Started rethinking my order on the DX 50-100mm f1.8. I may opt for the 150-600mm Sport with birding season coming. The one thing I need to research is how well the D7100 does with it given that it's only rated to f5.6 with the exception of the center focus point. I've seen no complaints and the f6 at the long end isn't too concerning, particularly once I pick up the D500 (looking more now like it'll be mid-late summer after the dust settles and raw support is available in Lr), but it's always good to be sure.