How Do They Do That?: Split Focus Diopters

In order to understand cinema at a deeper and more meaningful level, one must be willing to investigate the technology that makes the artistry of a film possible. And as film is an extremely technical medium to work with, it is essential to learn, in as much detail as possible, about its many technological components. 

This post will consider the split focus diopter, a device used to achieve a very specific and unique visual effect.

The split diopter, pictured above, attaches to the lens of the camera. Half of the diopter is comprised of convex glass (in this example, the right half), while the other half is left empty. This allows the camera to capture two different planes in the frame, both clearly defined and acceptably focused. The diopter is able to focus on the foreground while the camera lens simultaneously focuses on the background.


Use of the diopter achieves a similar effect to that of the traditional deep focus, in which there is a very large depth of field, in which the foreground, middle ground, and background are all in focus. However, the split diopter causes one extremely unsubtle difference: the middle-ground appears blurry, lacking the continuous depth of focus that traditional deep focus offers.

Traditional deep focus in Citizen Kane

Split diopter in Star Trek: The Motion Picture

Notice the there is an out of focus line of demarcation between the foreground and background in the frame from Star Trek: The Motion Picture.

Use of the split diopter became popular after its invention in the 1970s, and was favored especially by director Brian DePalma. Editor Vashi Nedomansky compiled all 15 of the split diopter shots contained within DePalma's 1981 Blowout in this video

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In fact, DePalma still uses it today, as is evidenced by the still below, taken from his recent film Passion:

Split diopter in Passion

Quentin Tarantino used the visual effect in his 1992 debut  Reservoir Dogs:

Reservoir Dogs