Mystery of Split Diopter

CITIZEN KANE must be the most analyzed film of all time. And the use of deep focus cinematography is the topic many of the cinephiles would love to discuss in great length. I do admire the look of the certain frames composed in tortuously forced perspective in Orson Welles’ films, especially in CITIZEN KANE. Film history books tell you that Gregg Toland was behind this technological wizardry in this 1941 RKO film. Then, I ran into this passage written by Paul Schrader:

Sometimes a film will get an undeserving reputation for a certain kind of technical innovation. A good case in point is CITIZEN KANE. Gregg Toland was using the new coated lenses, which allowed him to let about 10 percent more light into the lens. And therefore the focal length was a little deeper. In the mythology of Hollywood the mantra has become that Gregg Toland and Orson Welles created deep-focus photography. Well, yeah, sort of. The fact is, Toland had used deep-focus photography before, and some of the most stunning “deep-focus” scenes in Kane aren’t even deep-focus—they were done with a split diopter and two different planes of focus. Game Changers: The Birth of Narrative, Paul Schrader

Actually, if you have listened to the Roger Ebert’s audio commentary on CITIZEN KANE BluRay or have read “Making of Citizen Kane” by Robert L. Carringer, you must be familiar with the fact that a large part of “deeply-focused” scenes was not deep focus, i.e. shot in one take using a wide-angle lens with a small aperture. Some of them were multiple exposures and the other were processed using an optical printer in the RKO lab. According to Linwood Dunn, the RKO special effects engineer, “in some reels the percentage of optically printed work is as high as 80 percent”. As Schrader points out, some of the most stunning “deep-focus” scenes in CITIZEN KANE are not deep-focus as advertised. But I have never heard about the use of a split diopter.

Composite Shot in CITIZEN KANE
Multiple Exposure Shot in CITIZEN KANE

A split diopter is a device, with which a cameraman can have two different planes of focus in one shot. This half-lens has been utilized in many films since ’70s, when the particular scene calls for two distinctive planes of focus, one in background and the other in front, without resorting to post-production processes or back-projection techniques. Brian De Palma is known to be particularly fond of this technique. There is a technique called “rack focus”, in which the cameraman changes the plane of focus from background to foreground, vice-versa during one shot. This can guide viewer’s attention from one character to another or from a foreground action to a background smoothly, but the invisible hand is so obvious. It may appear too manipulative in some cases. Split diopters can bring two things in focus simultaneously without causing uneasiness. However, because of its construction, the lens tends to create blurred region in the middle of screen, if not properly composed. It is not an easy device to use when the camera or the characters are moving within the frame.

Split Diopter Shot in RESERVOIR DOGS
(Director Quentin Tarantino, Cinematographer Andrzej Sekula, 1992)

Effective Use of Rack Focus Shot in DOWNTON ABBEY
(Season 4 Episode 1, Directed by David Evans, Cinematography by Nigel Willoughby, 2013)

Diopter was originally a unit for expressing the refractive power of a lens. A lens whose focal length is 1 meter is one diopter. A split(-field) diopter lens has two different refractive powers hence its name. As far as I could gather, the use of split-field diopter in photography and cinematography had never been mentioned before 1970’s in the literature. The very exhaustive research on the topic by Paul Ramaeker (Film History, 19, 179-198 (2007)) places one of the earliest references for the technique in 1966. Ramaeker mentions another technique called “swing mount lens” used by cinematographer Hal Mohr in THE GREEN PASTURES (1936) and BULLETS AND BALLOTS (1936). With the “swing mount lens”, you can tilt the lens relative to the image plane, creating varying fields of focus within a shot. For example, there is a striking quasi-deep-focus shot in BULLETS AND BALLOTS. The gangster Nick Fenner (Humphrey Bogart) is making a phone call, when his boss (Barton MacLane) appears in background. Though we can see two actions occurring in one shot, it is not well-focused and rather distracting. Since the mechanism of the “swing mount lens” is astonishingly complicated, it might not have suited to the daily routine of cinematographic work. Even though it didn’t quite catch on as tools of choice among Hollywood cinematographers, Hol Mohr nevertheless obtained the patent for this device. U.S. Patent No. 2,177,737 and 2,195,750.


Swing Mount Lens Shot in BULLETS AND BALLOTS
(Director William Keighley, Cinematographer Hal Mohr, 1936)
Swing Mount Lens. U.S. Patent 2195750

At this point, the use of a diopter lens by Gregg Toland in CITIZEN KANE remains to be unconfirmed. And as can be seen, these other techniques have their own drawbacks as well. Crisp, sharp deep focus shots can be most effective. Toland had been interested in deeply focused cinematography for many years. And we can trace his keen interest as early as 1930.


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