Originally posted by erwinx
The guy with the red cap (which he needs to cover his lens when not in use because he's lost the overpriced canon lens cap) is carrying a full backpack in order to counterbalance the heavy weight of the canon lens. If he removed the backpack, he would simply fall forward. This is what Canon calls 'Image Stablisation'.
The Nikon 300 f/4 on the other hand, is a lightweight, maneuverable lens, no need such accessories.