Professional camera gear unfortunately is not light and, as a mountain and adventure sports photographer covering both summer and winter sports, I carry a fair amount of equipment. On top of camera bodies, lenses, flashes, light modifiers, stands, etc. I often need to pack enough outdoor gear (e.g. rain jackets, insulated jackets, sleeping bag, shelter, etc.) to stay safe in the environment I am working in.
All this equipment needs to be carried. Conventional rucksacks are fine but to protect my camera gear from trauma and rain, it's best if you place it inside your pack and protect it with padded and/or waterproof cases. This however adds to the weight and increases the time it takes you to stop, take the pack off, unload the contents, use the gear, put it away, etc. Multiply these stops over the course of a day and I'm a.) missing out on a lot of great shots and b.) potentially frustrating athletes / models / friends by slowing them down.
A few years ago, Lowepro introduced a pack for the outdoor photography market called the Lowepro Photo Sport 200 AW (view their current model here). It was fairly lightweight (1300g) and has an integral camera compartment which is accessed by swinging the pack round to your front on one shoulder.
The pack comes with an attached water-resistant rucksack cover (the AW in the title). There's also a separate compartment that sits against your back and houses a water bladder. Personally, I carry lightweight waterproof dry bags for my camera gear and prefer to use the back space for an iPad so I can show clients images from a shoot on location.
I've used the Lowepro Photo Sport for outdoor and adventure sports photography for over 4 years and have found it ideal for hiking, running and mountain biking photography or indeed any mountain sports photography where I need to move around quickly. (These days, I swap it out with an F-Stop Satori camera backpack I also own, depending on my requirements).
What I carry;
Camera body and lens (D4S and 17-35mm f2.8 lens)
Spare lens (e.g. 70-200mm f2.8)
Flash, stands and light modifiers - Reflector, softbox, grid, etc.
Tripod - Joby Focus
Outdoor clothing/equipment - Duvet jacket, hat, gloves, waterproofs, etc.
For winter mountain sports photography, e.g. mountaineering and ski touring, there's more outdoor gear required (e.g. a thick insulated jacket, ice axe, crampons, emergency shelter, etc.) so I commonly use the Photo Sport in combination with a Lowepro Toploader Pro 75 AW camera bag, wearing the Toploader Pro on the front and the Photo Sport on the back.
In the Toploader Pro, I carry my main camera body and lens along with spare memory cards, lens wipes, dry bag, map and a compass. I use the Photo Sport 200 to carry my spare equipment.
What I like about the Lowepro Photo Sport
Longevity - After 4 years heavy use it's battered and bruised (there's more than a few holes in the base and the external water bottle pocket, and the shoulder straps have started to twist) but it's still perfectly usable and comfortable.
Bottom straps - I use these for attaching crampons to the pack in the winter (If I'm not shooting mountaineering photography, it also serves as a useful home for a Joby Focus tripod, which I use as a light stand or for long exposures)
Zipped hip belt pockets - these are excellent. I fill them up with food items and batteries, memory cards, etc.
Cinch strap in camera compartment - this does an excellent job of keeping the contents from bouncing around
What would I change?
Size - I'd like the main compartment to be slightly bigger as it's difficult to put the camera back in the compartment when the main pack is full (Lowepro now make a Photo Sport 300 which I reckon is nearer 30 litres in size - see link below)
Shovel pocket - The shovel pocket on the front could do having more stretch in it so it could accomodate more gear. I generally use it to house a small reflector.
Buckles - The pack's small buckles are difficult to use in winter conditions with cold hands or gloves on