Finding right backpack is almost a science .. There is unfortunately absolutely no guarantee, that backpack that somebody else would recommend will fit to you and to your needs .. moreover the backpack that somebody else would praise as the best backpack in the world can easily turn out to be disastrous choice for you and wasted money ..
Let me share as a generic rant my opinion based upon my extensive experience with traveling, backpacking and hiking with my camera:
ad. big backpacks) that carry other gear and some camera stuff - 50L+ ::: I am tall guy, 6'5" (195cm) in height ... finding right backpack that would fit my camera gear + travel gear (or camping gear) and that would also fit properly to my torso length and that will have a decent carry system (hipbelt, shoulder harness, air ventilation and back panel) turned out to be impossible .. At least in segment of pure camera backpacks .. let me not bother you here with long story when I can do it short - based upon my experience I prefer to have a normal hiking backpack of my choice that fits me the best and I use a camera inserts instead .. There are many on the market, the most famous are probably f-stop ICU inserts .. (I have "small" and "medium") ..
FYI: if you're interested I was using for one year one of the best camera backpacks available -
Wandrd Fernweh ..
here is my review on their site, there are a lot of spoilers
ad. medium backpacks) they are camera backpacks primarily with a little space for something more, so 30L+ ::: here it really makes a sense to have some pure camera backpack. Not small, not big. Unfortunately even these backpacks are required to fit comfortably, Tripod, camera gear and some daily stuff will easily exceed 25 pounds (11kg) but it is often required that you have a quick access to your gear. Yes, the backpanel access is mega-cool - I won't lie you. Again to make long story short - due to my height and hiking experience I didn't find a proper backpack that I would like unconditionally .. I had
Mindshift BackLight 36L - sold after two years, I have
Tasmanian Tiger camera backpack - I hate it and like it in same time, I was testing out Shimoda Action X40 - probably the best pure camera backpack, but still, pff ++ I had several other older camera backpacks that fit into this category ... Result ? Similarly as with big backpacks I tend to use a smaller hiking backpack (my current small adventure backpack is
Ortlieb Atrack 35L) + I use my f-stop insert.
ad. small backpacks) here it probably doesn't matter .. if you like it, if it has a features that you're looking for, go for it
.. point here is, that these guys are just casual and you will not load them very heavily ... but this is not your question obviously ..
Summary: pure photography backpacks are in my opinion overpriced, they usually don't fit good as hiking backpacks and they tend to be heavy often loaded with a lot of features that nobody will use all. I've lost my faith that pure photography backpacks exists for a tall guy who wants: back access, simple and relatively light, with good carry system and balanced protection against mechanical damage and wet .. I use instead some hiking backpack (I have many) + ICU (internal camera unit) insert ... Small/casual photo bags don't matter. Get what you like.
Personal Recommendation: not going to give any regarding pure photo backpacks ... but I can share my experience that
Ortlieb Atrack is awesome piece of gear that will never let you down, that is not overpriced and that you can use for every adventure, but it is not primarily a photography backpack ..