When you see a wildlife photographer with a B.I.G white lens, we're willing to bet a half eaten packet of crisps that the lens will be a 500mm f/4. Simply put, that's because it's one of the best lenses you can buy. And it's ridiculously good for wildlife (and other long range stuff).

Need a bit more zoom? No problem. You can use either a 1.4x III Extender or 2x III Extender with the lens. The 1.4x is the popular option.. the 2x is there if you are feeling greedy.