The problem with all the backpacks for photo gear that actually have some kind of actual backpacking/hiking application in mind is they are designed to carry the photo gear, i.e., the heaviest stuff in my case, in the bottom. Anybody who does any serious backpacking knows that's the not the place for heavy stuff. Ideally it should be located high and near to your back.
That's why I use standard internal-frame packs (a couple of different Ospreys for light and medium outings and an ArcTeryx Bora 80 for multi-day excursions). I put my lenses in a waterproof stuff-sack, well-padded, and my camera body and accessory pack go in another, right near the top where I can get them (albeit not the easiest access, but side-zippers make this work a bit better). The balance is much better for scrambling and such and doesn't pull away from your back. My 'pod straps centrally in back in different fashions depending on the specific pack, again, for balance and optimum weight distribution.
My needs are not the same as most, perhaps, but I've yet to find a "photo pack" that comes even close to working for what I do. I'm sure a more serious pack-maker like Osprey, ArcTeryx or Mountain Hardwear could design one if they tried.