Hey Bryan,
As for the 15lb pack, it's mostly my gear. (I know this is a day-pack thread, but I could pretty much justify this as a day pack depending on what I was hiking that day)
1) I have a very light down bag (western mountaineering summerlite): 500 grams. It's ridiculous how small this packs down.
2) I have a very light tent (google henry shires tarp tent; I have the Squall II): 940grams (lighter than many bivy sacks!). Held up by either hiking poles or tie to a tree.
3) After that, it's just some dehydrated food, granola bars, oatmeal, and a 2L water bottle (I use water purifying drops: Pristine Water drops). I camp and hike in the Rockies around and above tree level and use the rivers for water. Haven't had any issues with bugs from the water. Sometimes I don't even treat it.
4) I have light, natural fibers for clothing; mostly merino wool base layer and wool sweater, etc. Outdoor Research Trance Down Sweater (400grams and packs super small) and eVent Integral Designs Thruhiker are my mainstays. I've been rained/sleeted on for 24 hours straight and stayed dry and warm, even with my down bag, which can be a pain if wet (totally loses insulating properties).
5) I pack all that in a very light (not the lightest, but light) 30 liter Lowe Alpine Summit Attack bag. Not the best bag and I'll likely get another one eventually, but it does.
I carry my knife on my belt with bear spray, a few other things in the bag (MSR Pocket Rocket stove, matches, one small pot and cup), and generally I come in around 15lbs with food (water I get from rivers using the Pristine drops). My Nikon D80 adds the most weight almost!
Upside, too, is that with a light pack I can wear runners instead of my heavy hiking boots, which allows me to go farther longer. I generally hike 16K in to my favourite site, explore a bit, and hike out a couple of days later. Sore feet is something I like to avoid

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Oh yeah, this is obviously summer! For winter trips I just go nuts and bring almost everything I can think of and a ton of food! Probably 40-50 pounds for a winter 2-3day trip. But we sleep in a snow cave, so that saves on carrying a big 3-4 season tent.
Statius, 3 days with water and food, I drink nearly a gal. of water a day. how do you do that? could you post a pic of your gear or at least give a list of it and what pack you use? I am guessing that is a summer camping trip. any more info on your set would be very helpful.
Thanks again all,
Bryan