[andre-mz5]

I freqeuently use Photome (see PhotoME - Exif, IPTC & ICC Metadata Editor) to check exif info, including lens info. The maker of this program has already pointed out that Sigma is using the same lens codeid for several of it's lenses so he can't disctinguish between e.g. a Sigma 70-200 HSM II and a Sigma 120-400 HSM.
However I assume lens makers do use specific lensid so it suprises me that your Tokina shows up as a Sigma.
Maybe you could check and see what Photome thinks your lens is.