Backpacks are an awful way to carry photo equipment where long walks are required, except for all the other ways... which are worse. As with camera bags, there is no perfect solution.
I haven't seen all the photo backpacks available, just the ones that the local shops display: Tamrac, LowePro,Think Tank, etc.
None of the ones I've seen in the store have anything approaching a proper internal-frame suspension system---an absolute requirement for carrying heavyish loads over any appreciable distance.
In 2013, I bought a
Photobackpacker P3 pack with the full size, customizable padded insert.
I used it on one trip and then decided I had gotten too large a model. It
does properly suspend the load and transfer the load to the hip belt.
http://www.photobackpacker.com/home.phpIf I had to do it again, I'd maybe get a smaller pack, and/or a smaller insert for the equipment.
The rational alternative is to get a really good non photo-oriented, internal frame day pack, and pack your equipment as needed.
Most of the time I use an ordinary Kelty Redwing 2650---a barely-acceptably-suspended, medium-sized day pack---and just dump the lenses in using individual LowePro lens cases or thin nylon stuff sacks.
In most of the places I hike with a camera, dust is a major concern, so having all the lenses exposed to the hostile sand/dust environment every time I open a pack that is employing the open, customizable padded partition-style inserts is not desirable.
Usually, I end up keeping the lenses in individual cases, or in nylon stuff sacks. The individual cases are a time-consuming hassle when changing lenses, but they do protect from shock as well as dust.
The stuff sacks provide no padding or shock resistance, but do keep the items clean and from grinding metal chips out of each other.