Some 'after market' non makers lenses have a reputation for not being compatible with more 'modern cameras and this may be the case with yours. The circuitry may not be quite what the lens/camera combination needs.
Originally Posted by bobwysiwyg
I have a Nikon D300 as well as my F models and one of my lenses a 400 F4 Tokina will not focus with the D300 at all. Manually it will but autofocus - NO! The same can be said about the Sigma 70-300 too, on the film cameras it is fine but at 300mm with the D300 it is useless unless used manually.