I found the depth of field picker to be already good, you click where you want the focus to be its easy.
That's a great solution, but what about making it a little more powerfull and user friendly.
Click where you want the sharp point (like the current picker)
Ctrl-Click a second time to define the far limit
Ctrl-Click a third time to define the near limit
There's no need for additional UI buttons, only use the current picker and if you want to set additional points use CTRL and click a second time and a third time. (CTRL is only an example, could by another key or something else)
Its only a visual way of doing it, its just like moving sliders in the interface. That mean you can get the exact same settings without using the picker.
It would be good for very subtle things, like you are rendering a watch and you want that particular point to be in focus, and you want that precise piece to be the start point the blur. It would be more easy to do than with regular sliders
