So I have my 200-400 and my 600 both tuned through the use of a test chart. Everything was set up and looked good although the 600 on its own required a setting of -20 and it still seemed to have a light bias towards rear focus but not enough to worry about. With the 1.4 attached it had a setting of -5 and was spot on. So yesterday I was out shooting and as the evening drew in I started taking photo's of some wild foxes that has become an ongoing project with a friend of mine. One of my first shots with the 600mm looked lovely, nice pose and framing of the fox and a nice surrounding. But then I look at the image on the screen and zoom in and notice the fox is 'just' out of focus even though the focus bracket was slap bang in the middle of him! And the front of a big hay bail is razor sharp, which is slightly in front of him. So I did a quick re-adjust of the focus fine tune and set it from -20 to -10 and carried on shooting. The rest of my images looked a lot better although I think there may still be a slight front focus but I've decided I won't fiddle with it again until I'm out in the field. I did all my original fine tunning with a test chart at min focus distance but I guess doing it with a target further away thats more like what you'll be shooting is a better way of doing it. The odd thing is, the 600 + 1.4 seems to be perfect regardless of how far away your subject is, and that was fine tuned at closest focus distance at the same time as the 600 on its own, which turned out to be off for further away subjects. Odd!