I'm having an issue with my 600mm f4 IS II, and wondering if anyone else has experienced this too. When using the lens with either version III teleconverter there is a small amount of play in the connections. If the camera and lens are rotated to portrait orientation a light pull backward on the top of the camera will cause the lens and camera to lose their connection. If IS is engaged it will stop with a sudden "clunk" and you will see a significant shift in the image if you are looking through the viewfinder, it appears to me that the lens is losing power. As soon as you stop pulling on the body the connection is restored and the IS kicks back in. In theory the same thing should happen in landscape mode by pulling back on the right hand side of the camera, but in practice I'm not able to duplicate the failure that way. I'm probably not pulling hard enough, in portrait mode gravity is assisting to break the connection.

I've tested this with multiple bodies (1DXII, 1DX, 1DsIII, 7DII), two copies of the 1.4x III, and a single copy of the 2x III and it is consistent across all of those variables. When mounting the body directly to the lens there is no issue, there is no play and I can't cause a break in the connection. Also, I have not been able to replicate the issue when mounting the TC's on other lenses (100-400 IS II, 70-200mm IS II).

Under field conditions I've only encountered this as an issue a few times. Most often when shooting from a vehicle at a downward angle with the lens cantilevered over the windowsill such that gravity is working to pull the lens away from the body. The other time I ran into this was shooting perched eagles off a tripod where the lens was aimed up, in this case it was gravity pulling the camera away from the lens. As mentioned above, in either case this only happened with the camera in portrait orientation.

Once I figured out the cause it became pretty easy to avoid the issue, but I'd like to know if anyone else has experienced this? Even if you haven't run into this under field conditions, maybe a couple people could test their lens and try to duplicate the issue? The part that confuses me is why it only happens with the TC's, if the mount on the lens is out of spec it seems like it should be an issue even when mounting just a body on its own. If it's an issue unique to my 600mm I'll send it to Canon for repair, just trying to figure out if it really is just my lens.