I use the focus limiter for BIF to stop the AF switching to the sky or background.
If I am shooting against the sky, I just set the maximum distance in the limiter to, say, 200m. If I am shooting against trees, I have to measure the "distance" to the trees first (see below) and set the maximum distance to that. I can then be certain that the focus will never switch to the sky or trees. It works very well for this.
You can use the camera to measure "distance" as follow:
- Use AF to focus on the point whose distance you want to measure
- Switch AF to PreMF and press the INFO button
- The distance will then be shown in the EVF. Use MF to change the focus if you wish.
- Press Menu to get out of the PreMF setup and set AF back to whichever mode you want to use
- In the Menu, set the far distance limit to the distance you have just measured.
As the other poster has said, this is very fiddly and is only useful when you are going use that distance for a considerable number of shots.
Note that the distance measurement is extremely inaccurate at longer distances with a fast lens like the 40-150mm f/2.8 and it moves very quickly to infinity. It is more usable with a slower lens like the PL100-400mm.