Right, let me start off by saying a couple things. As far as I understand it (I've never formally studied optics, all self taught) to focus a lens you move all the lens elements together either closer or further away from the sensor, to achieve infinity focus and MFD respectively. To zoom a lens (increase or decreases the focal length) you move specific groups of lens elements closer or further away from each other. Please correct me if any of the above is wrong.
Now, when using extension tubes you're moving the entire lens (and thus all the elements) further away from the sensor which in turn decreases the MFD. Then to focus the lens you do as you normally would, turn the focusing ring to precisely alter the lens to sensor distance. So, with bellows it should be the same as they're technically just adjustable extension tubes, however because you have the ability to precisely alter the length of the bellows you can use them instead to focus, correct?
My problem is this, using my 135mm on my bellows reduces the focussing distances so much that the total range using the focussing ring is literally a couple centimetres, so instead I'd be better off racking the bellows back and fourth to focus. However when I do this the focussing distance doesn't change much (no where near enough to make them viable for focussing, it's easier to just move the whole rig forwards and backwards), instead the magnification/zoom does. I can extend and retract the bellows a few centimetres without the focus changing however the image through the viewfinder get's magnified instead. Once I reach the extremes of extension/retraction the focussing distance does indeed change, but no more than a couple centimetres.
I've tested this with the camera mounted on a tripod making sure the body and thus sensor is stationary so only the lens is moving. So, what sort of voodoo is going on here and how can I get this thing to focus like it should (to my knowledge)?