Focal plane setup
When applying defocus sometimes it is great to see what would be in focus and what is in the near and far field. Using the focal plane preview option this is possible.
Only for depth and camera based
When using 2D, everything is set by the size knob, so there is no point in having a focal plane setup. In that case nothing will be rendered.
How to use
To activate it, in the render knob, select focal plane setup. This will activate the preview rendering. No defocus will be shown.