this https://www.pentaxforums.com/forums/photographer-s-marketplace/24445-fs-penta...f-1-7x-tc.html
from Frank. These are getting harder and harder to find and this is a vey good price considering that they generally sell on Ebay for over $400. Great item and It will always hold it's value.
04-06-2008, 01:28 AM   #6
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QuoteOriginally posted by Noisychip Quote
So that means buying an SDM/HSM lens now leaves no possibility to use it on a TC with AF?

I suppose the SDM converter from Pentax that is shown on the lens roadmap will do...?
According to my information, Sigma shall release HSM-enabled TCs by the summer. I'm personally looking forward it as I consider for a longer time buying Sigma's TC 2x (and then I'd like to have HSM enabled for future Sigma lenses). Well, I'll wait and see.
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