A camera bag is a must-have to protect my gear. I own several camera bags so that I can take the right bag with me for different occasions. If I want to travel light for a day or two, e.g. when on a city trip, I take my camera, one or two lenses (a 24-105mm, or the 20mm and the 50-300mm), maybe pola and ND filters: this fits perfectly well in the Lowepro Event Messenger 150. When going on a longer trip or out on a landscape shoot, I take my camera, two or three lenses (16-35mm, 20mm, 50-300mm), pola, ND + NDgrad (LEE) filters: all of this fits in the still relative small Tenba Messenger DNA 13. Finally, on a wildlife or sports shoot, I take my camera and one of two lenses (at least the 200-600mm, possibly the 24-105), pola and ND filters, and a hat to protect myself from the sun: the Tenba Axis v2 20L backpack can take this with ease.

Of course all of these options still have some space for spare batteries and memory cards.