I'm going to Peru for a week-long hike to Machu Picchu. I think the tour guides carry the large gear (tents, food). What backpack should I get? My local store suggested an Osprey 40 but I really have no idea what to get. Help! Thanks.
Well, Chris, first thing I'd do is double check to be sure the guides really carry the tent and food! That would have a significant impact on what you need, pack-wise.
The Talon 44
The Talon 44
That said, I would think that a pack with a capacity around 3,000 cubic inches would give you enough room for your gear—clothing, camera, and the like—as well as a little extra space in a case you need to pitch in a help with the party's load.
Your shop may have been referring to the Osprey Talon 44 ($159), a light backpacking pack that's suitable for trips where you're not carrying all of your own gear. It's a lightweight (2 pounds 10 ounces), comfortable pack that has 2,700 cubic inches of space inside. So, not a real big pack. But then, nature abhors a vacuum, and will find a way to fill whatever space you have.
Gregory's Z45 ($189) is a similarly sized pack that's a bit heavier-duty than the superlight Talon. The Gregory weighs a pound more, which won't really be felt on your back but shows itself in sturdier materials and a little more load-friendly suspension.
If you think you might need more capacity, 4,000 cubic inches would be the next step up and would give you more room without creating a huge pack. I really like REI's Flash 65 ($169), a well-designed light backpacking pack that is right at the size I just mentioned. It carries well, has lots of packing options, yet still weighs 3 pounds 2 ounces, so won't be a big burden to stuff into your luggage.