What is the distance a 420-800mm lens can shoot?

That's a tough question to answer when it comes to Camera Lens', but put it this way: a 55mm lens is the "normal" distance. So if you take a picture of a tree with a 55mm lens, it captures pretty much what you see with your own eyes, now if you use a 200mm lens, you can see the same shot if you were 4x the distance than you were with the first shot with the 55mm lens. So with a 400mm lens, you can see the same shot if you're 8x the distance, 600mm will be 16x the distance, and 800mm will be 32x the distance. So that should Kinda give you an idea as to how you can see with that lens.

Hope this helped!!