Sigma 1.4x TC question

I understand that Pentax lens SDM does not work on Sigma 1.4 TCs but HSM does, and HSM works on a Pentax body. Does anyone know why this is?

I don't think that even Sigmas HSM lenses AF on their 1.4x or 2.0x TC when mounted on Pentax. It may be something to do with how the TC is identified by the camera perhaps ?

Neither SDM or HSM work with the Sigma teleconverters.

Oh!

According to their table here some of their HSM lenses do retain AF. Are they using screw drive?

Sigma ar just as slow as Pentax when updating their web pages.
Heres a more up to date one LINK

Thanks Mike.

Looking at the various Sigma sites the allegedly AF compatible lenses are HSM, but not in their Pentax incarnation.

Mt brain hurts.