Dear all, i Just received my new lens. Tamron 18-400mm for canon. i want it to replace my canon lens 75-300 for bird photography. But while Trying the lens i noticed That i was able to get closer to the subject with my 300mm lens than the 400mm Tamron. I really do not understand how That is possible. What did i Miss? can someone explain to me how this works? Thank you so much in advance!