Teleconverter Question

ESiggins

I'm thinking about getting a 1.4 teleconverter when I get a mid-range telphoto later this year. My question is this: I have both Sigma and Nikkor lenses and would want to use the TC with both. Both the Nikon and Sigma auto TCs say they con only be used with lenses of the same brand, but is that just marketing/not-our-fault-if-it-doesn't-work, or a real limitation? Would love to hear from anyone who has either the Sigma or Nikon auto 1.4 TCs.

Oh, and it also seems like the new TCs are designed to work with lenses that have thier own AF motors (AF-S/HSM), does that mean they will absolutely not work with a lens that uses the camera's AF motor? Are there any older TCs that will work with both AF-S and normal AF lenses? Thanks!

Fred.