Split-focus Diopter
In the 70s, directors made frequent use of the split-focus diopter. With this invention it was possible to have one plane in focus in one part of the picture and a different plane in focus in the other half of the picture. This was and still is very useful for the anamorphic widescreen format, which has more depth of field.
A split diopter is half convex glass that attaches in front of the camera's main lens to make half the lens nearsighted. The lens can focus on a plane in the background and the diopter on a foreground. A split diopter does not create real deep focus, only the illusion of this. What distinguishes it from traditional deep focus is that there is not continuous depth of field from foreground to background; the space between the two sharp objects are out of focus. Because split focus diopters only cover half the lens, shots in which they are used are characterized by a blurred line between the two planes in focus.
Manipulated deep focus is not a new phenomenon, as it had also been created in other ways during the golden age of 20th century. The Diopter gave the opportunity for spectacular deep focus-compositions that would have been impossible to achieve otherwise. In the American New Wave, director Brian De Palma explored the possibilities of the split-focus diopter extensively, as did other 70s films such as Robert Wise's The Andromeda Strain and Star Trek: The Motion Picture.
Read more about this topic: Deep Focus