Remember "All the president's men"?

I'm not a big fan of dioptric lenses, there are movies I cannot stand because of the use of such unpredictable pieces of glass (think to the scenes on board of the Enterprise in "Star Trek: the motion picture") but... there are exceptions.

A dioptric (split) lens is an additional lens (looking as a filter) to be used in front of your main lens (usually prime) and shifting the focal plane for a selected portion of the frame only, so to have the focus set on two subjects at the same time.

"All the president's men" is worth noting for the use of that technique. A take everybody will remember is the 6 (six, I measured yesterday) minute long telephone call Robert Redford made from his office. In addition to a very talented actor you'll need a crew of four people minimum: camera, focus puller who constantly refocus the main subject as the camera approaches (in this case it seems to simply zoom in though), dolly, and someone who constantly moves the dioptric lens in front of the (moving) camera to keep both the main character and the background in focus, until the director says it's enough and wants a more traditional close up with a blurry background. Easy, isn't it?

A concrete pillar also worked (uncredited) in this take, and was an essential element.
By the way, we've also seen the "pillar trick" more than ten years before in "Il monaco di monza" by Sergio Corbucci.

Quiet! No mistakes allowed during these six minutes.

(Starting min. 1.30)