The 20L backpacks have much of the same functionality as the 30L backpacks. The 20L model is designed to carry a mid-sized DSLR with lens attached, along with other accessories in the camera compartment, as well as personal belongings or other accessories in the top compartment.
The camera compartment is completely removable so the backpack can be converted completely into a regular outdoor backpack. It also features straps to carry the Off Road walking sticks and tripod, which are easily removable without taking the backpack off your back.
The new 20L Off Road backpacks retail for $159.99. Check them out here at B&H Photo.