I use the lowepro flipside 400-plenty of capacity, tripod holder on the back, and you don't have to put it down. Also as it has access to the main compartment through the back, i feel it's a bit more secure from pickpockets than other bags I've had in the past
https://m.youtube.com/watch?v=DXCAMU2A2ws
http://www.amazon.co.uk/Lowepro-Flipside-400-AW-Backpack/dp/B001APJZ50
https://m.youtube.com/watch?v=DXCAMU2A2ws
http://www.amazon.co.uk/Lowepro-Flipside-400-AW-Backpack/dp/B001APJZ50