There was me thinking I'd be able to go trekking through Asia with a tiny backpack. Well I can if I don't take a big camera and don't plan to be too far off the beaten track. But the more research I do the more I realise that if I take the hammock (which I am) I'll need the sleeping bag as well. A few trial nights out on my balcony on hot nights has shown me that hammocks can be quite chilly, not to mention that mozzies will attempt to bit you through the fabric. Very impressed with the hammock otherwise!!
I have a 36L backpack that I took to Vietnam for 2 weeks in 2002. It was perfect, but I was staying in guesthouses the whole time. Throw in the sleeping bag and hammock and I just can't manage to fit everything else in. Although I'll usually be wearing my camera in a harness on my chest, I'd like to be able to pack it away on occasion. As much as people bang on about not making it obvious you've got an expensive camera, it seems bleeding obvious the moment you pull it out to take a picture. My solution to the security issue is a chest harness rather than an over the shoulder camera bag, the latter being easier to misplace or have taken. Someone would need to get pretty close and personal with me to nick my camera!!
I'd love to be able to just do carryon, but I think that's just not possible. The conundrum is that a carryon size could probably work if I used a squat short backpack, but that's a seriously uncomfortable option for serious trekking. Plus I'd have no room for extra purchases enroute (something I'll definitely need, though there's never a problem buying an extra bag).
So today I purchased a 55L trekking backpack. It has all the options I wanted, some of which my 36L pack lacked. Dual compartment, adjustable harness system, hydration bladder compatible, rain cover, snack/water compartments on the belt, and extendible/compressible. It also has a combination of top and side loading which is great, because top loading can be a real headache. And it's pretty light too. Everything fits, with room to spare!! Just got to do some measurements to see if I can get it into the carryon dimension requirements.