Its essentially a big lens pouch. I use it for both the Canon 70-200 2.8 IS L and the Canon 100-400 f4.5-5.6 L IS , both very big & heavy lenses. The reason you might want to think about one of these guys (or the smaller ones) is your bag may be already full and you want to bring just one more lens. No problem, throw it in this pouch and attach it to the underside of your backpack. Great little investment.