Which is more worthwhile telephoto addition? I shoot Nikon (have a Z7 and my old D610) and for birding my longest focal length lens is a 300mm f4 af-s. I’m of course like everyone else I’m wanting more focal length and am trying to decide between two lenses: the Z 400mm f4.5 and the older f-mount 500mm f5.6 PF. It’s not a difference in IQ between these lenses I’m inquiring about, it’s more…