For long backpacking trips or more technically oriented outdoor adventure where weight is critical, I’ll use the Lowepro TLZ 1 Toploader supported by a Lowepro TLZ Chest Harness. This system puts a Canon 5D Mark II with Canon EF 24-105mm f/4L IS right on my chest ready for immediate access, even while still wearing a full-size backpack.
In addition to the Toploader, I will usually put a Canon EF 16-35mm f/2.8L in a Lowepro belt pouch on my backpack waist strap, stuff a Sunpak RD2000 flash into a pants pocket and carry more small stuff in my backpack’s top pouch (shutter release cable, ND grad filters, spare batteries, Cokin P filter holder, memory cards, Lenspen, microfiber cloth).
This is the system I use for long backpack trips (5+ days), climbing and mountaineering.
I recently used this setup on an 8-day 65 mile back-country traverse of the Bailey Range in Olympic National Park. The photo gear on this trip weighed 7.5 pounds, plus a 3.5 pound tripod (Flashpoint F-1128 carbon fiber legs with Manfrotto 484RC2 ball head). My complete backpack, however, totaled 65 lbs, with food, camping gear, ice axe and crampons. “65 pounds of ultra-light-weight camping equipment,” we would joke to ourselves.
Hopefully I’ve fueled your camera bag addiction a bit. Now it’s time to find a bag to house my new Sony NEX-7 system.