I have found that people for the most part need 2 backpacks, one that is around 20L and one that is around 35L. My main 2 camera backpacks are a Pacsafe Camsafe V17 and the Wandrd Prvke 35L. I can highly recommend both (although I did modify the prvke to remove some Velcro, and added Molle so I could have a second water bottle holder). I do have a third which is not a camera backpack… it’s a hiking backpack from Osprey (stratos 34). It’s great because the sleeping bag compartment fits a 100-400 in its case. Basically perfect for hiking with a trinity of zooms and all the stuff you need for hiking.