when ever I buy a back pack I first look for features that makes the pack useful for carrying the load such as
waist straps
The majority of a backpack's weight, 80 percent or more, should be supported by your hips.
Load Lifter Straps
These are stitched into the top of the shoulder straps, and they connect to the top of the pack frame. Ideally, they will form a 45° angle between your shoulder straps and the pack. Kept snug (but not too tight), they prevent the upper portion of a pack from pulling away from your body, which would cause the pack to sag on your lumbar region.
Sternum Strap
This mid-chest strap allows you to connect your shoulder straps, which can boost your stability. It can be useful to do so when traveling on uneven cross-country terrain where an awkward move could cause your pack to shift abruptly and throw you off-balance.
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I look at what my intent is with the back pack
am I intending on carrying kit onto an air plane
Carry-On Size Chart: 170+ Airlines - Travel Made Simple
will it be stored under the seat ahead of me or in the overhead bin?
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am I using it to carry kit from A to B
how far
what type of terrain?
walking around a city or serious back packing/hiking
from room to car
car to where
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I have accumulated the following collection as my needs and the amount of kit has changed and I use which ever one fits my purpose at the time
Lowepro Fastpack BP 150AW II
Lowepro Flipside 400AW Backpack
Ruggard Thunderhead 75 DSLR & Laptop Backpack
reviews here
Backpacks - Pentax Camera Accessory Review Database