Cinematography: Split diopter

SPLIT FIELD DIOPTER
AUDIOVISUAL SPLIT FOCUS
 Star Trek: The Motion Picture (1979)

Star Trek: The Motion Picture (1979)

In this article, I want to introduce the idea and technique of split focus. In cinematography, the concept of split focus was explored mainly during 1970’s to the early 1980’s, namely by Brian De Palma and Robert Wise.



But even to this date, this technique is still used, probably as homage to these directors. Interestingly, the concept of split focus can also be found in sound .

What always interests me is the narrative motivation for specific concept or technique, so here are 3 possible narrative motivations for the use of audiovisual split focus:

to introduce multiple elements without editing
to intensify the viewer’s emotional response
to add sense of realism
To discuss this technique in more detail, we have to briefly introduce/remind few facts about depth of field (DOF).

Depth of Field (DOF)

Depth of field (DOF) is a great and very powerful tool how to direct or guide the audience where to look at, i.e. how to tell them what is important in the image.

If you look at the image below, your attention most probably goes to the girl, which is in focus.

The Enigma by Andrew Mohrer

The Enigma by Andrew Mohrer

Technically speaking, DOF is the distance range in the image along the Z axis that appears acceptably sharp (=area perceived as being in focus). Although the lens can truly focus only at one distance (plane) along the Z axis, the transition between area in focus and out of focus is gradual; therefore we can talk about “field” (instead of plane).

In cinematography, a large DOF is often called deep focus (think Orson Welles and Citizen Kane) and a small DOF is often called shallow focus (think Gus van Sant and Paranoid Park).

Unfortunately, things are little bit more complicated than this; DOF is influenced mainly by the aperture of the lens: Larger aperture produces shallow focus, while small aperture produces deep focus.

But at the same time, aperture controls how much light hits the film or digital sensor, so you have to find the balance between DOF and exposure by:

some kind of compensation (additional light for small aperture, ND filters for large aperture)
using optical tricks
using a device called split field diopter.
Split field diopter

Star Trek: The Motion Picture (1979)
Star Trek: The Motion Picture (1979)

With the invention of split field diopter, it is possible to achieve the look and feel of deep focus (=large DOF) without the need of using additional lights (to compensate for the small aperture).

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 element. (1)

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. 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.(1)

Note: The following text consists of excerpts taken from the article “Notes on the split-field diopter” by Paul Ramaeker, available after free registration at jstor.org.

The split-field diopter lens simply permits focusing on a very close object on one side of the frame, while a distant subject is photographed normally through the uncovered portion of the prime lens; in this way, the shot may be focused on both near and far subjects simultaneously.

Star Trek: The Motion Picture (1979)
Star Trek: The Motion Picture (1979)

The split-field diopter, while it could not provide true deep focus, could create an impression of considerable depth, without the requirement of any additional light. Previously, such effect was only feasible by combining two separate shots in an optical printer via a matting process.

While the edge of the glass at the split between the diopter portion of the lens and the flat glass is invisible, the diopter part itself has an extremely shallow depth of field. As a result, the region where the lens is split, in the center of the frame, will appear blurred due to the focal differences between the diopter and the prime lens. In order to maintain the illusion of depth and obscure the use of the diopter, this focal difference must be hidden in some way.

Star Trek: The Motion Picture (1979)
Star Trek: The Motion Picture (1979)

For example, the edge of the diopter lens may be positioned so that it lines up with a straight edge in the background – such as the corner of a room, the edge of a column or a bookcase. A neutral background, of uniform color and texture, may hide the split, but may not always be feasible.

Two examples from Dressed to Kill, 1980

Dressed to Kill (1980)
Dressed to Kill (1980)

Unbeknown to either detective Marino or Dr. Elliott, Peter (the young man sitting outside) is bugging the conversation inside detective’s office. The staging and use of split field diopter functions here to unite a disparate group of characters variously connected to a woman’s death and to denote their differential access to narrative information.

Dressed to Kill (1980)
Dressed to Kill (1980)

Peter times exits from Dr. Elliott’s office so as to judge what sort of delay to put on the camera he is hiding at the scene.

