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Originally posted by Samsungian I'll point out nikon never made a screw drive teleconverter. Pentax and Nikon both share this screwdrive autofocus technology. I know Tamron made screw drive af TC as I own it. But I have no SDM motor glass to see how it does on my K20D. I read it works fine with SDM glass. Its odd third party created a product that pentax cannot or will not. I think the tamron 1.4x for Pentax mount were discontinued in 2003. They are still available for other mounts.
I have the big-brother of the 1.5x Kenko tc, a 2x version. It works fine on all my screwdrive lenses from excellent on the FA50/1.4 to OK on the more darkish zoom lenses. Funny thing is that it didn't want to engage the SDM focus on the DA*50-135 despite that it had the SDM contacts (or once in a time long ago "power zoom" contacts). Not hunting like others were complaining on some other Kenko or Tammy tc's, but the SDM engine was completely dead.
So I oppened it, found a small electronical circuit inside, removed it, found a way to get the 7 A contacts to connect through the tc without the removed circuit. With this modification, the SDM was activated on the DA*50-135, but it hunted so much and never realy stopped in a good focus, so it was useless. When the SDM focus wont work on a f2.8 lense while the screwdrive focus work on f4 lenses or worse with the same tc, this makes me think that the SDM focus for some reason is more sensible to light losses than the screwdrive focus.
Thinking that since the screwdrive focus worked on slower lenses, it would be better than nothing if I could engage the screwdrive focus on the DA*50-135, so I tried disconnecting the SDM contacts and/or part of the A contacts, but no combination worked. Either, with A contacts, the camera (a K20D) does know that it is a SDM lens and refuse to activate the screwdrive, or, with the A contacts off, it believes it is a manual lens and refuse to activate any autofocus
Just to irritate me, the DA*50-135 and this 2x Kenko TC worked well (with screwdrive focus) on my old MZ5 and *istDS. It tells us that it is something in the new DSLRs that prevent it from activating screwdrive focus on a SDM lens.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/38514-converti...erter-sdm.html