Hi Guys
I did develop the system specifically for people using Zoom type lenses.The big problem is that you cannot zoom while the lens is on the bag, you do need to pick up the lens to zoom. Also with the focusing ring on many lenses fixed, or zoom the focus can go out the moment you pan because of the movement on the bag.
The system I developed keeps the lens away form the bag. the bag itself is also rather heavy (8kg) but is is purposely made that way to give good solid base. Like J-MS picture of is cut-up jeans. Well thought out there j-ms
The lens mounts on a panning plate which now allows you to follow through when panning and still maintian the functionality of the zoom.
Many guys with fixed lenses like 300 f.8 and 500 F4 and 600 F4 also found this sollution usefull , even if it is not the only sytem they use. It is aslo rather handy in awkward places like birdhides.
It does come out with a shoulder carry strap to make you walk to the hide a bit easier
Check out Eric Landsberg website he done a nice review on the system.
http://www.wildlife.co.za/bulletin/archive/ELWP_bulletin_007.htm
The Ergo rerst is also a very usefull tool but if you fit a head you must make sure to fit a head that suites your style of work . You dont want to loosen 3 levers before you try to compose. You do want freedom of movement.
The Ergo rest is rated well for lenses up to something like 50-500 but I will not go beyond that.
For me that uses a 600f 4 (6kg) and a 300 f2.8 (3kg) I use a bracket on the door with a gimbal when I do birds in fligh. For normal game phototography I utilize the bag and panning plate because its so easy to setup. Well there is no setup really , i just put the lens on the window sill and I am ready .