Originally Posted by ogl 
It's just great cooperation...
It's easy.
Kenko and Tokina has the same owner. Kenko and Tokina cooperate with Hoya. And it's all.
This companies are not in structure of HOYA.
That's it and Kenko/Tokina also builds Hoya Filters which seems to be a very small part of Hoya operations. That's also probably why THK includes Hoya in its name.
Now, it would be great if a Hoya/Pentax official would clear definitively the "Hoya owns Tokina" rumour.