The shorter working distance of the 100-400mm lens can be very useful when you happen to time a visit to the beach with the nesting of horseshoe crabs. I have two hours on the beach with just me, the birds and a great number of horseshoe crabs following nesting rituals that were already ancient when the first dinosaurs showed up. I wsa so close to this mollusk-bedecked female that I needed to rack back to 191mm to get the whole carapace and telson in the frame from where I was standing. 1/1000 F6.3 ISO 400.