First of all, I am a Canon user. But my father shoots Nikon. And I am looking for something for him for Christmas. He has a D7100 (I think) and a Sigma 150-600mm. I was thinking of getting him a 1.4 extender, but not sure which one. I see that Kenko makes a fairly good one and it gets good reviews. I don't think he needs the high end Nikon extender, he just doesn't use his big lens that often. Do any of you guys have suggestions on reasonably priced extenders for his setup? I appreciate it!