Originally posted by seadog I've had a Tamron SP400 f4 Adaptall-2 for a while but just tried it on my K-5.
I've been through this and the options get very few when you get into this territory. There are two bags from lowepro that should do the job the lens trekker pro which I just bought a used version of the first one and it is great for me. I think the second version is a little longer. Then they have the pro trekker which is sized for a 600 as well and allows you to carry some extra gear. I have some different inserts that I can throw into the lens trekker for carrying extra lens if I want to take more with me and then there are several daisy chains on the outside for adding extra gear.
I think Kata has a long lens backpack and the kinesis gear is pretty good. I think for this kind of thing you really need good shoulder straps and a sturdy waistbelt. Kinesis has the backpack platform that you can add some of thier long lens cases too and that might work for you but it is pretty pricey when you start adding it up.
Then there are the DIY options. I have tried a baseball bat equipment bag, a wide mouth tool bag from lowes or homedepot(added some upholstery craft store foam to both for the padding), there are external frame packs without the bag that you can try lashing things to. But in the end for me I am planning on sticking with the lens trekker for the cost and ease of use since it was made for this type of application I don't have to do as much work to get it ready and save the cost of modifying some of the DIY options.
Its been great to get out and hike and see what I can find, it gets old driving around and hoping you come across something close to the road. Found this guy out on my second hike with the lens trekker.