Other Examples



Reservoir Dogs (1992)
Reservoir Dogs (1992)

Equilibrium (2002)
Equilibrium (2002)

The Untouchables (1987)
The Untouchables (1987)

Pulp Fiction (1994)
Pulp Fiction (1994)

Valibation (2013)
Valibation (2013)



Blurry line between audio and visuals

By now you’ve probably noticed the blurry line between the two areas in focus. Well, the same blurry line can be found between audio and visuals. These two guys have more in common than you might think.

My favorite example is when Walter Murch talks about editing in relation to Beethoven’s music or when Janusz Kaminski talks about shutter angle and describes actor’s movement using musical term.

One of the most recent examples of the correlation between audio and visuals is when Phil Benson is discussing the balance between music, sound effects and dialogue at the Sundance Film Festival. To quote Phil Benson:

“Imagine if you could have shallow DOF with like 4 different areas on the screen, your audience wouldn’t know what to focus on […], so imagine sound as being somewhat similar in some ways to playing with DOF, visually.”


Now, we know that split field diopter enables us to have two subjects simultaneously in focus at different distances along the Z axis. In other words, it creates two separate DOFs and the look and feel of deep focus (=large DOF). And I was wondering, what would be the equivalent in sound?



If you've ever been watching a film, say Carrie or Reservoir Dogs, and you've seen a shot that had incredibly deep focus, then maybe you were looking at a shot that used a split focus (or split field) diopter. These lens attachments produce a signature look that puts objects near and far into focus, a look that made Gregg Toland a legend, and later swept over the filmmaking world in the 70s. Vashi Nedomansky of Vashi Visuals takes a look at the use of split focus diopters throughout cinema, and picks out all 15 shots from Brian De Palma's 1981 film Blow Out that use one.

 But, for those who aren't familiar with them or have never considered using them in your films, many, many movies, especially in the 70s and 80s, used split diopters to get that deep focus look. Probably the most famous use of diopters is in Orson Welles' Citizen Kane, but All the President's Men, The Thing, and many of De Palma's films also utilize the effect.

Since split focus diopters are half convex glass, when attached to your camera's lens, it makes one side nearsighted, and the other farsighted. What this allows you to do is focus on multiple planes -- you could have objects in the foreground and background completely in focus. This also means that the staging of your scene must be copacetic.

The way that many cinematographers use this is to add a little mystery or uncertainty to their shot. Since our eye doesn't see things in this way naturally, it can be a jarring and disorienting experience when it does. In fact, some shots look almost like two separate images juxtaposed together, which immediately adds a layer of fantasy.

In the shot from Reservoir Dogs above, you can see that the split diopter creates a clear line down the frame, which can be masked using several techniques, like shooting the split against vertical lines or darkness.

All the President's Men (1976)

A Split Focus 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 element. 

In the 1970’s and 1980’s, Brian De Palma championed the use of this tool to enhance the visual and emotional experience of his films.

The Split Diopter allows for Deep Focus cinematography but requires much less light. 


It also delivers a distinctive look that blends sharp and out-of-focus imagery all in one frame. 


Subjects in both foreground and background can be kept in focus. 

Reservoir Dogs (1992)

The Split Diopter has a clear line 

down the middle of the 

frame between foreground and 

background. - 

See more at: http://vashivisuals.com/splitting-focus-de-

palmas-blow/#sthash.fc09JJoc.dpuf




In the video below are all 15 Split Diopter shots from Brian De Palma’s film Blow Out (1981).







Vertical lines in the frame can be used to hide the shift between foreground and background focus

All 15 Split Diopter Shots in BLOW OUT from Vashi Nedomansky on Vimeo.




The bottom line is that Split Diopters help achieve the cinematic and story goal of emphasizing subjects as needed by the Director and DP. When utilized for decisive or important moments…they can visually amplify the dramatic tension for the viewer. If used too often, just like with any other technique, the magic of this tool will diminish and the effect will become obvious. - See more at: http://vashivisuals.com/splitting-focus-de-palmas-blow/#sthash.fc09JJoc.dpuf