How to choose the right backpack
There's a lot to consider when buying a travel backpack. All of the packs here meet the carry-on size for some airlines, but not necessarily all. It's important to check the size restrictions of the specific airline you'll be travelling with to insure you can bring your bag on board with you and save on baggage fees.
Backpacks are generally available as a top load pack or front load. Front load is often preferred by backpackers, as it opens more like a suitcase allowing you easy access to all of your belongings. Top load opens at the top only.
Your pack doesn't need to be water proof, but check to make sure it's made with a water resistant material. If the safety of your belongings is important, go for a bag with lockable zippers.
In order to get a comfortable fit, measure your torso (the base of your neck to your iliac crest) and choose a pack with a torso fit range that matches.
Women, especially those on the shorter side, frequently have trouble finding packs with a torso fit that's short enough. Thankfully more and more brands are coming out with women-specific backpacks, so this is becoming less of a problem.
Padded shoulder straps, back pads and a hip belt is also important to consider.
Whether you want an external frame or internal frame is up to personal preference, but internal frames are usually suggested because of the lighter weight and reduced bulk. A frame combined with a good quality hip belt is going to shift the weight of your pack from your shoulders to your hips, making both very important features for those planning on spending a long time wearing the bag.