100-500mm - too short. Just put my old Kenko 3xTC on the 100-400mm II and R5. The 3xTC is not supposed to allow AF, and it won't on a DSLR, but the R5 focusses fast at 1200mm and f/17. Here is quick shot of a geranium (cropped) at 1/160s. More seriously, the 100-400mm II is working so well, bare and with TCs, I am not going for the 100-500mm. I am also resisting buying a used 400mm DO II at the price I got for my old one. I'll wait until the next lenses come out.