I take either one or two bags on my excursions, depending on what location I am visiting, what kind of photos I want to take and how long I expect the tour to be.
I have a Crumpler Light Delight 6000.
Inside I can fit my NEX-6, the SEL-P1650 pancake lens, tactical gloves, a couple of lights, a Leatherman Multitool, the camera remote, spare batteries for the flashlights, maybe one extra lens and - if I'm really good, two small PMR radios.
If you can drive up to a location, this is more than enough to take with you - if you know what you will need.
I have decided for the Highlander Backpack New Forces 25 with a volume of - you guessed it - 25 liters.
I didn't want a too large backback since I always take my Crumpler bag anyway for the camera and small stuff. Also, it is much easier to maneuver through closed spaces if you're not impeded
by a too large backpack. I can also attach my tripod to this backpack (it's not designed for that, but it does the job). In the backpack, I can fit my tactical gloves, my bump cap, extra lights,
my gas mask and my steampunk goggles (accessories for photos), food and water, rain coat, first aid pack, disinfection agent, and maybe some more stuff that depends on the location and expected
I usually carry a rechargeable LED construction floodlight with me. This backback fits not only that big light, but also an 18-LED search light that I sometimes carry for longer walks through