I was wondering someone might help me. I would like to know and understand the relationship between Nikon and Tokina companies and mainly the differences in between the quality of their glasses. May I consider Tokina as second class glasses produced by a Nikon partner? I am trying to understand that because I want to buy a 500 mm and as I don't have cash enough for the Nikon prime, I am considering an used Tokina 150-500 mm. Also, comments about that specific glass are very welcome. Thanks.